,radioisotopes and ,life ,processes ,the ,understanding the ,atom ,series ,nuclear energy is playing a vital role in the life of every man1 woman1 and child in the ,united ,states today4 ,in the years ahead it will affect increasingly all the peoples of the earth4 ,it is essential that all ,americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them4 ,the ,united ,states ,atomic ,energy ,commission provides this booklet to help you achieve such understanding4 ,illustration3 ,edward ,j4 ,brunenkant ,edward ,j4 ,brunenkant1 ,director ,division of ,technical ,information ,,united ,,states ,,atomic ,,energy ,,commission ,dr4 ,glenn ,t4 ,seaborg1 ,chairman ,james ,t4 ,ramey ,wilfrid ,e4 ,johnson ,dr4 ,theos ,j4 ,thompson ,dr4 ,clarence ,e4 ,larson ,radioisotopes ,,and ,,life ,,processes by ,walter ,e4 ,kisieleski and ,renato ,baserga ,,contents ,,introduction #a ,,cell ,,theory3 ,,dna ,,is ,,the ,,secret ,,of ,,life #b ,,radioactive ,,isotopes3 ,,the ,,biological ,,detectives #aj ,,dna ,,synthesis3 ,,the ,,autobiography ,,of ,,cells #ae ,,rna ,,synthesis3 ,,how ,,to ,,translate ,,one ,,language ,,into ,,another #be ,,protein ,,synthesis3 ,,the ,,molecules ,,that ,,make ,,the ,,difference #ce ,,cell ,,cycle ,,and ,,gene ,,action3 ,,life ,,is ,,the ,,secret ,,of ,,dna #cg ,,isotopes ,,in ,,research3 ,,probing ,,the ,,cancer ,,problem #dc ,,conclusions #de ,,suggested ,,references #dg ,united ,states ,atomic ,energy ,commission ,division of ,technical ,information ,library of ,congress ,catalog ,card ,number3 #ff-faijh #aiff2 #aifg7,rev47 ,,the ,,cover ,the cover design portrays the inter-relationships suggested by the title of this booklet3 ,on a trefoil symbolizing radiation are superimposed a dividing cell1 a plant1 an animal1 and a double helix of a molecule of deoxyribonucleic acid1 a material unique in and fundamental to all living things4 ,,the ,,authors ,illustration3 ,,walter ,e4 ,,kisieleski is an ,associate ,scientist in the ,division of ,biology and ,medicine of the ,argonne ,national ,laboratory4 ,he was formerly associate professor of chemistry at ,loyola ,university in ,chicago4 ,his undergraduate studies were at ,james ,millikin ,university in ,decatur1 ,illinois1 and his graduate studies were at the ,university of ,chicago4 ,he received an ,honorary ,doctor of ,science degree from ,james ,millikin ,university in #aifb4 ,in #aieh he was a delegate to the ,second ,atoms for ,peace ,conference in ,geneva1 ,switzerland4 ,he was visiting lecturer in the department of biochemistry at the ,university of ,oslo in ,norway in #aifc4 ,dr4 ,kisieleski is shown operating an automatic windowless strip counter that scans paper chromatograms and thus locates labeled substances4 ,illustration3 ,,renato ,,baserga was born in ,milan1 ,italy1 and received a medical degree from the ,university of ,milan in #aidi4 ,he is presently research professor of pathology at the ,fels ,research ,institute at ,temple ,university ,medical ,school in ,philadelphia1 and associate editor of the journal1 ,cancer ,research4 ,formerly he was associate professor of pathology at ,northwestern ,medical ,school in ,chicago1 where he was the recipient of a ,research ,career ,development ,award from the ,national ,institutes of ,health4 ,radioisotopes ,,and ,,life ,,processes ,by ,,walter ,e4 ,,kisieleski and ,,renato ,,baserga ,,introduction ,here and elsewhere we shall not obtain the best insight into things until we actually see them growing from the beginning4 ,aristotle ,the nature of life has excited the interest of human beings from the earliest times4 ,although it is still not known what life is1 the characteristics that set living things apart from lifeless matter are well known4 ,one feature common to all living things1 from one-celled creatures to complex animals like man1 is that they are all composed of microscopic units known as cells4 ,the cell is the smallest portion of any organism that exhibits the properties we associate with living material4 ,in spite of the immense variety of sizes1 shapes1 and structures of living things1 they all have this in common3 ,they are composed of cells1 and living cells contain similar components that operate in similar ways4 ,one might say that life is a single process and that all living things operate on a single plan4 ,the past few years have been a time of rapid progress in our understanding of the mechanisms that control the function of living systems4 ,this progress has been made possible by the development of new experimental techniques and by the perfection of instruments that detect what happens in the tiny world of molecules4 ,prominent among the methods that have contributed to the explosive growth in our understanding of biology is the use of radioactive isotopes as laboratory tools4 ,in this booklet we shall attempt to give an account1 in chemical terms1 of the materials from which living matter is made and of some of the chemical reactions that underlie the manifestations and the maintenance of life4 ,to accomplish this1 we have chosen to describe three types of molecules that have become the basis of modern biology3 deoxyribonucleic acid 7,,dna71 ribonucleic acid 7,,rna71 and proteins4 ,we will show how radioactive isotopes can be used to pry into the innermost secrets of these substances4 ,before we can understand the function of these precious molecules1 however1 it will be necessary to review the structure of a cell and the physical nature of radioactive isotopes4 ,,cell ,,theory3 ,,dna ,,is ,,the ,,secret ,,of ,,life ,we have seen that all organisms are composed of essentially like parts1 namely cells2 that these cells are formed and grow in accordance with essentially the same laws2 hence that these processes must everywhere result from the operation of the same forces4 ,theodor ,schwann ,unit of ,life ,the cell theory1 based on the concept that higher organisms consist of smaller units called cells1 was formulated in #ahch by two ,german biologists1 ,mathias-,jacob ,schleiden1 a botanist1 and ,theodor ,schwann1 an anatomist4 ,the theory had far-reaching effect upon the study of biological phenomena4 ,it suggested that living things had a common basis of organization4 ,appreciation of its full significance1 however1 had to await more precise knowledge of the structure and activities of cells4 ,some organisms1#a for instance1 amoebae1 consist of a single cell each and are therefore called unicellular organisms4 ,higher animals are multicellular1 containing aggregations of cells grouped into tissues and organs4 ,a man1 for instance1 consists of millions of many different cells performing a variety of different functions4 ,cells of higher animals differ vastly from one another in size1 shape1 and function2 they are specialized cells4 ,illustration3 ,figure #a ,one of the earliest photographs of cells taken with a microscope4 ,this photomicrograph shows cells in the blood of a pigeon4 ,it was made by ,j4 ,j4 ,woodward1 ,u4 ,s4 ,army surgeon1 in #ahga4 ,woodward had made the first cell micrograph 7a graphic reproduction of the image of an object formed by a microscope7 in #ahff4 ,there is a remarkable similarity1 moreover1 in the molecular composition and metabolism#b of all living things4 ,this similarity has been taken to mean that life could have originated only once in the past and had a specific chemical composition on which its metabolic processes depended4 ,this structure and metabolism were handed down to subsequent living things by reproduction1 and all variations thereafter resulted from occasional mutation1 or changes in the nature of the heredity-transmitting units4 ,one of the most extraordinary of all the attributes of life is its ordered complexity1 both in function and structure4 ,it is agreed among biologists that the functional manifestations of life include movement1 respiration1 growth1 irritability 7reaction to environmental changes71 and reproduction and that these phenomena are therefore possessed by all cells4 ,the first four of these can be grouped under a single word3 metabolism4 ,we can therefore say that living things have two common properties3 metabolism and reproduction4 ,therefore1 when we say we are studying life processes1 we actually are studying the metabolism and reproduction of cells4 ,since metabolism is the sum of the biochemical reactions taking place in a living organism1 it properly belongs to the field of investigation of biochemists4 ,cell reproduction is the concern of both biochemists and morphologists#c since it can be studied by either biochemical or morphological techniques4 ,cell ,structure ,illustration3 ,figure #b ,generalized diagram of a cell1 showing the organelles1 or 8little organs01 of its internal structure4 ,the organelles that are labeled are important for this booklet4 ,cell membrane ,cytoplasm ,chromatin ,mitochondrion ,nucleolus ,endoplasmic reticulum ,nucleus ,nuclear membrane ,the basic structure of a cell is shown in ,figure #b4 ,each cell consists of a dense inner structure called the nucleus1 which is surrounded by a less dense mass of cytoplasm4 ,the nucleus is separated from the cytoplasm by a double envelope1 called the nuclear membrane1 which is peppered with perforations4 ,the cytoplasm contains a network of membranes1 which form the boundaries of countless canals and vesicles 7or pouches71 and is laden with small bodies called ribosomes4 ,this membranous network is called the endoplasmic reticulum and is distinct from the mitochondria1 which are membranous organelles 7little organs7 structurally independent of other components of the cytoplasm4 ,the outer coat of the cell is called the cell membrane1 or plasma membrane1 and forms the cell boundary4 ,illustration3 ,figure #c ,electron micrograph of a primary spermatocyte cell of a grasshopper1 showing the nucleus 7,n71 endoplasmic reticulum 7,,er71 mitochondrion 7,m71 chromatin 7,c71 nuclear membrane or nuclear envelope 7,,ne71 cell membrane 7,,cm71 and intercellular space 7,i74 ,the magnification is about #be1jjj times the actual size4 ,the nucleus1 which in many cells is the largest and most central body1 is of special importance4 ,it contains a number of threadlike bodies1 or chromosomes1 that are the carriers of the cell's heredity-controlling system4 ,these contain granules of a material called chromatin1 which is rich in a nucleic acid1 ,,dna 7deoxyribonucleic acid74 ,the chromosomes usually are not readily seen in the nucleus except when the cell1 along with its nucleus1 is dividing4 ,when the nucleus is not dividing1 a spherical body1 the nucleolus1 can be seen4 7,in some nuclei there may be more than one47 ,when the nucleus is dividing1 the nucleolus disappears4 ,not all cells possess all these structures4 ,for instance1 the red cells of the blood do not have a nucleus1 and in other cells the endoplasmic reticulum is at a minimum4 ,the diagram 7,figure #b7 is valid for a great majority of the cells of higher organisms4 ,the cell structures shown in ,figure #c are visible with an electron microscope4 ,they contain the chemical components of the cell4 ,the chief classes of these constituents are the carbohydrates 7sugars71 the lipids 7fats71 the proteins1 and the nucleic acids4 ,however1 a cell also contains water 7about #gj of the cell weight is due to water7 and several other organic and inorganic compounds1 such as vitamins and minerals4 ,carbohydrates serve mostly as foodstuff within the cell4 ,they can be stored in several related forms4 ,further1 they may serve a number of functions outside the cell1 especially as structural units4 ,in this way structure and function are correlated4 ,lipids in the cell occur in a great variety of types3 alcohols1 fats1 steroids1 phospholipids1 and aldehydes4 ,they are found in all fractions of the cell4 ,their most important functions seem to be to form membranes and to give these membranes specific permeability4 ,they are also important as stores of chemical energy1 mostly in the form of neutral fats4 ,illustration3 ,figure #d ,scientists using an electron microscope 7left7 and an optical microscope 7right7 in fundamental biochemical research4 ,both instruments are important tools in studies of life processes4 ,the proteins occur in many cell structures and are of many kinds3 ,enzymes1 the catalysts for the cell's metabolic processes1 are proteins1 for instance4 ,the nucleic acids are ,,dna and ,,rna 7ribonucleic acid71 which function together to manufacture the cell's proteins4 ,since a large share of the remaining pages will be devoted to a discussion of proteins and nucleic acids1 at this point we need only emphasize that these two types of materials are interrelated in their function and that both are essential4 ,the ,two ,nucleic ,acids ,it is not very fruitful to discuss whether proteins or nucleic acids are more important4 ,that question is something like the one about the chicken and the egg4 ,we cannot think of one without thinking of the other4 ,although our insight into the mutual dependence of these two materials has greatly increased in recent years and although we know the relation between them is a fundamental factor in such events as reproduction1 mutation1 and differentiation 7or specialization7 of cells1 our understanding of their interplay is far from complete4 ,real understanding of the relation between them would give us insight into the essence of growth--both normal and abnormal--or1 indeed1 one could almost say1 into the complexity of life itself4 ,illustration3 ,figure #e ,photomicrograph of ,paramecia1 one-celled animals1 magnified #aajj times4 ,many of the same structures that appear in ,figure #c can be seen here4 ,this photo was taken with an 8interference0 microscope designed to permit continuous variation of contrast in the subject under study4 ,practically all the ,,dna of most cells is concentrated in the nucleus4 ,,rna1 on the other hand1 is distributed throughout the cell4 ,some ,,rna is present in the nucleus1 but most of it is associated with minute particles in the cytoplasm known as microsomes1 some of which are especially rich in ,,rna and are accordingly named ribosomes4 ,these are much smaller particles than the mitochondria4 ,illustration3 ,figure #f ,stages of the mitotic cycle in a hypothetical cell with four chromosomes4 7a7 ,interphase 7b7 ,early prophase 7c7 ,prophase 7d7 ,metaphase 7e7 ,anaphase 7f7 ,telophase 7g7 ,interphase1 daughter cells ,mitosis ,one of the most remarkable characteristics of cells is their ability to grow and divide4 ,new cells come from preexisting cells4 ,when a cell reaches a certain stage in its life1 it divides into two parts4 ,these parts1 after another period of growth1 can in turn divide4 ,in this way plants and animals grow to their normal size and injured tissues are repaired4 ,cell division occurs when some of the contents of the cell have been doubled by replication1 or copying 7to be discussed later74 ,the division of a cell results in two roughly equal new parts1 the daughter cells4 ,the process of cell division is known as mitosis and is diagrammed in ,figure #f4 ,mitosis is a continuous process2 the following stages of the process are designated only for convenience4 ,during interphase the cell is busy metabolizing1 synthesizing new cellular materials1 and preparing for self-duplication by synthesizing new chromosomes4 ,in prophase the chromosomes1 each now composed of two identical strands called chromatids1 shorten by coiling1 and the nucleolus and nuclear membrane disappear4 ,during metaphase the chromosomes line up in one plane near the cell equator4 ,at anaphase the sister chromatids of each chromosome separate1 and each part moves toward the ends1 or poles1 of the cell4 ,during telophase the chromosomes uncoil and return to invisibility2 a new nucleus1 nucleolus1 and nuclear membrane are reconstituted at each end1 and division of the cell body occurs between the new nuclei1 forming the two new cells4 ,each daughter cell thereby receives a full set of chromosomes1 and1 since the genes are in the chromosomes1 each daughter cell has the same genetic complement4 ,illustration3 ,figure #g ,photomicrograph of cells of the ,trillium plant1 which has five chromosomes1 in anaphase4 ,note the duplicate sets of chromosomes moving to opposite poles of the cell4 ,all life processes use up energy and therefore require fuel4 ,the mitochondria have a central role in the reactions by which the energy of sugars is supplied for cellular activity4 ,the importance of this vital activity is obvious4 ,in this booklet1 however1 we are concerned with the processes1 involving nucleic acids and proteins1 that can be described as making up 8the gene-action system04 ,the gene-action system is the series of biochemical events that regulate and direct all life processes by 8transcription0 of the genetic 8information0 contained in molecules of ,,dna4 ,,radioactive ,,isotopes3 ,,the ,,biological ,,detectives ,man 444 has found ways to amplify his senses 444 and1 with a variety of instruments and techniques1 has added kinds of perception that were missing from his original endowment4 ,glenn ,t4 ,seaborg ,atomic ,structure ,practically everyone nowadays is to some extent familiar with the atomic structure of matter4 ,atomic energy1 nuclear reactors1 and radioisotopes are terms in everyday usage4 ,however1 to appreciate how radioisotopes can be applied to the study of life processes1 we must have at least a working knowledge of their properties1 their preparation1 and their limitations4 ,it is therefore appropriate to examine them in detail so that the succeeding chapters will be more easily understood4 ,according to present-day theory1 an atom consists of a nucleus#d that is made up of protons and neutrons#e and is surrounded by electrons4 ,in each atom there is an equal number of protons 7positively charged7 in the nucleus and electrons 7negatively charged7 moving concentrically around the nucleus2 since neutrons have no electrical charge and since protons and electrons cancel each other's charges1 the whole atom is electrically neutral1 or uncharged4 ,each atom is identified by an atomic number and an atomic weight4 ,the atomic number of an element 7for example1 carbon1 nitrogen1 oxygen7 is determined by the number of protons1 or positive charges1 carried by the nucleus 7or by the number of electrons surrounding the nucleus1 which is the same74 ,the atomic weight is the weight of an atom as compared with that of the atom of carbon1 which is taken as a standard4 ,the weight1 or mass1 of an atom is due chiefly to its protons and neutrons because the mass of its electrons is negligible4 ,isotopes ,atoms of the same element1 that is1 atoms with the same number of protons and electrons1 may vary slightly in mass because of having different numbers of neutrons4 ,since the chemical behavior of an element depends upon its electrons' electrical charges1 extra neutrons 7which do not have an electrical charge7 may affect the mass of an atom without disturbing its chemical properties4 ,atoms having the same atomic number but different atomic weights are called isotopes4 ,for example1 as shown in ,figure #h1 the isotope ,h1 or ordinary hydrogen1 consists of a nucleus containing a proton 7charge3 #a2 mass3 #a7 around which revolves an electron 7charge3 -#a2 mass3 negligible72 ,h1 known as deuterium1 contains an additional nuclear particle1 a neutron 7charge3 #j2 mass3 #a72 ,h1 or tritium1 contains two neutrons4 ,since the chemical behavior of an element depends upon the number of its electrons1 these three atoms1 although differing in weight1 behave identically in chemical reactions4 ,for convenience1 the atomic weight is written as a superscript to the left of the element's symbol4 ,for instance ,c is the isotope of carbon with an atomic weight of #ad 7ordinary carbon is the isotope with an atomic weight of #ab1 and it is written ,c74 ,illustration3 ,figure #h ,isotopes of hydrogen4 ,hydrogen 7,h7 ,deuterium 7,h7 ,tritium 7,h7 ,practically all elements have more than one isotope4 ,there are two general classes of isotopes1 stable and radioactive4 ,stable isotopes have no distinguishing characteristic other than their mass2 radioactive isotopes not only differ from their brothers in mass but also are characterized by unstable nuclei4 ,when the nucleus of an atom is unstable1 because of an unbalanced number of protons and neutrons1 a redistribution occurs sooner or later1 and the atom decomposes spontaneously and emits one of several kinds of radiations4 ,because of their common mode of action and effects on living organisms1 these different kinds of radiations are known collectively as ionizing radiations4 ,all radioactive elements emit one or more of three types of penetrating 7ionizing7 rays4 ,alpha rays or particles are double-charged helium nuclei1 ,he 7atomic number3 #b2 mass3 #d74 ,they are emitted by many heavy radioactive elements1 such as radium1 uranium1 and plutonium4 ,beta rays or particles can be either positive or negative4 ,negative beta particles are high-speed electrons and are emitted by many radioactive elements4 ,positive beta particles are positively charged electrons 7positrons71 have only a transitory existence1 and are less common4 ,gamma rays are electromagnetic radiations1 a term that also describes radiowaves1 infrared rays1 visible light1 ultraviolet light1 and ,x rays4 ,gamma rays are usually emitted after the emission of alpha or beta particles4 ,in our studies of life processes1 we are interested only in the radioactive isotopes that emit gamma rays or beta particles4 ,radioactive ,isotopes ,radioactive isotopes occur as minor constituents in many natural materials1 from which they can be concentrated by fractionation procedures4 ,in a very limited number of cases1 more significant amounts of a radioactive isotope1 for example1 radium or radioactive lead1 can be found in nature4 ,most radioactive isotopes in use today1 however1 are prepared artificially by nuclear reactions4 ,when a high-energy particle1 such as a proton1 a deuteron1 an alpha particle1 or a neutron1 collides with an atom1 a reaction takes place1 leading to the formation of a new1 unstable compound--a man-made radioactive isotope4 ,the great usefulness of radioactive isotopes1 as we shall see later1 is that they can be detected and identified by proper instruments4 ,biochemists have long recognized the desirability of 8tagging0 or 8labeling0 a molecule to permit tracing or keeping track of the 8label0 and consequently of the molecule as it moves through a reaction or process4 ,since the radiations emitted by radioactive isotopes can be detected and measured1 we can readily follow a molecule tagged with a radioactive atom4 ,illustration3 ,figure #i ,a laboratory technologist preparing dissolved biological materials as part of a study of the uptake of radioactive substances in living organisms4 ,note the radiation-detection instrument at right4 ,the earliest biochemical studies employing radioactive isotopes go back to #aibd1 when ,george de ,hevesy used natural radioactive lead to investigate a biological process4 ,it was only after ,world ,war ,,ii1 however1 when artificially made radioactive isotopes were readily available1 that the technique of using isotopic tracers became popular4 ,in our investigations of life processes1 we are especially interested in three radioactive isotopes3 ,h1 the hydrogen atom of mass #c2 ,c1 the atom of carbon with atomic weight #ad2 and ,p1 the atom of phosphorus with atomic weight #cb4 ,these radioactive isotopes are important because the corresponding stable isotopes of hydrogen1 carbon1 and phosphorus are present in practically all cellular components that are important in maintaining life4 ,with the three radioactive isotopes1 therefore1 we can tag or label the molecules that participate in life processes4 ,illustration3 ,figure #aj ,a visiting scientist at an ,,aec laboratory uses radioactive tritium 7,h7 to study the effect of radiation on bean chromosomes4 ,the famous scientist1 ,george de ,hevesy1 also used beans in conducting the first biological studies ever made with radioisotopes4 ,hydrogen-#c is a weak beta emitter2 that is1 it emits beta particles with a very low energy 7#j4jah ,mev#f7 and therefore with a very short range4 ,carbon-#ad is also a weak beta emitter 7#j4aed ,mev71 although the beta particles emitted by ,c have a higher energy and therefore a longer range than those emitted by ,h4 ,the beta particles emitted by phosphorus-#cb are quite energetic 7#a4fi ,mev7 and have a longer range4 ,radioactive ,isotopes' ,value in ,biological ,studies ,to biologists1 then1 the essential feature in the use of radioactive isotopes is the possibility of preparing 8labeled0 samples of any organic molecule involved in biological processes4 ,with labeled samples it is possible to distinguish the behavior and keep track of the course of molecules involved in a particular biological function4 ,in this capacity the isotope may be likened to a dynamic and revolutionary type of 8atomic microscope01 which can actually be incorporated into a living process or a specific cell4 ,just as a real microscope permits examination of the structural details of cells1 isotopes permit examination of the chemical activities of molecules1 atoms1 and ions as they react within cells4 7,neither optical nor electron microscopes are powerful enough for us to see anything as small as a molecule clearly47 ,,dna ,,synthesis3 ,,the ,,autobiography ,,of ,,cells ,here1 surely1 is the prime substance of life itself4 ,isaac ,asimov ,the many characteristic features of each living species1 its complex architecture1 its particular behavior patterns1 the ingenious modifications of structure and function that enable it to compete and survive--all these must pass1 figuratively speaking1 through the eye of an ultramicroscopic needle before they are brought together as a new1 individual organism4 ,the thread that passes through the eye of this needle is a strand of the filamentous molecule1 deoxyribonucleic acid 7,,dna74 ,let us now outline the research that led to these conclusions4 ,,dna in ,somatic and ,germinal ,cells ,one of the fundamental laws of modern biology--which states that the ,,dna content of somatic cells is constant for any given species--was first set forth in a research report of #aidh4 ,this finding means that in any given species1 such as a mouse or a man1 all cells except the germinal cells contain the same amount of ,,dna4 ,germinal cells1 that is1 the sperm cells of the male semen and the female egg1 contain exactly half the amount of ,,dna of the somatic cells4 ,this must be the case1 since ,,dna is the hereditary material1 and each individual's heredity is shaped half by his father and half by his mother4 ,one ten-trillionth of an ounce of ,,dna from a father and one ten-trillionth of an ounce of ,,dna from the mother together contain all the specifications to produce a new human being4 ,a large amount of ,,dna must be manufactured by an individual organism as it develops from a fertilized egg 7one single cell7 to an adult containing several million cells4 ,for instance1 a mouse cell contains about #g picograms of ,,dna 7one picogram is one millionth of a microgram1 or one millionth of one millionth of a gram74 ,a whole mouse contains in its body approximately #be milligrams 7#be thousandths of a gram7 of ,,dna1 and all this ,,dna was synthesized by the cells as the mouse grew to adulthood4 ,since the amount of ,,dna per cell remains constant and since each cell divides into two cells1 it is apparent that each new cell receives the amount of ,,dna characteristic of that species4 ,once we realize that a cell that is making new ,,dna 7as most cells do7 must divide to keep the amount of ,,dna per cell constant1 it follows that a cell that is making ,,dna is one that is soon destined to divide4 ,if we can now mark newly made ,,dna with a radioactive isotope1 we can actually mark and thus identify cells that are preparing to divide4 ,the task can be divided into two parts3 7#a7 to label the newly made ,,dna and 7#b7 to detect the newly made1 labeled ,,dna4 ,replication of ,,dna ,figure #aa is a diagram showing the essential structure of the large ,,dna molecule4 ,according to the ,watson-,crick model1#g the molecule consists of two strands of smaller molecules twisted around each other to form a double helix4 ,each strand consists of a sequence of the smaller molecules linked linearly to each other4 ,these smaller molecules are called nucleotides1 and each consists of three still smaller molecules1 a sugar 7deoxyribose71 phosphoric acid1 and a nitrogen base4 ,each nucleotide and its nearest neighbor are linked together 7between the sugar of one and the phosphoric acid of the neighbor74 ,this leaves the nitrogen base free to attach itself1 through hydrogen bonding1 to another nitrogen base in the opposite strand of the helix4 ,in the ,,dna of higher organisms1 there are only four types of nitrogen bases3 adenine1 guanine1 thymine1 and cytosine4 ,adenine in either strand of the helix pairs only with thymine in the opposite strand1 and vice versa1 and guanine pairs only with cytosine1 and vice versa1 so that each strand is complementary in structure to the other strand 7see ,figure #ab74 ,the full structure resembles a long twisted ladder1 with the sugar and phosphate molecules of the nucleotides forming the uprights and the linked nitrogen bases forming the rungs4 ,each upright strand is essentially a mirror image of the other1 although the two ends of any one rung are dissimilar4 ,illustration3 ,figure #aa ,diagrammatic structure of the ,,dna molecule as proposed by the ,watson-,crick model4 ,,dna3 ,diagrammatic single-strand chemical formula ,,dna3 ,structural arrangement ,when ,,dna is replicated1 or copied1 as the organism grows1 the two nucleotide strands separate from each other by disjoining the rungs at the point where the bases meet1 and each strand then makes a new and similarly complementary strand4 ,the result is two double-stranded ,,dna molecules1 each of which is identical to the parent molecule and contains the same genetic material4 ,when the cell divides1 each of the two daughter cells gets one of the new double strands2 each new cell thus always has the same amount of ,,dna and the same genetic material as the parent cell4 7,all that has been said so far about ,,dna replication depends upon an assumption that the ,,dna molecule is in some way untwisted to allow separation of two helical strands1 but there is no compelling reason to believe that such an untwisting does indeed take place1 nor do we know1 if the untwisting does take place1 how it is accomplished4 ,much that has been said in the last few paragraphs is therefore purely speculative4 ,it is1 however1 based on sound observation and is a more logical explanation than others that have been advanced47 ,illustration3 ,figure #ab ,the pairing of the nucleotide bases that make up ,,dna4 ,illustration3 ,figure #ac ,the ,,dna molecule and how it replicates4 7a7 ,the constituent submolecules4 7b7 ,assembly of subunits in complete ,,dna molecule4 7c7 8,unzipping0 of the double nucleotide strand4 7d7 and 7e7 ,the forming of a new strand by each individual strand4 7f7 ,,dna molecule in twisted double-strand configuration4 ,adapted from ,viruses and the ,nature of ,life1 ,wendell ,m4 ,stanley and ,evans ,c4 ,valens1 ,e4 ,p4 ,dutton ,co41 ,inc41 #aifa1 with permission4 ,labeling ,,dna with a ,radioactive ,isotope ,of the four bases in ,,dna1 three are also found in the other nucleic acid1 ,,rna2 but the fourth1 thymine1 is found only in ,,dna4 ,therefore1 if thymine could be labeled and introduced into a number of cells1 including a cell in which ,,dna is being formed1 we would specifically label the newly synthesized ,,dna1 since neither the old ,,dna nor the ,,rna would make use of the thymine4 ,we could in this way mark cells preparing to divide4 7,actually1 thymine itself is not taken up in mammalian cells1 but its nucleoside is4 ,a nucleoside is the base plus the sugar1 or1 in other words1 the nucleotide minus the phosphoric acid47 ,the nucleoside of thymine is called thymidine1 and we say that thymidine is a specific component of ,,dna and can be used1 both in laboratory studies and in living organisms1 for labeling ,,dna4 ,thymidine labeled with radioactive compounds is available as ,c-thymidine 7thymidine with a stable carbon atom replaced by a radioactive carbon atom7 and as ,h-thymidine 7thymidine in which a stable hydrogen atom has been replaced by tritium74 ,thus1 when cells actively making ,,dna are exposed to radioactive thymidine1 they incorporate it1 and the ,,dna becomes radioactive4 ,we have thus found a way to complete the first part of the task1 the labeling of new ,,dna4 ,we still must find out how to distinguish labeled ,,dna among the many components of the cell4 ,we might do it with a system based on measuring the amount of radioactivity incorporated into the ,,dna of cells exposed to radioactive thymidine1 as an approximation of the frequency of cell division in the group of cells4 ,however1 a better method for studying cells synthesizing ,,dna1 and thus preparing to divide1 is the use of high-resolution autoradiography4 ,detecting ,,dna with ,autoradiography ,autoradiography is based on the same principle as photography4 ,just as photons of light impinging on a photographic emulsion produce an image1 so do beta particles 7or alpha particles7 emitted by decomposing radioactive atoms4 ,a photographic emulsion is a suspension of crystals of a silver halide 7usually silver bromide7 embedded in gelatin4 ,when crystals of silver bromide are struck by beta particles1 the silver atoms are ionized and form a latent image1 so called because it is invisible to our eyes4 ,after the emulsion is developed and fixed1 each little aggregate of reduced silver atoms becomes a visible black speck on the emulsion4 ,the distribution and combination of the specks make up the photographic image 7see ,figure #ad74 ,in ordinary photography such an image is a negative1 which has to be converted into the positive photograph by printing4 ,in autoradiography we are satisfied to look at the negative image since the clusters of developed silver atoms1 appearing under a light microscope as black dots1 supply all the information we need4 ,illustration3 ,figure #ad ,schematic diagram of a radioautograph4 ,the distinction of having made the first autoradiograph belongs to the ,french physicist1 ,antoine ,henri ,becquerel2 and to another ,frenchman1 ,a4 ,lacassagne1 goes the credit for having introduced this technique into biological studies4 ,lacassagne used autoradiography to study distribution of radioactive polonium in animal organs4 ,after ,world ,war ,,ii1 when radioactive isotopes were first available in appreciable quantities1 autoradiography was further perfected through the efforts of such scientists as ,c4 ,p4 ,leblond in ,canada1 ,s4 ,r4 ,pelc in ,england1 and ,p4 ,r4 ,fitzgerald in the ,united ,states4 ,today autoradiography is sufficiently precise to locate radioactively labeled substances in individual cells and even in chromosomes and other structures within the cell4 ,two conditions must be met to achieve this high resolution3 7#a7 ,the radiation from the radioactive element in the cells must be of very short range4 7#b7 ,the cells must remain in close contact with the photographic emulsion throughout the various experimental manipulations4 ,when these conditions are met1 the black dots will appear in the emulsion directly above the cell or cell part from which the radiation came 7see ,figure #ad74 ,shortness of range is satisfied by use of tritium1 since its beta particles travel only about #a micron 7one thousandth of a millimeter7 and the diameters of mammalian cells range from #ae to #dj or more microns4 ,a mammalian-cell nucleus is at least #g to #h microns in diameter4 ,illustration3 ,figure #ae ,cells being prepared for autoradiography4 7a7 ,cells being coated with a photographic emulsion4 7b7 ,coated cells being exposed to produce a latent image4 ,the condition of close contact between cells and emulsion is achieved by the technique of dip-coating autoradiography4 ,in this process the glass slide on which the cells are carried is dipped into a melted photographic emulsion 7see ,figure #ae;a71 a thin film of which clings to the slide4 ,after it has been dried1 the slide is placed in a lighttight box and kept in a refrigerator for the desired period of exposure1 usually several days or weeks4 ,during this period disintegrating radioactive atoms within the cells continue to emit beta particles1 which1 in turn1 produce a latent image in the overlying emulsion4 ,after the exposure is complete1 the slide is developed and fixed like a photographic plate1 and a stain is applied which penetrates the emulsion so that the outlines of the cells and their internal structures can be seen4 ,the fixing process removes all silver bromide that has not been ionized so that the emulsion is reduced to a thin1 transparent film of gelatin covering the stained cells and containing only the clusters of silver grains that were struck by the beta particles4 ,illustration3 ,figure #af ,radioautographs of tumor cells4 ,above1 tumor cells and blood cells4 ,below1 magnification of tumor cells4 ,when the finished autoradiograph is examined under the microscope1 it will look like the radioautographs of tumor cells in ,figure #af4 ,in the upper micrograph the tumor cells are the larger ones and the smaller ones are blood cells4 ,the dense structures in the center of the tumor cells are nuclei4 ,the cells were exposed to tritium-labeled thymidine1 and those synthesizing ,,dna at the time of exposure took up the thymidine and became radioactive4 ,they can be identified by the black dots overlying the nuclei2 the dots are the aggregates of silver grains struck by the beta particles4 ,notice that only the nuclei contain radioactivity2 the reason for this is that radioactive thymidine is incorporated only into ,,dna localized in the nuclei of cells4 ,this picture identifies not only the cells that were making ,,dna at the time the label was administered but also the cells that were destined to divide in the immediate future1 since cells synthesize ,,dna in preparation for cell division4 ,if we want to compare two populations of cells to find out which is proliferating 7dividing7 more actively1 counting the fraction of cells labeled will give the number of cells synthesizing ,,dna in preparation for cell division4 ,of course1 a rough approximation of the proliferating activity can be obtained by simply counting the number of cells actually dividing4 ,but with tritiated thymidine we can obtain not only much more accurate measurements but also considerable information that cannot be obtained by simply counting the number of cells in mitosis4 ,we shall discuss the cell cycle later on1 but for the moment we should emphasize that much of our knowledge of the cell cycle stems from the use of high-resolution autoradiography4 ,it is clear that autoradiography enables us to find out which cells are dividing in a cell population and how many of them do so4 ,for instance1 in a given tissue or organ1 not all cells are capable of dividing into two daughter cells4 ,in the epidermis1 which is the thin outer layer of the skin1 only cells in the deepest portion can divide4 ,the other cells1 although originating from cells in the deep layer1 have lost the capacity to divide1 and eventually die without further division4 ,if we take a bit of skin1 expose it to tritiated thymidine1 and determine the amount of radioactivity incorporated into the skin cells' ,,dna1 we obtain a fair measurement of the amount of ,,dna being synthesized4 ,however1 this purely biochemical investigation cannot possibly give any information on which specific cells are synthesizing ,,dna4 ,for this1 autoradiography provides the information we need4 ,,rna ,,synthesis3 ,,how ,,to ,,translate ,,one ,,language ,,into ,,another ,mathematicians are like ,frenchmen3 whatever you say to them they translate into their own language1 and forthwith it is something entirely different4 ,wolfgang von ,goethe ,we have mentioned previously that there are two main types of nucleic acids3 ,,dna1 the genetic material itself1 and ,,rna1 the molecule that translates the genetic message from ,,dna into terms the cell can use as 8instructions0 for making protein4 ,cells differ from each other on the basis of kinds of proteins they contain1 and1 since differences among cells determine differences among organisms1 it follows that differences in the composition of ,,dna serve to explain the variety in living organisms populating the world4 ,however1 if differences between two organisms can be explained by differences in the chemical composition of their respective ,,dna's1 how can we explain differences between cells of the same organism8 ,how can we explain that cells of the human pancreas secrete insulin1 whereas other cells in man produce no insulin8 ,or how can we explain that certain cells make bone and others make fat8 ,if indeed all cells in the same organism contain the same amount and kind of ,,dna 7since all ,,dna in an organism derives from the duplication of the ,,dna of the fertilized egg cell and its descendants71 it would seem1 at first glance1 that ,,dna is not the molecule responsible for differences among the cells4 ,the clarification of this apparent contradiction is found in the remarkable properties of the other nucleic acid1 the translator molecule1 ,,rna4 ,three ,kinds of ,,rna ,in the first place1 there are at least three different kinds of ,,rna4 ,the largest quantity is a special kind called ribosomal ,,rna1 or r-,,rna4 ,it is found in close conjunction with proteins and makes up the structural frame upon which the protein-synthesizing machinery is built4 ,the r-,,rna and the proteins to which it is firmly bound form the ribosomes1 the ,,rna-rich microsomes that are attached to the endoplasmic reticulum4 ,proteins are synthesized on ribosomes4 ,we shall see later what determines the differences among proteins and how these differences are dictated directly by ,,rna and indirectly by ,,dna4 ,besides r-,,rna1 there is a kind of ,,rna called soluble ,,rna1 or transfer ,,rna1 or s-,,rna4 ,it combines with r-,,rna to complete the sequence of events that synthesizes the proteins4 ,a bond between r-,,rna and s-,,rna is established by a third ,,rna molecule called messenger ,,rna1 or template ,,rna1 or m-,,rna4 ,this m-,,rna molecule is truly the messenger that carries the genetic message from ,,dna to the protein-synthesizing apparatus4 ,dr4 ,michael ,shimkin1 a ,temple ,university scientist1 in his analogy has compared the ,,dna ,,rna protein sequence to the activities of a newspaper staff4 ,,dna is the editor2 m-,,rna molecules are copyboys who carry the editorials to the typesetters1 the r-,,rna and s-,,rna1 who then take the 8letters0 of nucleic acid and set them into slots in accordance with the editor's directions4 ,there are also workers who melt down outworn letters and still other workers who make new letters for further use2 these are the enzymes1 special kinds of proteins4 ,if we wish to continue the analogy1 we may say that each kind of cell in the organism has a different subeditor1 who writes that cell's own editorial4 ,actually we might say that all cells have the same board of editors in common1 but only one editor functions in any given type of cell4 ,in biological terms this means that only a portion of all the cellular ,,dna is active in each cell4 ,the active ,,dna is the ,,dna that makes m-,,rna that will carry instructions to the protein-synthesizing machinery of that type of cell4 ,cells of the same organism therefore differ from each other on the basis of the segment of ,,dna that is active in making m-,,rna4 ,let us now see how we can use radioactive isotopes to investigate the synthesis of ,,rna4 ,labeling ,,rna with a ,radioactive ,isotope ,,rna synthesis is investigated with radioactive tracers in the same way as ,,dna synthesis4 ,if we can mark1 with a radioactive atom1 a small molecule that is incorporated into newly formed ,,rna1 we can then trace the course of the labeled ,,rna molecule with a radiation-detection device4 ,,dna had one advantage in this regard--the fact that one compound1 thymidine1 was a precursor of ,,dna1 a specific material that could be incorporated only into ,,dna4 ,we do not know similar specific precursors of ,,rna4 ,but we know several precursors that are predominantly incorporated into ,,rna2 the most common of these are the nucleosides adenine1 cytidine1 and uridine1 and the smaller molecule1 orotic acid4 ,all these precursors can be labeled with either ,h or ,c1 and their incorporation into ,,rna can be measured4 ,detecting ,,rna with ,autoradiography ,as in ,,dna synthesis1 we can use autoradiography to follow the incorporation of precursors into ,,rna4 ,by proper treatment of the tissues1 we can make sure that all the radioactivity visible by autoradiography is due to labeled ,,rna1 even though some of the precursor also enters ,,dna molecules4 ,even so1 the kind of information obtained from autoradiographs of tissues exposed to ,,rna precursors is different from that obtained with ,,dna precursors4 ,the advantage of high-resolution autoradiography in ,,dna studies is the possibility of identifying particular cells that are synthesizing nucleic acid4 ,this advantage is apparently lost in the case of ,,rna4 ,the reason is that1 at any given time1 only a few cells are making ,,dna1 whereas practically all cells are synthesizing ,,rna constantly4 ,the only exceptions are cells in the midpoint of mitosis4 ,at the beginning 7prophase7 and at the end of cell division 7telophase71 ,,rna is synthesized4 ,if we want a quantitative measurement of ,,rna synthesis1 other methods1 to be examined presently1 are considerably more precise4 ,but autoradiography can still give us valuable information4 ,other ,methods of ,detecting ,,rna ,if we look at cells soon after they have been exposed to an ,,rna precursor1 we find that the radioactivity detectable by autoradiography is only in the nuclei of the cells4 ,no radioactivity can be detected in the cytoplasm1 although we know that the cytoplasm of living cells contains large amounts of r-,,rna and s-,,rna4 ,one or two hours later1 however1 radioactive ,,rna appears in the cytoplasm as well as in the nucleus4 ,what autoradiography is telling us is that ,,rna is made in the nucleus and then is slowly transferred to the cytoplasm4 ,autoradiography cannot tell us whether the ,,rna that has been newly synthesized in the nuclei of cells is m-,,rna1 s-,,rna1 or r-,,rna4 ,the methods necessary to make this distinction are based on the chemical fractionation of the tissue1 isolation of ,,rna1 determination of its amount by quantitative analysis1 and determination of the amount of radioactivity by physical methods4 ,let us examine these steps separately4 ,illustration3 ,figure #ag ,injecting a mouse with a radioactive solution4 ,chemical ,fractionation of ,tissue ,after an animal has been injected with a radioactive precursor of ,,rna1 some of it will be incorporated into ,,dna as well as into ,,rna 7remember that the precursors of ,,rna lack specificity71 and part of the precursor will be broken down into smaller molecules4 ,the injected animal can be sacrificed1 and an organ or another tissue1 for instance1 the liver1 can be removed4 ,then the liver is homogenized1 that is1 ground to a pulp with a modern version of the mortar and pestle4 ,the homogenate 7pulp7 is treated with cold 7weak7 acid4 ,proteins and nucleic acids are insoluble in cold acids and therefore precipitate to the bottom of the test tube4 ,all molecules that are soluble in a cold acid are left in the supernatant 7the remaining liquid72 among these are small molecules1 like those of the ,,rna precursor4 ,the precipitate 7the solid material that settles to the bottom71 now containing proteins and nucleic acids1 is then treated with a strong alkali1 for instance1 sodium hydroxide4 ,alkali will digest ,,rna into smaller molecules but does not affect ,,dna4 ,if we now add acid to the solution1 ,,dna1 being insoluble in acid1 will precipitate again2 ,,rna1 having been broken down into small molecules1 will remain in the supernatant4 ,,dna can then be extracted from the precipitate by boiling in strong acid4 ,proteins from the tissue remain in the final residue4 ,we have now fractionated the tissue into four portions3 the acid-soluble fraction 7containing small molecules71 ,,rna1 ,,dna1 and proteins4 7,the cell's lipids and sugars come out during alcohol rinses between the weak acid and the alkali steps47 ,chemical analysis allows us to measure precisely the amount of ,,rna or ,,dna in its respective fraction and therefore in the tissue or organ4 ,the amount of radioactivity in the ,,rna fraction can then be determined by a technique known as liquid scintillation counting4 ,liquid ,scintillation ,counting ,liquid scintillation counting is the preferred method for the measurement of low-energy beta-emitting radioisotopes commonly used in cell-fractionation studies 7see ,figure #ah74 ,it is convenient1 sensitive1 and rapid for routine measurement of radiation in hydrocarbons1 other organic compounds1 and aqueous solutions containing such isotopes as ,h1 ,c1 and ,p4 ,liquid scintillation solutions share with other scintillating materials the property of converting into visible light the energy deposited in them by ionizing radiation4 ,in theory1 if a sample of a beta emitter is dissolved in a liquid scintillator solution1 every beta particle emitted will be absorbed completely because the range of penetration of beta particles in liquids is quite short 7ranging from #j4jjh millimicron for ,h to #g4i millimicrons for ,p in a medium of unit density74 ,the kinetic energy of the beta particles is largely used up in the ionization and excitation of the most abundant molecular species present1 the solvent in which the scintillating material was dissolved4 ,a fraction of the energy thus expended by each beta particle is transferred from excited solvent molecules to scintillator molecules2 thus the electrons in the atoms of the scintillator molecules are raised to an excited 7higher energy7 state4 ,when these electrons return to the ground1 or unexcited1 state1 a fraction of them emit a photon of light4 ,thus each beta particle produces a burst of photons4 ,illustration3 ,figure #ah ,technician placing a tray of samples in a liquid scintillation counter4 ,the radioactivity of each sample is recorded as the trays revolve4 ,if a vessel containing the liquid scintillator and the radioactive sample is placed near a suitably sensitive instrument known as a photomultiplier tube1 each burst of scintillator photons activates this device and causes it to release a burst of photoelectrons4 ,each burst of photoelectrons is multiplied successively in a series of electronic steps2 as a result1 there is a suitably large electrical-output pulse to be recorded4 ,one of the principal advantages of the liquid scintillation method is the ease of sample preparation4 ,we need only transfer a known volume of a liquid sample or weigh a given mass of a solid sample into a sample bottle1 add a known amount of the liquid scintillator solution1 and stir until there is a homogeneous solution4 ,samples thus prepared are placed in a refrigerated counting apparatus4 ,after a short waiting period to allow time for the samples to cool and for a natural1 short-lived phosphorescence 7due to exposure to room light7 to subside1 the samples are ready to be measured4 ,illustration3 ,figure #ai ,placing radioactive samples in a refrigerated unit for liquid scintillation counting4 ,one disadvantage of liquid scintillation counting is that different compounds show different degrees of quenching 7loss of emitted photons71 and the effect must be checked for each class of compounds in each concentration range4 ,this checking is usually done with an internal standard technique1 the sample being counted before and after a standard1 or known1 emitter is added4 ,another difficulty is that the best scintillating solvents are not the best chemical solvents for most biological materials4 ,the solubility problem is also aggravated by the low temperatures at which liquid scintillation counters are usually operated for more effective instrument performance4 ,with the method we have described1 we can obtain a fairly accurate idea of the rate of ,,rna synthesis in a given tissue4 ,there are other things we would like to know about ,,rna4 ,the first of these is the kind of ,,rna being synthesized4 ,during alkaline digestion all kinds of ,,rna are broken down into their component nucleotides2 we must therefore use other methods if we wish to know the kind of ,,rna in which the radioactivity of the precursor has been incorporated4 ,isolation of ,,rna ,native ,,rna1 that is1 ,,rna not broken down into its smaller constituents1 can be obtained in a variety of ways1 but the most popular one makes use of phenol extraction1 which removes ,,dna and proteins and leaves ,,rna in solution4 ,if this phenol-purified ,,rna is dissolved in a concentrated sugar solution and spun in a centrifuge at a very high velocity1 it will separate into three major components4 ,these components separate because they have different molecular weights1 and the larger the molecule1 the faster it forms a sediment in the centrifugal field4 ,two of these components are s-,,rna1 the lightest of all1 and r-,,rna1 which is divided into two subfractions4 ,we can also identify a third component1 m-,,rna1 with the centrifuge system but only with some difficulty and only after labeling it with a radioactive precursor1 because the amount of m-,,rna in a cell is very small4 ,illustration3 ,figure #bj ,diagram of ascending paper chromatography4 ,quantitative ,analysis ,another important feature of ,,rna 7or ,,dna1 for that matter7 is its base composition1 that is1 the percentage of each of the nucleotides that make it up4 ,the four bases that1 with ribose and phosphoric acid1 comprise the ,,rna molecule are guanine1 adenine1 cytosine1 and uracil4 ,it will be noted that three of the four--guanine1 adenine1 and cytosine--are the same as those in ,,dna1 but thymidine in ,,dna has been replaced by another base1 uracil4 ,to determine the percentage of each base in a given ,,rna molecule1 we must digest ,,rna with alkali to produce mononucleotides1 which are smaller molecules1 each consisting of a base1 ribose1 and phosphoric acid4 ,we can now separate the four nucleotides by using paper chromatography 7see ,figure #bj74 ,illustration3 ,figure #ba ,a paper chromatography showing separation of amino acids in two directions4 ,radioactivity in samples then produced this record by radioautography4 ,in this technique a mixture of compounds is deposited on the edge of a special type of paper4 ,this edge is then immersed in a solvent that slowly permeates the paper 7at a constant speed7 by capillary action4 ,as the solvent moves from the immersed edge toward the other edge1 which is hanging freely1 it carries the mixture of nucleotides with it4 ,each of the compounds in the mixture travels at a different speed1 however2 thus1 as the solvent front moves along the paper1 the dissolved compounds are separated from each other and appear as distinct spots on the paper4 ,to locate the nucleotides on the paper and to determine the percentage composition1 we can use a chromatogram scanner1 a device that scans the paper chromatograms1 measures the radiation from them1 and thus locates the labeled substances 7see ,figure #bb74 ,illustration3 ,figure #bb ,recording of radioactivity in a sample by radioautography and paper chromatography4 ,the peaks of the trace prepared by a chromatogram scanner coincide with the areas of separated components on the same chromatogram1 as revealed by radioautography4 ,the radioautograph is superimposed on the chromatogram recording4 ,another technique used to separate the nucleotides of ,,rna is column chromatography4 ,in this method mixtures of nucleotides are separated as they pass down a column of chemicals 7see ,figure #bc74 ,illustration3 ,figure #bc ,students visiting ,argonne ,national ,laboratory listen to a scientist explain the column chromatography process1 in which mixtures of nucleotides are separated as they pass down a column of chemicals4 ,we have now learned how to use radioisotopes to investigate the synthesis of ,,rna1 the molecule that translates the ,,dna message into the language of proteins4 ,let us now see what we can learn about the synthesis and function of proteins4 ,,protein ,,synthesis3 ,,the ,,molecules ,,that ,,make ,,the ,,difference ,if a man will begin with certainties he shall end in doubts2 but if he will be content to begin with doubts he shall end in certainties4 ,francis ,bacon ,proteins occupy a central position in the structure and functioning of living matter and are intimately connected with all the metabolic reactions that maintain life4 ,some proteins serve as structural elements of the body1 for instance1 hair1 wool1 and the scleroproteins of bone and collagen1 the latter an important constituent of connective tissue4 ,other proteins are enzymes1 which are extremely important since they regulate all metabolic reactions4 ,most of the proteins in the tissues of actively functioning organs1 such as the liver and the kidney1 are enzymes4 ,other proteins participate in muscular contraction1 and still others are hormones or oxygen carriers4 ,special proteins called histones are associated with gene function1 and the antibodies that an organism produces to defend itself from bacteria are also proteins4 ,the differences in proteins1 especially in enzymes1 account for differences among cells4 ,it is now appropriate to ask what makes one protein different from another4 ,we know that the structure of a protein depends upon several factors1 such as the molecular weight4 ,but the main differences among proteins depend upon the sequence1 or order1 of the amino acids that are linked together in the protein molecules4 ,amino ,acids and ,protein ,structure ,amino acids are the fundamental structural units of proteins4 ,there are #bj amino acids found frequently in mammalian proteins1 and these molecules may be linked to one another to form a chain called a polypeptide chain4 ,the structure of a protein then depends on3 7#a7 the quantity of each amino acid present2 7#b7 the sequence of amino acids in the polypeptide chain2 7#c7 the length of the polypeptide chain1 that is1 the molecular weight2 and 7#d7 the folding and the side 7nonlinear7 arrangement of the polypeptide chain molecules1 that is1 the secondary and tertiary structures4 ,how can we investigate protein synthesis by using radioactive isotopes8 ,since proteins are made up of amino acids1 the logical conclusion1 after what we have learned about ,,dna synthesis and ,,rna synthesis1 is that the best way would be to mark an amino acid and follow its incorporation into a molecule of protein4 ,we could label a mixture of several amino acids1 but1 for the sake of clarity1 we will describe the incorporation of a single labeled amino acid4 ,labeling an ,amino ,acid with a ,radioactive ,isotope ,suppose we have the amino acid leucine labeled with ,c and we inject a solution containing it into an experimental animal4 ,since leucine is incorporated into proteins1 if we isolate the proteins and determine both the amount of proteins and the amount of radioactivity1 we can measure fairly accurately the rate of protein synthesis4 ,autoradiography1 by the way1 is of little help in studying most protein synthesis because all cells are always synthesizing proteins and so are all labeled after a single exposure to a radioactive amino acid4 ,with ,,rna precursors autoradiography at least told us where ,,rna was being made1 but with amino acids we do not even get this information because proteins are synthesized both in the nucleus and in the cytoplasm4 ,under these circumstances radiochemical methods are better for studying protein synthesis4 ,proteins are isolated from the residue left after a nucleic-acid extraction process similar to that described previously1 and the amount of protein is determined by a simple colorimetric analysis based on comparison of the color of the solution with a standard color4 ,the amount of radioactivity 7remember that we are now using a precursor labeled with ,c7 can be determined with a gas-flow counter1 which is probably more widely used at present than any other instrument for counting beta emitters1 chiefly because of its reliability and low cost4 ,illustration3 ,figure #bd ,a college chemistry major analyzing a sample of radioactive materials with an instrument known as a proportional beta counter4 ,,cell ,,cycle ,,and ,,gene ,,action3 ,,life ,,is ,,the ,,secret ,,of ,,dna ,some circumstantial evidence is very strong1 as when you find a trout in the milk4 ,henry ,david ,thoreau ,for a biologist interested in the mechanism of cell proliferation1 the most important event in the life of a cell was1 until very recently1 cell division4 ,as we mentioned1 when a cell divides into two daughter cells1 it undergoes a process called mitosis2 mitosis itself is subdivided into four stages called prophase1 metaphase1 anaphase1 and telophase4 ,mitosis in most cells takes less than one hour4 ,between one mitosis and the next1 there can be an interval1 from a few hours to several days in length1 during which a cell is said to be in interphase4 ,the entire period between the midpoints of two successive mitoses is called the cell cycle4 ,interphase ,until a few years ago1 we knew very little about interphase4 ,in fact1 in one classic book on histology1#h while a description of mitosis required almost #ab pages1 interphase was dismissed in less than six lines6 ,the reason for this lack of interest was1 of course1 the fact that no adequate methods were available for studying metabolic activities of cells in interphase4 ,the methods of high-resolution autoradiography and radiochemical analysis of synchronized cell populations have become available only in the past few years4 ,we now know that metabolic activities during interphase are of primary importance in understanding the mechanism of cell division4 ,it is1 in fact1 the orderly sequence of metabolic events occurring in interphase that leads from one mitosis to the next4 ,the ,cell ,cycle ,figure #be is a diagram of the cell cycle4 ,try to imagine the cell cycle as a race track and individual cells as cars that race around it4 ,you are sitting at the finish wire1 which is mitosis 7we chose mitosis because it is easy to recognize when the cell is observed with the aid of a microscope74 ,at a certain time during the race1 all the cars in a portion of the track1 say a #bjj-yard sector of the backstretch1 are sprayed with a blue dye as they race by4 ,these cars are now marked1 just as cells synthesizing ,,dna are marked if briefly exposed to tritiated thymidine1 the common radioactive precursor of ,,dna4 ,as soon as these cars have been sprayed1 you observe all the cars as they pass the finish line in front of you4 ,at first1 you will see cars that were nearest the wire and were not sprayed2 then the dye-marked cars will pass2 and finally more unmarked cars1 those that had passed the finish line but had not reached the spray area when the marking was done1 will come by4 ,if you replace the words spray1 cars1 and wire with the words radioactivity1 cells1 and mitosis1 you have described the cell cycle and the flow of cells in the cycle4 ,now1 if all cars were going at the same speed1 you could calculate with great accuracy the time taken for any one car to go around the track1 or from the finish line to the backstretch1 or through the spray sector1 and so on4 ,however1 since cars move at different speeds1 you can only obtain an average time for all sprayed cars4 ,similarly1 since individual cells behave differently1 you can only obtain averages of the times these cells spend in the various portions of the cell cycle4 ,illustration3 ,figure #be ,,the ,,cell ,,cycle ,these cell-cycle portions are four in number1 according to nomenclature originated by ,a4 ,howard and ,s4 ,r4 ,pelc1 two ,english investigators who first described the cycle3 7#a7 mitosis2 7#b7 ,g1 which is the period between mitosis and ,,dna synthesis2 7#c7 ,s phase1 which is the period during which ,,dna is replicated2 and 7#d7 ,g1 which is the period between ,,dna synthesis and the next mitosis4 ,only cells in the ,s phase 7,,dna synthesis7 are marked when exposed to a radioactive precursor of ,,dna4 ,,dna ,synthesis and the ,cell ,cycle ,because it has several important implications in biology and medicine1 it is important to remember that ,,dna synthesis occurs only during the short1 well-defined ,s period of the cell cycle4 ,other synthetic processes go on throughout the cycle4 ,we mentioned1 for instance1 that all cells can be labeled by a brief exposure to a radioactive amino acid1 a precursor of proteins2 this means that protein synthesis occurs throughout the entire cell cycle1 including mitosis4 ,when we use a radioactive ,,rna precursor1 all cells except those in anaphase and metaphase are labeled2 this means that ,,rna synthesis occurs throughout the entire cycle except during anaphase and metaphase4 ,but a radioactive tag on a ,,dna precursor reveals that only during the ,s phase is there ,,dna synthesis4#i ,it is also important to remember that a cell that has synthesized ,,dna is a cell that1 with a few exceptions1 will divide in the very near future4 ,thus1 for an understanding of the mechanisms that control cellular proliferation1 it is important to investigate the factors that control ,,dna synthesis4 ,our recent knowledge of the cell cycle has therefore led to a shift in the focus of investigation from mitosis to ,,dna synthesis4 ,another point to remember is that not all cells keep going through the cell cycle indefinitely4 ,as shown in ,figure #be1 when a cell divides1 the daughter cells have two alternatives1 either to go through another cycle or to leave it altogether4 ,cells that leave the cycle are called differentiated cells and will eventually die without any further division4 ,many cells in an adult organism also have lost the capacity to make ,,dna and therefore the capacity to divide4 ,these cells often have other specialized functions in the body2 examples are nerve cells and muscle cells4 ,the synthesis of other macromolecules 7giant molecules1 like ,,dna7 connected with the gene-action system is another field of active investigation4 ,we have described how we can investigate the synthesis of proteins and ,,rna with radioactive isotopes1 and we have given some information on the gene-action system1 which is also shown in ,figure #bf4 ,the genetic material of a cell is ,,dna4 ,the ,,dna molecule is in the form of a double-stranded helix that is supported by a protein backbone4 ,genes are often described as simply segments of ,,dna4 ,they differ from each other only in the order in which the four nucleotide bases that make up ,,dna are arranged4 7,look at ,figure #ac again47 ,since a single gene is usually made up of several hundred bases1 it is easy to imagine the infinite variety of genes that could exist by simply changing the order of the four bases several hundred times4 ,illustration3 ,figure #bf ,,the ,,gene-,,action ,,system ,not all genes in the cells of a living organism are active4 ,in fact1 most of them are inactive1 or1 as geneticists say1 repressed4 ,what represses genes to make them inactive is not known1 but many investigators believe the activity1 or lack of it1 is regulated by proteins called histones4 ,if a gene is repressed1 nothing happens2 it remains inactive1 presumably until something removes the repressing factor4 ,but an active gene sets in motion a train of events that results in activation of one of the processes of life3 ,the gene's ,,dna directs the manufacture of ,,rna1 which in turn brings about the synthesis of a specific protein to carry out a specific metabolic process4 ,in other words1 all the activities of the cell are dictated by active genes 7the ,,dna molecules7 through the mediation of ,,rna and are executed by proteins4 ,here is what happens as nearly as scientists can reconstruct it3 ,translation of the ,genetic ,message ,the ,,dna of a particular active gene manufactures a molecule of m-,,rna by the same kind of replication that it uses for making more ,,dna4 ,in m-,,rna the sequence of bases is the same as in the parent ,,dna segment2 for this reason1 m-,,rna is also called ,,dna-like ,,rna4 ,as shown in ,figure #ab1 a cytosine molecule in m-,,rna corresponds to a cytosine molecule in ,,dna1 a guanine to a guanine1 and so on1 except that the m-,,rna has uracil in all the places where thymine occurs in ,,dna4 ,the order of the nucleotides in the m-,,rna is the same as that in the ,,dna1 so the m-,,rna carries the genetic code of the gene that made it4 ,this process1 all of which occurs in the cell nucleus1 is one of copying1 or transcription1 rather than translation1 since the same 8codewords0 7the nucleic-acid bases7 are reproduced4 ,the new m-,,rna molecule then travels from the nucleus to the cytoplasm and attaches itself to an unoccupied ribosome 7see ,figure #bg74 ,here it fits to a molecule of r-,,rna and blends its shape geometrically1 or spatially1 with the shape of the r-,,rna in lock-and-key1 or jigsaw-puzzle1 fashion4 ,the combined new ,,rna molecule is now capable of manufacturing a specific protein4 ,illustration3 ,figure #bg ,protein synthesis in a ribosome 7microsome71 and its control by ,,dna in the nucleus1 using ,,rna as an intermediary4 ,adapted from ,principles of ,biology1 ,neal ,d4 ,buffaloe1 ,prentice-,hall1 ,inc41 #aifb1 with permission4 ,at this point an s-,,rna molecule arrives1 bringing with it one amino-acid molecule1 which then combines with other amino acids in the specific order dictated by the ,,rna to form a specific protein4 ,after the amino acids have been formed into the protein molecule1 they detach themselves from the s-,,rna molecule4 ,the s-,,rna molecule has two recognition sites by which it matches up to its neighbors3 ,one recognizes1 or 8fits01 the amino acid1 and the other recognizes a corresponding triplet of bases on m-,,rna4 ,there is thus a particular s-,,rna molecule for each amino acid and a particular triplet of bases on the m-,,rna molecule for each triplet of bases that is specific to the s-,,rna molecule4 ,in this process the machinery has translated the nucleic-acid code into the protein code2 that is1 it has translated a sequence of the bases into a sequence of amino acids4 ,this process is therefore called translation of the genetic message4 ,once the protein has been synthesized1 it will become active in performing some of the cell's metabolic activities4 ,the gene-action system actually is somewhat more elaborate than this4 ,there are feedback mechanisms1 genes that control the activity of other genes1 either directly or through the production of specific proteins1 and so on4 ,however1 the scheme just outlined gives a fair1 if simplified1 idea of how the genetic message is carried to the entire cell and how it is translated into actual life processes4 ,,isotopes ,,in ,,research3 ,,probing ,,the ,,cancer ,,problem 444 a riddle wrapped in a mystery inside an enigma4 ,winston ,churchill ,the various procedures in which radioactive isotopes play a major role have been applied to many studies and investigations in the fields of biology and medicine4 ,in fact1 most of the concepts of modern biology that we have been discussing in this booklet owe their discovery to the judicious use of radioisotopes4 ,to illustrate how radioisotopes can be used to solve a practical problem1 we have chosen a typical example1 the investigation1 at a molecular level1 of the effectiveness of an anti-cancer drug4 ,several drugs that exert a beneficial effect1 at least temporarily1 on the course of certain cancers have been used by doctors for several years4 ,most of them were discovered empirically1 that is1 by accident1 during routine trials against cancers4 ,doctors know they work but do not always know how4 ,they would also like to know the mechanism of the drugs' action at the molecular level so that the knowledge might open the way to the discovery of other drugs more effective against cancer and less toxic against normal cells4 ,the following experiment shows how the molecular effect of an anti-cancer drug is studied4 ,illustration3 ,figure #bh ,technician preparing tissues for comparative studies4 ,cells growing in tissue cultures are often used to test anti-cancer drugs 7see ,figure #bh74 ,these cells1 derived from human cell lines1 are grown in glass or plastic bottles as a suspension in a nutrient medium4 ,to begin1 a culture is divided into halves4 ,to one half is added the anti-cancer drug ,actinomycin ,d4 ,the other half will continue to grow without addition of other substances and will serve as a control1 or comparison4 ,after a suitable time has elapsed for the drug to act on the cultured cells1 similar portions of the drug-treated cells and the control cells will be tested in several ways4 ,one portion of each kind of cells is incubated with ,h-thymidine to determine the effect of the drug on ,,dna synthesis4 ,two other portions are incubated with ,h-cytidine to study the effect on ,,rna synthesis4 ,another pair will be tested with ,c-leucine to investigate protein synthesis4 ,the effect of the drug1 of course1 is determined by comparing the untreated control with the drug-treated culture4 ,the biochemical1 autoradiographic1 and counting techniques that we described previously are all used to determine the uptake of the radioisotopes into the cell's components4 ,chromatography is used to ascertain if the drug has changed the concentration of precursors 7thymidine1 cytidine1 or leucine7 in the nutrient medium1 since a change in these could produce misleading results4 ,finally1 if the drug is found to have an effect on ,,rna1 we can investigate the type of ,,rna that is affected by centrifuging phenol-purified ,,rna4 ,the results will disclose the primary site 7,,dna1 ,,rna1 or proteins7 of the drug action on cell metabolism4 ,more elaborate experiments can pinpoint more intimately the mechanism of action4 ,by studying the life processes of cells1 we can advance toward a common denominator in anti-cancer drugs that will lead to an effective anti-cancer treatment4 ,,conclusions ,thus1 the task is1 not so much to see what no one has seen yet2 but to think what nobody has thought yet1 about what everybody sees4 ,arthur ,schopenhauer ,the use of radioactive isotopes in the study of life processes is of importance in understanding them4 ,with the use of autoradiographic and radiochemical techniques1 it is possible to obtain valuable information regarding the life of cells and the intimate mechanisms by which life processes determine the fate of the entire organism4 ,our knowledge of the cell cycle and of the gene-action system has been useful in determining how organisms grow and how cancer cells behave4 ,it has been determined that certain normal adult cells divide more frequently than some cancer cells and that the growth of cancers depends not so much on the speed of cellular proliferation as on the number of cells actually dividing4 ,illustration3 ,figure #bi ,radioautograph showing ,,dna synthesis during chromosome replication4 ,chromosomes from cells in the root tip of the ,tradescantia plant were labeled with ,h-thymidine4 ,in ,a and ,b1 the midportion of ,,dna synthesis1 the radioisotope is distributed throughout the chromosome arms2 in ,c1 near the end of ,,dna synthesis1 it is confined mainly to the end of the arms4 ,knowledge of the cell cycle has also brought new insight to the control of cell division1 as in studies related to the therapy of cancer4 ,the most important problem now is1 not the control of cell division1 but the control of the synthesis of ,,dna4 ,our information on the gene-action system provides broad new opportunity for the investigation of many life processes4 ,hormone action1 processes by which the body develops immunity to disease1 and even cell division itself are apparently regulated through the gene-action system4 ,this1 in turn1 offers possibilities for investigations meant to control these processes4 ,it is difficult to chart the future course of modern molecular biology1 but it is not difficult to predict that the next few years will bring to biology the same kind of sweeping advances that revolutionized physics a few decades ago4 ,the ,,dna molecule has been called the atom of life4 ,when we have harnessed it1 the harnessing of the uranium atom will seem1 in comparison1 a result of scientific adolescence4 ,when man has mastered the genetic code1 he'll hold a vast power in his hands--power over the nature of coming generations4 ,,suggested ,,references ,books ,the ,cell1 ,carl ,p4 ,swanson1 ,prentice-,hall1 ,inc41 ,englewood ,cliffs1 ,new ,jersey1 #aifd1 #aad pp41 #a4ge4 ,inside the ,living ,cell1 ,j4 ,a4 ,v4 ,butler1 ,basic ,books1 ,inc41 ,new ,york1 #aiei1 #agd pp41 #c4ie4 ,life and ,energy1 ,isaac ,asimov1 ,doubleday ,company1 ,inc41 ,garden ,city1 ,new ,york1 #aifb1 #chj pp41 #d4ie4 ,applied ,nuclear ,physics1 ,ernest ,c4 ,pollard and ,william ,l4 ,davidson1 ,john ,wiley ,sons1 ,inc41 ,new ,york1 #aief1 #ceb pp41 #f4jj4 ,adventures in ,radioisotope ,research1 the collected works1 with recent annotations1 of ,george de ,hevesy1 ,pergamon ,press1 ,inc41 ,new ,york1 #aifa1 #ajdg pp4 7#b volumes71 #cj4jj4 ,the ,biochemistry of ,nucleic ,acids1 ,j4 ,n4 ,davidson1 ,john ,wiley ,sons1 ,inc41 ,new ,york1 #dth edition1 #aifj1 #bhg pp41 #d4be4 ,the ,machinery of the ,body1 ,a4 ,j4 ,carlson and ,c4 ,johnson1 ,the ,university of ,chicago ,press1 ,chicago1 ,illinois1 #aifa1 #geb pp41 #f4ej4 ,life3 ,an ,introduction to ,biology1 ,george ,g4 ,simpson and ,william ,s4 ,beck1 ,harcourt1 ,brace ,world1 ,inc41 ,new ,york1 #bnd edition1 #aife1 #hfi pp41 #h4ie4 ,from ,cell to ,test ,tube1 ,robert ,w4 ,chambers and ,alma ,payne1 ,charles ,scribner's ,sons1 ,new ,york1 #aifb1 #baf pp41 #a4de4 ,isotopic ,tracers in ,biology1 ,m4 ,d4 ,kamen1 ,academic ,press ,inc41 ,new ,york1 #crd edition1 #aieg1 #dgd pp41 #i4ej4 ,autoradiography in ,biology and ,medicine1 ,g4 ,a4 ,boyd1 ,academic ,press ,inc41 ,new ,york1 #aiee1 #cii pp41 #aj4jj4 ,a ,tracer ,experiment3 ,tracing ,biochemical ,reactions with ,radioisotopes1 ,martin ,d4 ,kamen1 ,holt1 ,rinehart ,winston1 ,inc41 ,new ,york1 #aifd1 #abg pp41 #a4bh4 ,molecular ,biology3 ,genes and the ,chemical ,control of ,living ,cells1 ,j4 ,m4 ,barry1 ,prentice-,hall1 ,inc41 ,englewood ,cliffs1 ,new ,jersey1 #aifd1 #aci pp41 #c4ce4 ,elementary ,biophysics3 ,selected ,topics1 ,herman ,t4 ,epstein1 ,addison-,wesley ,publishing ,company1 ,inc41 ,reading1 ,massachusetts1 #aifc1 #abb pp41 #b4ie 7hardback71 #a4ge 7paperback74 ,articles ,autobiographies of ,cells1 ,r4 ,baserga and ,w4 ,kisieleski1 ,scientific ,american1 #bji3 #ajc 7,august #aifc74 ,electrons1 ,enzymes1 and ,energy1 ,michael ,g4 ,del ,duca and ,john ,m4 ,fuscoe1 ,international ,science and ,technology1 #ci3 #ef 7,march #aife74 ,scientific ,american1 #bje 7,september #aifa74 ,this is a special issue on the living cell4 ,the two articles cited below are of particular interest3 ,how ,cells ,divide1 ,daniel ,mazia1 #bje3 #aja4 ,the ,living ,cell1 ,jean ,brachet1 #bje3 #ej4 ,reports ,liquid ,scintillation ,counting3 ,proceedings of a ,conference ,held at ,northwestern ,university1 ,august #bj-bb1 #aieg1 ,c4 ,g4 ,bell1 ,jr4 and ,f4 ,n4 ,hayes 7,eds471 ,pergamon ,press1 ,inc41 ,new ,york1 #aieg1 #bib pp41 #aj4jj4 ,atomic ,energy ,research3 ,life and ,physical ,sciences2 ,reactor ,development2 and ,waste ,management1 ,a ,special ,report of the ,u4 ,s4 ,atomic ,energy ,commission 7,december #aifa71 ,superintendent of ,documents1 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c4 #bjdjb1 #ccc pp41 #b4be4 ,booklets ,radioisotopes in the ,service of ,man1 ,fernand ,lot1 ,national ,agency for ,international ,publications1 #cag ,east #cdth ,street1 ,new ,york #ajjaf1 #aieh1 #hb pp41 #a4jj4 ,science and ,cancer1 ,m4 ,b4 ,shimkin1 ,public ,health ,service ,publication ,no4 #aafb1 ,superintendent of ,documents1 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c4 #bjdjb1 #aifd1 #acg pp41 #j4fj4 ,motion ,pictures ,the ,cell3 ,structural ,unit of ,life1 #aj minutes1 sound1 color or black and white1 #aidi1 ,coronet ,films1 ,inc41 #fe ,e4 ,south ,water ,street1 ,chicago1 ,illinois #fjfja4 ,continuity of ,life3 ,characteristics of ,plants and ,animals1 #aa minutes1 sound1 color or black and white1 #aied1 ,audio-,visual ,center1 ,indiana ,university1 ,bloomington1 ,indiana #dgdje4 ,,dna3 ,molecule of ,heredity1 #af minutes1 sound1 color 7,no4 #ahbe71 black and white 7,no4 #ahbf74 #aifj1 ,encyclopaedia ,britannica ,films1 ,inc41 ,wilmette1 ,illinois #fjjia4 ,the ,science of ,genetics1 ,,aibs ,secondary ,school ,film ,series1 ,no4 #acbhj1 #be minutes1 sound1 color1 #aifb1 ,mc,graw-,hill ,book ,company1 ,inc41 #ccj ,west #dbnd ,street1 ,new ,york #ajjcf4 ,available for loan without charge from the ,,aec ,headquarters ,film ,library1 ,division of ,public ,information1 ,u4 ,s4 ,atomic ,energy ,commission1 ,washington1 ,d4 ,c4 #bjede1 and other ,,aec film libraries3 ,tracing ,living ,cells1 ,challenge ,film ,no4 #aa1 #bi minutes1 sound1 black and white1 #aifb4 ,produced by ,ross-,mc,elroy ,productions for the ,national ,educational ,television and ,radio ,center under a grant from ,argonne ,national ,laboratory4 ,this nontechnical film demonstrates some of the uses of radioisotopes in the study of cell division and in medical therapy4 ,the ,eternal ,cycle1 #ab minutes1 sound1 black and white1 #aied4 ,produced by the ,handel ,film ,corporation4 ,this nontechnical film illustrates the use of radioisotope tracers in biological research and is suitable for intermediate- through college-level audiences4 ,chromosome ,labeling by ,tritium1 #ae minutes1 sound1 color1 #aieh4 ,produced by the ,jam ,handy ,organization for the ,u4 ,s4 ,atomic ,energy ,commission4 ,this technical film discusses the advantages of tritium over other radioisotopes as labeling material in autoradiography4 ,a is for ,atom1 #ae minutes1 sound1 color1 #aiec4 ,produced by the ,general ,electric ,company4 ,this nontechnical film explains the structure of the atom1 natural and artificially produced elements1 stable and unstable atoms1 principles and applications of nuclear reactors1 and the benefits of atomic radiation to biology1 medicine1 industry1 and agriculture4 ,it is suitable for elementary- through high-school audiences4 ,,footnotes #a;,an organism is a complete living plant or animal4 #b,metabolism is the sum of the life-sustaining activities in a living organism1 including nutrition1 production of energy1 and synthesis 7building7 of new living material4 #c,morphologists are biologists specializing in the structure of organisms or in the study of whole organisms4 ,biochemists1 by contrast1 study chemical reactions of biological materials4 #d,this is not to be confused with a cell nucleus4 ,this word was borrowed from biology for atomic theory1 however4 #e;,an exception is the hydrogen atom1 which has no neutron in its nucleus4 #f,mev is the abbreviation for million electron volts4 #g;,a concept for which ,james ,d4 ,watson of the ,united ,states and ,francis ,h4 ,c4 ,crick of ,england shared a ,nobel ,prize in #aifb4 #h,the study of tissues4 #i,there are additional1 more subtle metabolic events that lead to the synthesis of ,,dna1 but they are not important in this discussion4 ,,photo ,,credits ,figure #a ,armed ,forces ,institute of ,pathology ,negative ,no4 #daef ,figure #c ,dr4 ,t4 ,tahmisian1 ,argonne ,national ,laboratory ,figure #d ,oak ,ridge ,national ,laboratory 7photo on right7 ,figure #e ,oscar ,w4 ,richards1 ,american ,optical ,company ,figure #g ,brookhaven ,national ,laboratory ,figure #i ,battelle-,northwest ,laboratory ,figure #aj ,oak ,ridge ,national ,laboratory ,figure #ai ,argonne ,national ,laboratory ,figure #bc ,argonne ,national ,laboratory ,figure #bd ,argonne ,national ,laboratory ,figure #bh ,argonne ,national ,laboratory ,figure #bi ,brookhaven ,national ,laboratory ,this booklet is one of the 8,understanding the ,atom0 ,series4 ,comments are invited on this booklet and others in the series2 please send them to the ,division of ,technical ,information1 ,u4 ,s4 ,atomic ,energy ,commission1 ,washington1 ,d4 ,c4 #bjede4 ,published as part of the ,,aec's educational assistance program1 the series includes these titles3 ,accelerators ,animals in ,atomic ,research ,atomic ,fuel ,atomic ,power ,safety ,atoms at the ,science ,fair ,atoms in ,agriculture ,atoms1 ,nature1 and ,man ,books on ,atomic ,energy for ,adults and ,children ,careers in ,atomic ,energy ,computers ,controlled ,nuclear ,fusion ,cryogenics1 ,the ,uncommon ,cold ,direct ,conversion of ,energy ,fallout ,from ,nuclear ,tests ,food ,preservation by ,irradiation ,genetic ,effects of ,radiation ,index to the ,,uas ,series ,lasers ,microstructure of ,matter ,neutron ,activation ,analysis ,nondestructive ,testing ,nuclear ,clocks ,nuclear ,energy for ,desalting ,nuclear ,power and ,merchant ,shipping ,nuclear ,power ,plants ,nuclear ,propulsion for ,space ,nuclear ,reactors ,nuclear ,terms1 ,a ,brief ,glossary ,our ,atomic ,world ,plowshare ,plutonium ,power from ,radioisotopes ,power ,reactors in ,small ,packages ,radioactive ,wastes ,radioisotopes and ,life ,processes ,radioisotopes in ,industry ,radioisotopes in ,medicine ,rare ,earths ,research ,reactors ,,snap1 ,nuclear ,space ,reactors ,sources of ,nuclear ,fuel ,space ,radiation ,spectroscopy ,synthetic ,transuranium ,elements ,the ,atom and the ,ocean ,the ,chemistry of the ,noble ,gases ,the ,elusive ,neutrino ,the ,first ,reactor ,the ,natural ,radiation ,environment ,whole ,body ,counters ,your ,body and ,radiation ,a single copy of any one booklet1 or of no more than three different booklets1 may be obtained free by writing to3 ,,usaec1 ,p4 ,o4 ,,box #fb1 ,,oak ,,ridge1 ,,tennessee #cghcj ,complete sets of the series are available to school and public librarians1 and to teachers who can make them available for reference or for use by groups4 ,requests should be made on school or library letterheads and indicate the proposed use4 ,students and teachers who need other material on specific aspects of nuclear science1 or references to other reading material1 may also write to the ,oak ,ridge address4 ,requests should state the topic of interest exactly1 and the use intended4 ,in all requests1 include 8,zip ,code0 in return address4 ,printed in the ,united ,slates of ,america ,,usaec ,division of ,technical ,information ,extension1 ,oak ,ridge1 ,tennessee ,transcriber's ,notes --,retained publication information from the 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