STRUCTURE AND FUNCTION OF DNA F. H. C. CRICK asc. For a number of years a new scientiJc unity has been developing frotn a synthesis itz which chemistry and physics have become closely lirlked with biology. On the biological side cytologists are concerned with what happens to the nucleo-proteins and the nucleic acids present in plant and animal cells. These compounds are of great interest to biochemists atld biophysicists, and during I953 importarrt advances have been made on this front as a resrrlt of discorwries in British, U.S. nrtd Canadian loboratories. One of the fundamental problems of biology is the manner in which the hereditary factors are copied and passed on from one generation to another. In particular we should like to know in terms of atoms and molecules just how these factors carry the genetical information, how the cell produces an exact copy of them, and how they exert their influence on the cell. The hereditary factors are believed to be carried by the chromosomes, the rather fibrous bodies found inside the nucleus of a living cell. Now chromosomes mainly consist of two kinds of substance-protein and nucleic acid-and much experimental work has been carried out to discover their chemical nature. As far as nucleic acid is concerned this work has been conspicuously successful, and its general chemical formula is now known. Until recently, however, hardly anything has been found out about its `structure'- the arrangement of the molecule in space-but within the last year there have been exciting developments in this direction, and it is with these that this article is mainly concerned. Two sorts of nucleic acid are found in living cells. The one in which we are interested i31 this article is called deoxyribonucleic acid, or DNA for short. Its general formula is very simple to grasp. It consists of a very long chain made up of alternate sugar, and phosphate groups. The sugar is always the same sugar (known as deoxyribose, or ribose with one oxygen missing) and it is always joined on to the phosphates in the same way-by ester linkages-so that this long chain is perfectly regular, repeating the same phosphate-sugar sequence over and over again. This chain is only part of the molecule, however, for every sugar has a `base' attached to it, as shown in Fig. 1, but the base is not always the same base. Commonly four different types are found. They are all flat heterocyclic rings-two purines, known as adenine and guanine, and two pyrimidines, known as thymine and cytosine: (their formulae appear later, in Figs. 5 and 6). As far as is known the order in which they follow one another along the chain is irregular, and a typical bit of DNA might have the formula shown in Fig. 2, in which the names of the bases have been written in at random. It iS because the exact sequence of the bases is not known that one can only say that the general formula of DNA is established. It should not be thought that this rather simple formula was found in a day. It has taken more than twenty-five years' work by organic chemists to prove it, and it should be reckoned as one of the major achievements of organic chemistry applied to biology. It is the foundation for all the ideas described in the rest of this article. During the last few years biochemists, using improved modern methods (in particular, chromatography and 12 ultra-violet absorption), have tackled the problem of the relative amounts of the four bases in DNA from different species. The leaders in this field have been Dr. E. Chargaff and his colleagues at Columbia University, New York, and Dr. G. R. Wyatt in Canada. They have shown that the relative amounts of the various bases can vary from species to species, but appear to be fixed (within the limits of experimental error) for a given species, irrespective of which individual or which organ the DNA is taken from. The chemical formula does not by itself tell one the shape of the molecule. This is because there are many single bonds in the phosphate-sugar chain, and as rotation is in theory possible about all of them, one might expect \ \ \ / SUGAR -BASE PHOSPHATE UGAR- BASE PHOSPHATE' \ 'SUGAR- / BASE PHOSPHATE' \ / SUGAR- BASE PHOSPHATE, \ ./ SUGAR- BASE , , / F I G . 1. The general chemical for- mula of a single chain of DNA. JANUARY 1954 DISCOVERY I/IC chain to coil about in a rather random manner. ( nriously enough, both measurements of the viscosity and It&-scatteringofDNA insohition, andpicturesofdry DNAin I he clcctron microscope all suggest that the molecule is long, tlnn, and fairly straight, rather like a stiff bit of cord. The w~dtll of the molecule, as measured for example by the Icng~h of the `shadow' in the electron microscope, is about .`I) A. The length of the DNA inside the cell is perhaps very c:~caI indeed, and even after it has been extracted, a process which may break it up somewhat, it is still fairly long. A ~YIWII figure would be, say, 30,000 A, or 3~.* None of these methods tells usanythingabout the detailed .II I anbement in space of the atoms inside the molecule (the ~vp~cal distance between atoms bonded together in organic Inolccules is 12 A). For this it is necessary to use X-ray Ihll'raction. The DNA from a tissue-the favourite is the ~hvmus gland of the calf-can be extracted by mild t~t~thods, and then drawn into long fibres. The fibres can 1~ mounted in an X-ray diffraction camera in the usual m6umcr, and the diffraction pictures recorded on a photo- tir.rphic plate. The pioneer work was done by Professor W. I hstbury and Dr. Florence Bell before the war, but almost 1111 the recent work has been done by Dr. M. H. F. Wilkins, In Rosalind Franklin and their co-workers at King's ( ~~llcgc, London. The diffraction pictures they have ob- (iuncd are of an extremely high quality. If the structure ~rluld be deduced unambiguously from the X-ray photo- yr,cphs the solution would have been easy, but as is well LII~WI~, this is not possible. From a postulated structure *WC can work out mathematically the expected diffraction pictlcrn, but there is no direct way of goingfiom the X-ray ~(~IIII'C lo the structure. In mathematical language this is ~~IIISC the diffraction pattern gives the amplitudes of the 1 11ut icr components of the electron density, but not their trh~ivc phases. Nevertheless, certain facts emerged straight away from ~iw X-ray work. Firstly, it was found that there were two thrtwt X-ray patterns, depending upon the humidity. One tat these, which occurred when the water content was about JO",,, was crystalline; that is, there was three-dimensional turlcr present. When the humidity was raised the fibres IIQIL up more water, increased in length by about 30%, and w\r it ditferent pattern which tended to be paracrystalline; 11~1 IY. the molecules were all parallel to each other, but tic hod side by side in a less regular manner. ~U-AD- pr \pH ,SU-CY----GU-SU( PH PH 6 'SU-AD----TH-SU( p>U-Gu-- / v JANUARY 1954 DISCOv'fRY o ? ? ???? ? ?????? PH h4 )SU-CY----GU-SU' \ * P\*~~----Tfi-~~~ P c PH /' ,SU-GU----GY-SU( (a) (b) , --TH+Xl( PH --GU-SU( PH --TH-SU( PH - - CY-su( '\ `\ 'SU-AD----TH-SU/ PH Pk ;su-CY )P* ----(JJ-S PH Y \ PH --TH-SU ?SU-GU----GY-Su( *' '\ (d) FIO. 7 (A) A typical stretch of the DNA structure. (B) The two chains separate. (c) The formation of two new chains from loose nucleotides. (D) The process complete. Note that the sequence of the bases has been copied exactly. The letters represent the first two letters of the words PHosphate, Sugar, ADenine, GUanine, THymine and CYtosine. 16 JANUARY 1954 DISCOVERY %pcrm heads and bacteriophage, so that there seems little doubt that the structure is biologically significant. The present position is therefore that while the details of IIIC structure remain to be worked out-and until this is (lone the model cannot be considered as proved-it seems very probable that the following statements will stand the test of time: I. The structure consists of two chains. 7. The chains are helical and wound round a common axis. .\. They are held together by hydrogen bonds between specific pairs of bases. 4. The structure occurs in biologically intact material. A POSSIBLE REPLICATION MECHANISM Now the exciting thing about a model of this type is that II immediately suggests how the DNA might produce an r\:tct copy of itself. This is because the model consists of I\VO parts, each of which is the complement of the other. I he basic idea is that the two chains in the structure unwind ~114 separate. Each chain then acts as a sort of mould on to \\h~cIr a new complementary chain can be synthesised. When this process is complete there will be fwo pairs of t IIJII~S where we only had one before. Moreover, because I 11 111~' specific pairing of the bases the sequence of the pairs 1 +I 1~~1ses will have been duplicated exactly. !\F an analogy consider two photographic films, one a rllj\itrve and the other a negative of the same scene. Now if 1 IIIC gives the positive to one person, and asks him to print .I rrcgative from it, and also gives the original negative to .cr\lrther person, and asks him to print a positive from it, ~lrcv will end up with two pairs of photographs, each pair l~hc rhe original pair. We shall, in effect, have made an r\,rc`t copy of our original pair in one step. II) see how this works out in the case of DNA let us con- v!!c~ the process in rather more detail. Since we have to ~~trthcsise two new chains we require some new material. I IIC exact precursors of DNA are not known, but let us .\\uII~~ fdr simplicity that it is built up from nucleotides, \* tr1~21 is the name given to the small molecules which I I~IILIII~ one phosphate, one sugar and one base. Irn;rgine, then, that we have a single helical chain of DNA, *rr~~l Ihat floating around it, inside the cell, there is a supply 111 IIIC four sorts of nucleotides. Every now and then a loose ~11~ Icotitle will attach itself by its base to one of the bases 111 IIIC DNA chain. Now if this happens to two adjoining bases, and if the loose nucleotides are the type which can form specific pairs with those already there, they will be in just the right position to be joined together, and, event- ually, to form part of the new chain. If one or both of them is not the correct type to go in at that point, it will be impossible to join them together and before long they will diffuse elsewhere. Thus only the nucleotides with the proper bases will get joined together to form the new chain. While this process is going on, the other single chain of the original pair will also be forming, in a similar manner, a new chain complementary to itself. The whole process is illustrated in Fig. 7. In Fig. 7~ there is shown a small stretch of the original pair of chains. In Fig. 7~ these have separated. In the next figure new chains are being formed from loose nucleotides, and in Fig. 7~ the process is com- plete; it can be seen that the original pair has now been duplicated. At the moment this idea must be regarded simply as a working hypothesis. Straight away it raises a number of questions. How do the two chains unwind? What holds a single chain in a helical configuration? (Watson and I suspect that the replication starts almost as soon as the unwinding, so that only a very short stretch is ever in the `single' state at one time.) Most important of all, how does the DNA influence the rest of the cell? We believe that the sequence of the bases along the DNA is the code that carries the genetical information, but how does it produce its effect? We can see how the code may be copied, but as yet we cannot read it. In favour of the idea one can only say that it seems rather an odd coincidence to find in the one material which is most closely associated with replication a structure of exactly the type one would need to carry out a specific replication process, namely, one showing both variety and complementarity. The process is also attractive in its simplicity. While it is obvious that whole chromosomes have a fairly complicated structure it is not unreasonable to hone that the molecular basis underlying them may be rather simple. If this is so it may not prove too difficult to devise experiments to unravel it. It is, after all, remarkable that X-rays, which only show clearly the regular parts of a structure should tell us anything at all about a material whose main purpose, we suspect, is to embody variety. In any event we now have for the first time a well-defined model for DNA and for a possible replication process, and this in itself should make it easier to devise crucial experi- ments. REFERENCES I *U ~crrcral background, consult The Biochernisfry of the Nucleic Natrcre (1953), 172, 759. (This also gives all the other X-ray refer- @. +,/I I,y 1. N. Davidson (Methuen monograph). ences.) f'..P I % <,,I wcent research Electron microscope. R. C. Williams, Biqchim. et Biophys. Acta I ,u!`. The helical structure of crystalline deoxypentose nucleic (1953), Y, 237. H. Kahler and B. J. Lloyd, Biochim. et Biophys. ,I -1. \t II. F. Wilkins, W. E. Seeds, A. R. Stoker and H. R. Wilson, Acta (1953), 10, 355.