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Molecular Structure of Nucleic Acids
Author: J.D. Watson,F.H.C. Crick
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"MOLECULAR? STRUCTURE OF NUCLEIC ACIDS A Structure for Deoxyribose Nucleic Acid j3 W wish to suggecit 8 structure for the salt of deoxyribose nuclei0 void (D.N.A.). ThiR structure has novel f@8tUIW which 8IX3 of considereblc biological intewst. A structure for nucleic acid has already been proposed by p8uling and Corey*. They kindly m8de their manuscript 8vailablo to us in .8dvence of publication. Their model conhtR of three intar- twined cheins, with t.ho phosphates near lhe Abre 8x& and the bases on the outside. In our bpinion, this structure is un.tiefectory for two reasons : (1) We believe thnt the meteri which gives the X-ray diagr8ms is the s8lt. not the free noid. Without t.he ecidio hydrogen 8toms it is not clear Whet forces would hold the structure t.ogethcr, especially 88 the nega?ivcly cherged phosphates ne8r t.he 8ti? will repel each otlor. (2) Some of t,ho v8n der Waals dist8nces 8ppcsr to be too smell. Another thrcc-chein structure h8s also been sug- gcstod by Fro&r (in t,he press). .Tn his model the phospkatcs 8~) on the out&do 8nd Yhc bases on tho inside, -link4 together by hydrogen bonds. This structure BR described is tathor ill-dcflned, nnd for This llgurc Is purely cllaprammrtic. The two ribbonm nymbollse the two pho@phatwu&w chains. and the horl- mntal zoda the paim of basea boldln~ the chalm touether. The vertical lhc marka the fibre asln this mason we shall not comment on it. We wish to put forw8rd a mdic8lly diffcrcnt structure for the eelt of deoxyribose nucleic acid. This structure haa two helical chains each coiled round the -me axis (see di8gram). We have made the usuel chemical ARsumptions, nsmely, thet each chein consists of phoqhate di- e&x groups joining p-D-deoxy- ribofumnose r&&dues with 3?,5? hk8gCS. The two chains (but not their b8s~) 8re rel8ted by 8 dyad perpendicular to the fibre 8XiS. Both chains follow right- handed helices, but owing to the dyad the scquencoe of the taoma in the t.wo. ohains run in opposita directions. Each Clulin loosely resembles Fur- berg?s* model No. 1 ; th8t is, the bases 8re on the inside of thcr helix and t,he phosphet@s on the ciutside. The cor@ur8tion of the sug8r end t,he 8toms ne8r it is close to FUrberg?s ?standard cotiguration?, the sugar being roughly perpendi- cular to t.he aMached base. Them is a residue on each chain every 34 A. in the t-dim@ tion. We have assumed an 8&e of 36? between edjeccnt reaiducs in the same chain, so that the structure repeats after 10 residues on each ohain. that is, after 34 A. The dietenoo of 8 phosphorus atom tirn the flbre exis is 10 A. AE t.he phosphates erc on the outside, cations have esey 800888 to them: The structure is an open one, 8nd its w8tM Content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more comp8ct. The novel feature of the structure is the manner in which the two cheins are held %ogether by the purine and pyrimidine beses. The plenea of the ba6es are perpendicular to the fibro oxis. They are joined together in pairs, a single beee from one chain being hydrogen-bonded to a single base from the other chain, sc that the two lie side by side with identical z-co-ordinato8. One of the pair must be 8 purine and the other 8 pyrimidine for bonding to occur. The hydrogen bonds are made 8s follows : purine pcsition 1 to pyrimidine position 1 ; purine position 6 to pyrimidino position 6. If it is 8ssumed that the bases only occur in the &ucturo in the most plausible tautomeric formrr (thet is, with the keto rather than the onol oon- Qurations) it is found that only qx&ic pairs of b8ses c8n bond together. Thes8 pairs are : ad&nine (purine) with thymine (pyrimidinc), and gu81~i.1~ (purine) with cytosine (l~yrimidine). In other words, if 8n adenine forms one member of 8 pair, on oither chain, then on these assumptions the other member must be t.hymine ; similarly for gu8nine and oytosine. The sequence .of base8 on 8 sing10 chein does not 8ppe8r.to be restricted in 8ny way. However, if only specifli~ peirs of baeea can be formed, it follows thet if the sequence of b+?ea on one oh8in in given, then tho seqtronce on the other &bin is 8utomatic8lly deterniined. ?. ! It has been found experimentally?~4 thrrt the ratio of the amounta of adenine to thymine, and the ratio Of gUeIliIle t0 CytOSilU3,8I?e 81W83?8 VW&? OloSe t0 unity for deoxyribosc nucleic 8cid. It is probably impossible to build .this etruature with a ribose sug8r in pleoo of the deoxyribcee, as the extn, oxygen atom would make too close a van c&x Waals conttbct. The previously published.X-ray data@& on deoxy- riboee nucleic acid am insufilcient for a rigorous test of our structure. So far as we can tell, it is roughly compatible with the experimental do&, but it muat be regarded as unproved until it has been oheoked against more exact results. Some of these m given in the following communic8tiona. We wcm not (swam of t,he details of the results pmeented them when F devkd our structure, which resta mainly though not entimly on published experimental date cad attain- chemical e?rguments. ?t It has not escaped our nkioe thet the! apeoiso pairing we h8ve poatul8ted imme&tely &eetS? 8 possible copying meohaniem for the? n&o m&Al. Full details of the atructum, ino uding tlw tmn- ?ff ditions assumed in building it, tqether with a set of co-ordinates for the atoms, will be pubkhed elsewhere. We ew much indebt& to Dr. Jerry Doaohua for oonatsnt advice and orNoiam, orpsorally on inter- 8tOmiC distances. We have also been 8tim&te& 8 knOWledg@3 Of tb (Isarsti -turr, Of the experimental multi aad idaas of Dr., . H..F. T Wilkins, Dr. R. E, Franklin and their oo-wotha at 738 NATURE King?s CdAege, London. One of ua (J. D. W.) haa b&m aided by s fellow&p from the National Foundation for Infantile Persly&. J. D. WATSON s. lx. a. Cmoa tiedi- Research ~ounoil Unit for the Study of the MoIeouk Structure of Biological Systems, .$&wndiah L&oratory, Cambridge. April 2. .? Panlint?. I,., and Cmey. ?it. B.. Nature, 17l, 346 (1958) ; Pme. U.S. Nd. dad. Sci.. 28, 84 (1963). ? Fruberp, 8.. dds Chms. scord.. 6, 634 (1952). ? churpd. E.. for Mfwencu eea 55amcllbofl 9.. BMmnnall. 0.. md . chum& E.. Bfahin. St Biophm da, 0. 402 (lM2). ?Wstt. 0. B., J. C2m. PhwioZ.. 80, 201 (1952). ? Aatbw, W. T.. Symp. SOC. Exp. Bbl, 1, Nucleic Acid, 66 (Camb. Univ. PrM# 1947). * W~ikh. M. H. F., aad Bamh& J. T.. Bio&m. et Efephy8. Ado. 10, 192 WMS). -April?25;1953 VOL. 171 "
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Hemant kumar Joshi
08/21/2010
it was a marvelous invention in human history .
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Genetics
Gene Inheritance and Transmission
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Nucleic Acid Structure and Function
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