Deoxyribonucleic acid (DNA)
J.D. Watson and F.H.C. Crick showed in 1953 that DNA has a double helical structure with two polynucleotide chains connected by hydrogen bonds and running in opposite directions. When DNA samples are analysed from different organs of body of the same individual or from different individuals belonging to the same species, no differences in the relative composition of different bases were observed. The following generalizations could also be made :
Number of adenine molecules always equaled number of thymine molecules.
Number of cytosine molecules always equaled number of guanine molecules.
Number of adenine molecules did not necessarily equal that of guanine and similar relationship was observed between cytosine and thymine.
Although A = T and C = G, there is no restriction on the ratio (A + T)/(C + G); nor is there any restriction on sequence of bases in one polynucleotide chain. But, since A is always linked to T and C to G as determined from the above evidences, sequence of bases in one polynucleotide should determine the sequence of bases in the other polynucleotide of the double helix.
Besides chemical analysis, DNA was also studied by X-ray crystallography by M.H.F. Wilkins and his associates. Such studies demonstrated that DNA was a helical structure with a diameter of 20 Å and a pitch (one round) of about 34 Å. On the basis of these studies of chemical analysis and X-ray crystallography, following features of the DNA molecule are now known:
Fig. 25.15. A double helix model of DNA as proposed by Watson and Crick.
DNA consists of a double helix, in which two polynucleotide chains are coiled about the same axis in such a fashion that they can separate from one another only by uncoiling andthat lateral separation would not be possible.
Bases are set in a plane at right angle to the long axis.
Two polynucleotide chains which run in opposite directions have complementary base : sequences, since adenine is linked to thymine and cytosine to guanine.
Distance between two base pairs is 3.4 Å and there are 10 base pairs in each turn.
Choice of base pairing is understandable because of a constant diameter of 20 Å in the molecule; two purines would need too much space and two pyrimidines would occupy too little space to pair. When a pyrimidine always pairs with a purine, space could remain constant. The pattern of coiling of two polynucleotide chains is shown in' Figure 25.15.