DNA STRUCTURE Watson and Crick Model of DNA

THE WATSON AND CRICK’S MODEL OF DNA DOUBLE HELIX

In 1953, James Watson and Francis Crick deduced the three dimensional structure of DNA and immediately inferred its mechanism of replication, Watson and Crick analyzed X-ray diffraction photographs of DNA fibres taken by Rosalind Franklin and Maurice Klilkins and derived a structural model that has proved to be essentially correct.

The salient features of their model are:-

1. Two helical polynucleotide chains are coiled around common axis, the chains run in the opposite directions.

2. The purine and pyrimidine bases are on the inside of the helix, where as the phosphate and deoxyribose units are on the outside the planes of the bases are perpendicular to the helix axis. The planes of the sugars are nearly at right angles to those of the bases.

3. The diameter of the helix is 20 A⁰, adjacent bases are separated by 34 A⁰ along the helix and related by a rotation of 36⁰, hence the helical structure repeats after in residues on each chain, i.e., at interval of 34A⁰.

4. The two chains are held together by hydrogen bonds between the pairs of bases adenine is always paired with thymine guanine is always paired width cytosine.

5. The sequence of bases along a polynucleotide chain is not restricted in any way, the precise sequence of bases carries the genetic information.

6. The ratio of A+G/C+T always equals to one

7. In every organism, the sequence of nucleotides in constant. The ratio of A=T/G=C is also specific to organisms.

8. Each pitch of DNA has two major and two minor groves.

The most important, aspect of the DNA double helix is the specificity of the pairing of the bases. Watson and Crick deduced that adenine must pair with thymine and guanine with cytosine.

The steric and hydrogen bonding factors restriction is imposed by the regular helical nature of the sugar-phosphate backbone of each polynucleotide chain the glycosidic bonds that are attached to a bonded pair of bases are always 10.85 A apart a  pair of pyrimidine base pair fits perfectly in this shape, in contrast, there is insufficient room for two purines. There is more than enough space for two pyrimidines but they would be too far apart to form hydrogen bonds Hence, one member of a base pair in a DNA helix must always be a purine the other a pyrimidine, because of steric factors. The base pairing is further restricted by hydrogen bonding requirements. The hydrogen atoms in the purine and pyrimidine bases have well-defined positions. Adenine can not pair with cytosine because there would be two hydrogens near one of the bonding positions and none at the other likewise guanine can’t pair with thymine, whereas guanine forms three bonds with cytosine. The orientation and distance of those hydrogen bonds are optimal for achieving strong interaction between the bases. The base pairing scheme was strongly supported by the base compositions of DNA’s from different types. In 1950, Erwin chargalff found that the ratios of adenine to thymine and guanine to cytosine were nearly 1 in all the samples studied.