In 1953 Watson and Crick surprised the scientific world with a concise one page paper in the British Journal Nature. The paper reported their molecular model of DNA, the double helix, which has since become the symbol of molecular biology. The beauty of the model was that its structure suggested the basic mechanism of DNA replication. Watson and Crick suggested ladder type organization of DNA. Each molecule of DNA is made up of two poly nucleotide chains which are twisted around each other and form a double helix. The uprights of the ladder are made up of sugar and phosphate part of nucleotide and the rungs are made up of paired nitrogenous bases. The pairs are always as follows:
Adenine always pars with thymine and cytosine with guanine. There is no other alternate possible two polynucleotide chains which are complimentary to each other, are held together by hydrogen bonds. There are two hydrogen bonds between A = T, and three between C = G. Both polynucleotides strands remain separated by 2OA´´ distance. The coiling of double helix is right handed and complete turn occurs after 34A´´.
Since each nucleotide occupies 34A´´ distance along the length of a polynucleotide strand be 10 mononucleotide occur per complete turn. Watson Crick model explained chargaff’s rules. Wherever one strand of DNA molecule has an A, the partner strand has a T and G is one strand is always paired with a C in the complimentary strand. Therefore in DNA of any organism, the amount of academic equals the amount of thymine and the amount of guanine equals the amount of cytosine. Although the base-pairing rules dictate the combinations of nitrogenous bases that form the rungs of the double helix; they do not restrict the sequence of nucleotide along each DNA strand. Thus the linear sequence of four bases can vary in countless ways and each gene has unique order, or base sequence.
DNA REPLICATION
Watson Crick model suggested that the basis for occupying the genetic information is complimentary one chain of DNA molecule may have any conceivable base sequence but this sequence completely determines that of its partner in the duplex. If the sequence of one chain is ATTGCAT, the sequence of its partner in duplex must be TAAGGTA. Each chain in duplex is a complimentary mirror image of the other. To copy the DNA molecule one need only unzip it construct new complimentary chain along each naked strand.
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