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प्रश्न
With the help of a neat and labelled diagram describe Watson and Crick's model of DNA.
उत्तर
(i) In 1953 James Watson and Francis Crick proposed DNA structure based on X-ray crystallographic studies.
(ii) DNA molecule consist of two long stands coiled around a common imaginary central axis to form a double helix. It look like a twisted ladder.
(iii) Each strand consist of number of nucleiotides. Each nucleotide is made up of deoxyribose sugar, phosphate group and nitrogen base.
(iv) The combination of sugar and nitrogen base is called.nucleoside. Together with phosphate called nucleotide.
(v) The sucessive nucleotide of the same strand are linked by 3'— 5' phosphodister linkage i.e. phosphate attached to 5th carbon of sugar of nucleotide is joined to 3rd carbon of another.
(vi) The two stands are equidistant and parallel all along their length. Each transverse step is made up of nitrogen bases which are purines and pyrimidines.
(vii) Double helix : The strands of DNA molecule are spirally twisted (coiled) around each other. The side arms of ladder are called banister or railing and the steps are called rungs.
(viii) The base pairs forms rungs or steps and a back bone of strand forms railing. Backbone of strand is formed by sugar phosphate chain.
(ix) Base pairing : The nitrogen bases present on one strand, pair with the nitrogen bases of opposite strand.
(x) Purine base A always pairs with pyrimidine base T by two hydrogenbonds and purine base.
(xi) G always pair with the pyrimidin.e base C with 3 hydrogen bonds.
(xii) This pairing is termed as complementory base pairing.,
Purine : Pyrimidines
Adenine = Thymine (A T or T = A)
Guanine = Cystosine (G =.C or C = G)
Purines (adenine and Guanine) has double ring and Pyrimidines (Cystosine and thymine) has single ring.
