WATSON & CRICK MODEL OF DNA
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Though different authors had describe the structure of DNA differently but the most acceptable & convincing structure of DNA molecule had been illustrated by J. D. Watson & FHC Crick by using x-ray diffraction photograph of DNA. The model as illustrated by them known as Watson & crick structural model of DNA which provides an idea how the various constituents of DNA are attached to one another,
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The model has the form of a double helix from by polynucleotides. The helix has a diameter of about 20A along its length. Each term consist of a stack of 10 nucleotides & show distance between nucleotide is 3.4A. In DNA molecule the adjacent deoxyribonucleotides are join in a strand by Phosphodiester Bridge or bones which linked the sugar of one nucleotide to the sugar of other adjacent nucleotide. The sugar & phosphate chain remains on the inner side of the helix & face each other. The 2 polynucleotide chains are held together by H-bonds.
The pairing of the nitrogenous bases of the two strands is very peculiar. The dimension of the purine(A,G) ring is greater than the pyrimidine(T, C) ring. So, to maintain constant diameter of the 2 same helix molecule of DNA. The Purine always pair with Pyrimidine bases. Adenine can attached to Thymine by 2 H-bonds. Similarly Guanine can be attached with Cytosine by 3 H-bonds. If one chain contains Adenine than the another chain will be have Thymine, like wise if the one chain contains Guanine then there will be Cytosine in the other chains. So the sequence of the nucleotides of one chain will determines the sequence of nucleotides of the other chains.
Thus whatever the sequence of
base is present on one strand of the DNA molecule, a complementary sequence is
present on the other strand. Due to this type of base pairing the two strand of
DNA aren’t identical but complementary to each other.
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