In 1953 James Watson and Francis Crick suggested a model of DNA. They proposed that DNA molecule is double helix looks like a ladder. Each molecule of DNA is made up of two polynucleotide chains, which are twisted around each other form a double helix. The uprights of ladder are made 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 pairs with thymine and cytosine with guanine. There is no other alternative possible. These bases are hold together by weak force of hydrogen bonds. There are two hydrogen bonds between A=T, and three between C=G, both polynucleotide strands remain separated by 20 A distance. The coiling of double helix is right handed and complete turn occurs after 34 A. There are 10 mononucleotide occur per complete turn in double helix of DNA.
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