# Describe Watson-Crick model for structure of DNA.
ANS ::
* In 1953, Watson and Crick proposed a model showing molecular structure of DNA.
* In the molecule of DNA two polynucleotide chains run in antiparallel directions and twist around each other to form a spiral helical structure.
* One nucleotide is linked with another nucleotide by a phospho-diaster bond.
* In the chain, sugar and phosphate are linked with their nitrogenous bases.
* Purine type nitrogenous base of one nucleotide links with pyrimidine type of nitrogenous base by weak hydrogen bonds.
* Adenine(A) links with Thymine(T) by two weak hydrogen bonds whereas Guanine(G) links with Cytosine(C) by three weak hydrogen bonds. Such pairing is known as complimentary pairing.
* The purine to pyrimidine ratio in each DNA molecule is always 1 : 1.
* The length of one helix is 34 A° and each helix possesses 10 nitrogenous base pairs.
* The distance between two adjacent nucleotides is 3.4 A° and the distance between two opposite chains is 20 A°.
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