STRUCTURE OF DNA
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WATSON AND CRICK MODEL OF DNA
- James Watson &Francis Crick (1953) proposed the helical strucure of DNA and are awarded a Nobel Prize for it.
- he two strands are helically coiled in in such a fashion that base molecules of two strands are linked together by weak hydrogen bonds to constitute base pairs.
- Each turn of double helix contains ten paired nucleotides.Distances between two paired nucleotides is 3.4 armstrong.
- Adenine is always paired with thymine and Guanine with cytosine.Adenine and thymine are linked by two hydrogen bonds whereas Guanine and cytosine are linked by three hydrogen bonds.
- The two chains of DNA molecules run in antiparallel directions.This means that carbon atom at 5' position in the deoxyribose component is in one directionin one chain and is in the opposite direction in the other chain.That is, the the two chains are parallel but their 5' -3' directions are opposite.
- In each chain,the phosphate component carried by the carbon atom at position 5' of the sugar in one nucleotide unit is joined by phosphodiester bond to the hydroxyl component of the carbon atom at position 3' of the sugar in the next nucleotide unit.
- Thesa alternating sugar phosphate components form the backbones of DNA double helix molecule.
- The nitrogenous base molecules are attatched with the deoxyribose sugar molecules by glycosidic bonds.
- The glycosidic bond appears b/w 1' carbon of the sugar and the nitrogen at position 1' in case of pyramidine and at position 9'for nitrogenous base.