Structure of DNAWatson and Crick Model of DNA1 DNA consists of two polynucleotide chains (strands).2 These chains form a double helix like a spiral staircase.3 The sugar phosphate unit forms the backbone and nitrogen base forms the centre.4 Both the strands are joined together by weak hydrogen bonds.5 The two strands are anti parallel ie if one runs in 3-5 direction the other runs in 5-3direction.6 The width of DNA molecule is 20 angstrom or ( 2 nanometers).7 The helix takes a complete turn after 34 angstrom or (3.4 nanometers)8 There are 10 base pairs (3.4 A for each) in a complete turn (34A) of DNA molecule9 One chain can have any sequence of base but the other chain has to becomplementary. If there is a purine base in one helix it has to be pyramidine onthe other . This is called base pairing .It is very specific and the only patternfollowed is A double bond T (2 H bonds ) and G triple bond C (3 H bonds).10 The nucleotide in the helix are joined together by phosphodiaster bonds
