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12th Biology 2012 Set1 Outside Delhi Board Paper Solution

Question 25

List the salient features of double helix structure of DNA.
OR
How are the structural genes activated in the lac operon in E.coli?

Answer
The salient features of double helix structure of DNA are-
 
(i) The double helix structure is made of two polynucleotide chains; the backbone is constituted by sugar phosphate.
 
(ii) These two chains possess anti-parallel polarity, means one chain has polarity 5’-3’ and the other has 3’-5’.
 
(iii) The two strands of DNA are joined together by hydrogen bonds between purines and pyrimidines. Adenine always pairs with Thymine by two hydrogen bonds and Guanine always pairs with Cytosine by three hydrogen bonds.
 
(iv) The two chains are coiled in a right- handed manner where the pitch of the helix is 3.4 nm and distance between two base pairs is 3.4 A°.
 
(v) The backbone of DNA double helix is formed by sugars and phosphates where bases are projecting inwards.