Base Pairing in DNA and Hydrogen Bonding



Question:
Base pairing in double helix structure of DNA between adenine(A) and thymine(T) forms two(2) hydrogen bonding only while that one between guanine(G) and cytosine(C) forms three(3) hydrogen bonding between them. But if you analyze the structure of thymine you will find there is –oxo- group attached to carbon number six (6) of pyrimidine (thymine) and structure of purine (adenine). Also, there is hydrogen atom attached to carbon number two (2) which can participate in hydrogen bonding but this is not so. What is the reason(s) behind it?







Answer:
This happens because carbon (carbon number two) in which hydrogen is attached to, which can participate in hydrogen bonding is slightly electronegative thus C-H Bond forms a weak polar bond hence cannot participate in hydrogen bonding.

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