Molecular Basis of Inheritance Question 2
2. Which of the following statements is correct?
[2020]
(a) Adenine pairs with thymine through one $H$-bond
(b) Adenine pairs with thymine through three $H$-bonds
(c) Adenine does not pair with thymine
(d) Adenine pairs with thymine through two $H$-bonds
Show Answer
Answer : d
Hints & Solution
(d) Based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one. Adenine pairs with thymine through two $H$-bonds i.e., $A=T$ and guanine pairs with cytosine with three $H$-bonds. Adenine and Thymine also have a favorable configuration for their bonds. They both have to $-OH / NH$ groups which can form hydrogen bridges. When one pairs Adenine with Cytosine, the various groups are in each others way. For them to bond with each other would be chemically unfavourable.
