Question

Which one is true?
(1) $A=T,G=C$
(2) $A=G,T=C$
(3) $C=G,T=A$
(4) $T=U,G=A$

• A. 1,2 and 3 are correct.
• B. 1 and 2 are correct.
• C. 2 and 4 are correct.
• D. 1 and 3 are correct.

Answer 1

Answer: (D)

Nitrogen bases are of two types:
(1) Purines base - (a) Adenine (b) Guanine
(2) Pyrimidines - (a) Cytosin (b) Thymine
The specific pairing between purine and pyrimidine bases proposed by Watson and Crick not only fitted very well with the radius of DNA determined from X-ray diffraction pictures but also had necessary complementary lock and key shape for efficient hydrogen bonding, which happens to be two between $A$ and $T(A=T)$ and three betwen $G$ and $C(G\equiv C)$.