The compound 'A' is a complementary base of _______________ in DNA stands.

Solution

Chemistry ma’am · 3 steps

Spot the methyl group on that pyrimidine ring: A is thymine, and thymine always pairs with adenine.

1. Read the ring. A is a six-membered ring with two N atoms, so it is a pyrimidine base. It carries two C=O groups (at C2 and C4) and a CH₃ group at C5.
2. Name the base. A pyrimidine with two C=O groups and no methyl is uracil. Adding the 5-methyl group makes it thymine (5-methyluracil), the base found in DNA.

   Base in DNA	Type	Complementary base	Hydrogen bonds
   Thymine (A)	pyrimidine	Adenine (purine)	2
   Cytosine	pyrimidine	Guanine (purine)	3

3. So. In DNA, thymine pairs with adenine through two hydrogen bonds. Compound A is the complementary base of adenine, option (c).

The idea to keep. In DNA a purine always pairs with a pyrimidine: A with T (2 H-bonds), G with C (3 H-bonds). Thymine is just uracil with a CH₃ at C5.

Answer from the NTA final answer key. Solution by PrepWiser.