Q. In the DNA molecule,
A) the proportion of adenine in relation to thymine varies with the organism
B) there are two strands which run antiparallel-one in 5′ → 3′ direction and other in 3′ → 5′
C) the total amount of purine nucleotides and pyrimidine nucleotides is not always equal
D) there are two strands which run parallel in the 5′ → 3′ direction.
Explanation
A DNA molecule has two unbranched complementary strands which are spirally coiled. The two chains are antiparallel, i.e., they run parallel but in opposite direction. One chain has the polarity 5′ → 3′ whereas, other has 3′ → 5′. Both are held together by hydrogen bonds between their bases, i.e., A = T and G ≡ C and the amount of adenine is equal to thymine and guanine equals to cytosine. The base ratio A + T / G + C may vary from one species to another but is constant for a given species. The purine and pyrimidines are always in equal amount (A + G = T + C) but A + T is not necessarily equal to G + C.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.