Question

If there are 120 adenine molecules in a DNA double helical structure showing 20 coils, what is the number of pyrimidine nucleotides forming three hydrogen bonds in it?

• A. 80
• B. 100
• C. 120
• D. 140

Answer 1

The correct option is A 80

DNA shows 10 nucleotides per turn (coil) of the helix. If there are 20 coils then the total number of nucleotides is 200 out of which 120 are adenines (equal amount of thymines). So, the number of guanines (equal amount of cytosines) nucleotides is 80. Adenine and guanine form a base pair consisting of two hydrogen bonds, whereas guanine and cytosine form a base pair with three hydrogen bonds. Among guanine and cytosine, cytosine is the pyrimidine. Thus, there are 80 pyrimidine nucleotides which contain 3 H-bonds.