How many base pairs are present in one complete turn of DNA?

The correct answer is Option (1) → 10

In the Watson-Crick double helix, the two strands coil round each other in a right-handed spiral. One complete turn, the pitch, is 3.4 nm long, and the base pairs are stacked about 0.34 nm apart. So the number of base pairs in one turn is 3.4 ÷ 0.34 = 10.

Note: A similar question was also asked in CUET (UG) 2026 on 22-05-2026 (Slot 1).

NCERT Extract:
Structure of Polynucleotide Chain – “… (iv) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm (a nanometre is one billionth of a metre, that is 10^-9 m) and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm.”