# DBT JRF 2020 — Question 133

Exam: DBT JRF 2020 · Section: B · Topic: Biochemistry · Difficulty: Medium

## Question

In a DNA molecule, two antiparallel strands that are complementary in their nucleotide sequence are paired to form a

## Options

- **A.** right handed double helix with 8 nucleotide pairs per helical turn
- **B.** none of the given options
- **C.** left handed double helix with 10 nucleotide pairs per helical turn
- **D.** right handed double helix with 10 nucleotide pairs per helical turn

## Answer

Option D

## Explanation

Watson-Crick B-form DNA double helix parameters (derived from X-ray crystallography by Franklin, Watson, Crick, Wilkins, 1953): Handedness: RIGHT-HANDED helix (the helix coils clockwise when viewed from either end). Helical repeat: 10.5 bp per complete 360° turn (commonly approximated as 10 bp/turn for calculations). Pitch: 35.7 Å (~34 Å with the 10 bp/turn approximation). Rise per base pair: 3.4 Å. Diameter: ~20 Å. Base pairs are anti-parallel (5'→3' on one strand, 3'→5' on the other). Major groove (~22 Å wide) and minor groove (~12 Å wide). Z-form DNA is left-handed; A-form DNA is right-handed but more compact (11 bp/turn). Answer: right-handed double helix with approximately 10 nucleotide pairs per helical turn.

Source: https://mypyqbuddy.com/questions/in-a-dna-molecule-two-antiparallel-strands-that-6a05a8a02adafec47029329d