NEET UG 2020 — Zoology Molecular Basis of Inheritance

2020mcqeasy

Which of the following statements is correct?

Official previous-year question

Held on 30 Apr 2020 · Verified 9 Jul 2026.

Options

A
Adenine pairs with thymine through one H-bond
B
Adenine pairs with thymine through three H-bonds
C
Adenine does not pair with thymine
D
Adenine pairs with thymine through two H-bonds

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Solution

Erwin Chargaff’s rules: Chargaff’s along with his colleagues, performed base composition studies and put forward certain generalizations for double-stranded DNA, called Chargaff’s rule (Not applicable for single-stranded DNA).

i) Purines and pyrimidines occur in equal amounts.

ii) Purines found in DNA are adenine and guanine. Pyrimidines of DNA are thymine and cytosine.

A + G = T + C

iii) $\frac{A+G}{T+C}=1$ this value is constant for all species.

iv) Base ratio $\frac{A+T}{G+C}$ is specific for a species. It is used to identify the species. It is less than one in prokaryotes, e.g., E. coli = 0.92, and more than one in eukaryotes, e.g., Human = 1.52.

v) Sugar deoxyribose and phosphate residues occur in equal number.

vi) Purine adenine is equimolar with pyrimidine thymine

vii) Purine guanine is equimolar with pyrimidine cytosine.

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