Base Pairing Rules and Hydrogen Bonds — NEET Biology
✅ Asked in NEET 2022If A and C make 30% and 20% of DNA, respectively, what will be the % composition of T and G? (NEET 2022 Phase 2)
If A and C make 30% and 20% of DNA, respectively, what will be the % composition of T and G? (NEET 2022 Phase 2)
If Adenine makes 30% of DNA, what is % of Thymine, Guanine, and Cytosine? (NEET 2021)
Which of the following statements is correct? (NEET 2020)
Answer & NCERT explanation
Correct answer: C — T: 30%, G: 20%
Using Chargaff's rules: A = T and G = C. Given A = 30% and C = 20%, then T = A = 30% and G = C = 20%. Total should be 100% (30% + 30% + 20% + 20% = 100%). This is a fundamental principle of DNA base pairing where purines equal pyrimidines, maintaining the double helix structure.
Read more NCERT concept on the PYQ
The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.
NTA tests whether you know Chargaff's base pairing rules: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds), and purines always pair with pyrimidines. Students often confuse the number of hydrogen bonds (2 vs. 3) or incorrectly pair bases like A with C. The key to getting it right is remembering that A-T has 2 bonds and G-C has 3 bonds, and this pairing maintains uniform helix diameter. This concept appeared 3 times in recent NEETs because it's fundamental to understanding DNA structure and is directly asked in questions about strand complementarity and stability.
What does NCERT say about bases two strands are?
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Through deep analysis of NEET and NTA, 88 of 90 questions from the NEET 2026 paper were matched straight from the MedicNEET Biology question bank.
88/90MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.