Chargaff's Rules & Base Pairing — NEET Biology
📚 Practice ConceptIf a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.
Which of the above statements regarding DNA structure and base pairing are correct? S1: Guanine and Cytosine form three hydrogen bonds, making their interaction stronger than Adenine-Thymine pairing. S2: The complementary base pairing rules ensure that a purine always pairs with a pyrimidine, maintaining a uniform diameter of the DNA double helix. S3: Base stacking interactions between adjacent base pairs also contribute significantly to the stability of the double helical DNA structure. S4: The strength of hydrogen bonds between base pairs is the sole factor conferring stability to the DNA double helix. S5: In a double-stranded DNA molecule, the ratio of purines to pyrimidines is always one, according to Chargaff’s rule.
View solution & NCERT explanation
Correct answer: B — S1, S2, S3, S5
The core concept relates to the structure and stability of the DNA double helix, specifically focusing on hydrogen bonding and base pairing rules. S1 is correct: The NCERT states, 'Guanine is bonded with Cytosine with three H-bonds.' Adenine forms two hydrogen bonds with Thymine, so G-C interaction is indeed stronger. S2 is correct: As per NCERT, 'As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix'. This ensures a uniform diameter. S3 is correct: The NCERT mentions, 'The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability of the helical structure'. Thus, base stacking is a significant contributor to stability. S4 is incorrect: Stability is conferred by both hydrogen bonding and base stacking, not solely by hydrogen bonds. The statement 'sole factor' makes it false. S5 is correct: Chargaff's rule states that for a double-stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one (A=T, G=C). This implies that the total amount of purines (A+G) equals the total amount of pyrimidines (T+C), so their ratio is one. Based on the analysis, S1, S2, S3, and S5 are correct.
What does NCERT say about If double stranded DNA?
Has this concept appeared in NEET?
Which chapter is this from?
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