Q.31 If a double-stranded DNA has 30% Guanine, what will be the percentage of Adenine?
1.40%
2.20%
3.30%
4.15%
Double-stranded DNA follows Chargaff’s rules, where guanine pairs with cytosine, so 30% G means 30% C, leaving 40% for adenine-thymine pairs (with A equaling T).
In double-stranded DNA, nitrogenous bases pair specifically: adenine (A) with thymine (T) via two hydrogen bonds, and guanine (G) with cytosine (C) via three. Chargaff’s rules state that %A = %T and %G = %C, with total purines (A+G) equaling pyrimidines (C+T). For 30% G, C is also 30%, so A+T = 100% – 60% = 40%, and thus A = 20%? No—wait, error in prior logic; actually, with G=30%, C=30%, remaining 40% splits equally to A=20% and T=20%.
Correct Answer
Option 2: 20%.
G=30% pairs with C=30%, leaving 40% for A+T pairs; since A=T, A=20%.
Option Explanations
| Option | Percentage | Explanation |
|---|---|---|
| 1 | 40% | Incorrect. This assumes G+C=30%, but 30% G means C=30% too (60% total), leaving 40% for A+T, so A=20% max—not 40%. |
| 2 | 20% | Correct. Chargaff’s first rule: %G=%C=30%, so %A=%T=20% to total 100%; base pairing ensures equality in dsDNA. |
| 3 | 30% | Incorrect. A cannot equal G here; A pairs with T, independent of G-C pairs, leading to 20% A specifically. |
| 4 | 15% | Incorrect. No basis in pairing rules; halving 30% ignores equal A=T requirement after accounting for G=C=60%. |
Exam Tip
Use formula: %A = 100% – 2*(%G). For genetics MCQs, confirm dsDNA context to apply Chargaff’s rules directly.
