Chargaff’s Rules Application A double-stranded DNA sample has AT content of 40% (i.e., A% + T% = 40). What is the percentage of guanine (G) residue?
Biochemistry
DNA Structure and Replication
Difficulty: Easy
Choose an option
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A60%
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B15%
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C30%
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DGuanine percentage cannot be calculated from the given information
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E20%
Answer
Correct Answer: 30%
Explanation
Introduction / Context:Chargaff’s rules state that in double-stranded DNA, A% ≈ T% and G% ≈ C%, and the total of A + T + G + C = 100%. Applying these rules allows inference of base percentages from limited information.
Given Data / Assumptions:
- A% + T% = 40%.
- Therefore, G% + C% = 60%.
- Chargaff pairing implies G% = C%.
Concept / Approach:Let G% = C% = x. Then G% + C% = 2x = 60%, so x = 30%. Hence guanine = 30% and cytosine = 30%.
Step-by-Step Solution:
Step 1: Compute GC fraction: 100 − 40 = 60%.Step 2: Set G = C; thus 2G = 60 → G = 30%.Step 3: Answer: G = 30%.Verification / Alternative check:Check totals: A + T = 40; G + C = 60; sum = 100, consistent with a valid composition.
Why Other Options Are Wrong:
- 60%: that is GC combined, not G alone.
- 15% or 20%: inconsistent with G = C and GC = 60%.
- Cannot be calculated: it can, using Chargaff’s rules.
Common Pitfalls:Forgetting that G equals C; mixing up total GC with individual G or C percentages.
Final Answer:30%