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?

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:

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%