Question:
In a sample of double-stranded DNA, if adenine is 28 percent, what is the amount of cytosine present?
In a sample of double-stranded DNA, if adenine is 28 percent, what is the amount of cytosine present?
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A equals T and G equals C, and all four bases add up to 100 percent.
Updated On: Jun 23, 2026
- 23 percent
- 25 percent
- 46 percent
- 22 percent
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The Correct Option is D
Solution and Explanation
Step 1: Apply Chargaff's rule for double-stranded DNA: A equals T and G equals C, and the four bases sum to 100 percent.
Step 2: Adenine is 28 percent, so thymine is also 28 percent. Together A plus T equals \(28 + 28 = 56\) percent.
Step 3: The remaining \(100 - 56 = 44\) percent is shared equally between guanine and cytosine. So cytosine is \(44 / 2 = 22\) percent. Option (d) is correct.
Note: A-T pairs share two hydrogen bonds and G-C pairs share three, which stabilizes the double helix along with base stacking.
Ref: Lippincott's Illustrated Reviews Biochemistry, 5e, Structure of DNA, p 397.
Step 2: Adenine is 28 percent, so thymine is also 28 percent. Together A plus T equals \(28 + 28 = 56\) percent.
Step 3: The remaining \(100 - 56 = 44\) percent is shared equally between guanine and cytosine. So cytosine is \(44 / 2 = 22\) percent. Option (d) is correct.
Note: A-T pairs share two hydrogen bonds and G-C pairs share three, which stabilizes the double helix along with base stacking.
Ref: Lippincott's Illustrated Reviews Biochemistry, 5e, Structure of DNA, p 397.
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