Thymine makes up 28% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be guanine?单项选择题
A
72%
B
22%
C
16%
D
8%

Question

Thymine makes up 28% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be guanine?

BlackTom AI · Accepted Answer

In DNA, bases pair specifically: adenine pairs with thymine (A with T) and guanine pairs with cytosine (G with C). This means A and T are present in equal amounts, and G and C are present in equal amounts.

Option 1: 72% — If thymine is 28%, adenine must also be 28%. That would total 56% for A and T together. The remaining percentage for G and C would be 44%, which would split as 22% each if G pairs with C. A value of 72% would violate the A=T pairing and leave insufficient room for the other bases, so this option is inconsistent with base-pairing rules.

Option 2: 22% — This choice fits the logic: after assigning 28% to thymine, adenine is also 28%, leaving 44% for the two purine/pyrimidine pairs G and C combined; since G equals C, each would be 22%. This aligns with Chargaff’s rules of base pairing.

Option 3: 16% — If G were 16%, then C would also be 16%, totaling 32% for G+C together. That would leave 68% for A+T, which contradicts the established 28% T (and thus 28% A) requirement, making this option inconsistent with the pairing rule.

Option 4: 8% — An 8% fraction for guanine would force a matching 8% for cytosine, summing to only 16% for G+C. That leaves 84% for A+T, which cannot accommodate T at 28% plus the required A of 28% without exceeding the total, violating base-pairing balance.

To determine the correct value, apply base-pairing ratios: T equals A, G equals C, and A+T together equal 2A (or 2T). With T = 28%, A = 28%, leaving 44% to be split equally between G and C, yielding 22% for each. The option that matches this result corresponds to the standard Chargaff’s rule outcome.

Thymine makes up 28% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be guanine?单项选择题

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