If a DNA molecule contains 20% thymine, what percent would cytosine contain?

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Multiple Choice

If a DNA molecule contains 20% thymine, what percent would cytosine contain?

Explanation:
In double-stranded DNA, base-pairing rules require that complementary bases occur in equal amounts: A pairs with T, and G pairs with C. If thymine is 20%, adenine must also be 20%, leaving 60% for the two purines/pyrimidines that pair as guanine and cytosine. Since G and C occur in equal amounts, each must be 30%. Therefore, cytosine is 30%. The other options would violate either the equal pairing of G with C or the total 100% of bases.

In double-stranded DNA, base-pairing rules require that complementary bases occur in equal amounts: A pairs with T, and G pairs with C. If thymine is 20%, adenine must also be 20%, leaving 60% for the two purines/pyrimidines that pair as guanine and cytosine. Since G and C occur in equal amounts, each must be 30%. Therefore, cytosine is 30%. The other options would violate either the equal pairing of G with C or the total 100% of bases.

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