A nitrogenous base found in both DNA and RNA; pairs only with cytosine.

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Multiple Choice

A nitrogenous base found in both DNA and RNA; pairs only with cytosine.

Explanation:
Complementary base pairing in nucleic acids depends on specific hydrogen bonding that keeps the double-helix width uniform. Guanine is present in both DNA and RNA, and it pairs with cytosine through three hydrogen bonds, making it the partner for cytosine. Adenine pairs with thymine in DNA or uracil in RNA, not with cytosine, so it’s not the right match. Thymine is found only in DNA (RNA uses uracil), and cytosine pairs with guanine rather than with itself. So the base that both appears in DNA and RNA and partners with cytosine is guanine.

Complementary base pairing in nucleic acids depends on specific hydrogen bonding that keeps the double-helix width uniform. Guanine is present in both DNA and RNA, and it pairs with cytosine through three hydrogen bonds, making it the partner for cytosine. Adenine pairs with thymine in DNA or uracil in RNA, not with cytosine, so it’s not the right match. Thymine is found only in DNA (RNA uses uracil), and cytosine pairs with guanine rather than with itself. So the base that both appears in DNA and RNA and partners with cytosine is guanine.

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