In double-stranded DNA, adenine (A) forms complementary base pairs with which nitrogenous base?

• A. Uracil (U)
• B. Cytosine (C)
• C. Thymine (T)
• D. Guanine (G)

Answer: (C)

Verified step by step guidance

1. Recall the base pairing rules in double-stranded DNA, which are based on complementary nitrogenous bases pairing via hydrogen bonds.
2. Identify that adenine (A) is a purine base, and in DNA, it pairs specifically with a pyrimidine base to maintain the uniform width of the DNA double helix.
3. Remember that in DNA, adenine pairs with thymine (T), forming two hydrogen bonds, which is a key feature distinguishing DNA from RNA.
4. Note that uracil (U) replaces thymine in RNA, so adenine pairs with uracil only in RNA, not in DNA.
5. Conclude that in double-stranded DNA, adenine (A) forms complementary base pairs with thymine (T).