In double-stranded DNA, guanine (G) always base-pairs with which nitrogenous base?

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

Answer: (A)

Verified step by step guidance

1. Recall the base-pairing rules in double-stranded DNA, which are based on complementary nitrogenous bases.
2. Understand that in DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
3. Note that uracil (U) is found in RNA, not DNA, so it does not pair with guanine in DNA.
4. Therefore, guanine (G) always base-pairs with cytosine (C) in double-stranded DNA.
5. This complementary base pairing is due to hydrogen bonding: guanine forms three hydrogen bonds with cytosine, stabilizing the DNA structure.