In double-stranded DNA, guanine always forms hydrogen bonds with which base?

• A. Adenine
• B. Cytosine
• C. Thymine
• D. Uracil

Answer: (B)

Verified step by step guidance

1. Recall the base pairing rules in double-stranded DNA, which are based on complementary hydrogen bonding between specific nitrogenous bases.
2. Identify the purines and pyrimidines: Adenine (A) and Guanine (G) are purines, while Cytosine (C), Thymine (T), and Uracil (U) are pyrimidines.
3. Remember that in DNA, Adenine pairs with Thymine via two hydrogen bonds, and Guanine pairs with Cytosine via three hydrogen bonds.
4. Note that Uracil is not found in DNA; it replaces Thymine in RNA, so it does not pair with Guanine in DNA.
5. Conclude that Guanine always forms hydrogen bonds with Cytosine in double-stranded DNA.