Which purine base forms 3 hydrogen bonds when binding its complementary nucleotide base?

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

Answer: (A)

Verified step by step guidance

1. Recall that DNA base pairing involves purines pairing with pyrimidines through hydrogen bonds.
2. Identify the purine bases: Adenine (A) and Guanine (G). The pyrimidine bases are Cytosine (C) and Thymine (T).
3. Remember the specific base pairing rules: Adenine pairs with Thymine, and Guanine pairs with Cytosine.
4. Understand that the number of hydrogen bonds differs between these pairs: A-T pairs form 2 hydrogen bonds, while G-C pairs form 3 hydrogen bonds.
5. Since Guanine is a purine that pairs with Cytosine via 3 hydrogen bonds, the purine base forming 3 hydrogen bonds is Guanine.