DNA Base Pairing Hydrogen Bonds

Steps:

• Adenine (A) complements thymine (T) in DNA.
• A-T pairing involves two hydrogen bonds between their nitrogen and oxygen atoms.
• Cytosine (C) complements guanine (G) in DNA.
• C-G pairing involves three hydrogen bonds between their nitrogen and oxygen/amino groups.

Why B is correct:

• DNA base pairing rules specify A-T forms 2 hydrogen bonds and C-G forms 3, as defined by Watson-Crick model.

Why the others are wrong:

• A: Cytosine-Guanine forms 3 bonds, not 2.
• C: Adenine-Thymine forms 2 bonds, not 3.
• D: Both pairings are miscounted; A-T is 2, not 3.

Final answer: B