DNA Base Pairing Rules

Steps:

• Recall that DNA bases pair specifically: adenine (A) with thymine (T), guanine (G) with cytosine (C).
• Note that A-T pairs form two hydrogen bonds, while G-C pairs form three.
• Identify the pair with two hydrogen bonds as A-T.
• Confirm the order in the choices matches the standard A-T designation.

Why A is correct:

• Adenine and thymine pair via two hydrogen bonds, as defined by Watson-Crick base pairing in DNA.

Why the others are wrong:

• B: Cytosine-guanine is a G-C pair, which forms three hydrogen bonds.
• C: Guanine-cytosine is a G-C pair, which forms three hydrogen bonds.
• D: Thymine-adenine reverses the standard A-T order, not matching the diagram's identification.

Final answer: A