The correct base pairing for DNA is

Step 1: Analyze the question You are asked for the correct complementary base pairing in a DNA molecule.

Step 2: The concept DNA is a double helix made of two strands held together by hydrogen bonds between nitrogenous bases. The bases fall into two groups:

• Purines (double-ring): Adenine (A) and Guanine (G)
• Pyrimidines (single-ring): Thymine (T) and Cytosine (C)

Each base pair must join one purine with one pyrimidine, so the width of the helix stays constant. Specific pairing rules apply (Chargaff's rules):

• Adenine pairs with Thymine (A=T), joined by 2 hydrogen bonds
• Guanine pairs with Cytosine (G≡C), joined by 3 hydrogen bonds

Step 3: Check each option

• A. A-T and G-C: Correct, this matches the base pairing rules.
• B. A-G and T-C: Wrong. These pair purine with purine and pyrimidine with pyrimidine, which is not allowed.
• C. A-C and G-T: Wrong. These are purine-pyrimidine pairs, but they are not the correct complementary partners.
• D. A-A and C-C: Wrong. Identical bases do not pair.

Step 4: Answer The correct option is A.

Exam tip: Remember "Apple Tree, Grass Cow": Apple-Tree, Grass-Cow. Also, in RNA, thymine is replaced by uracil, so A pairs with U. JAMB often tests this difference.