Which option shows a correctly paired DNA double-stranded segment according to Watson-Crick base-pairing rules?

• A. 5'–A G C T–3' paired with 3'–U C G A–5'
• B. 5'–A G C T–3' paired with 3'–T G C A–5'
• C. 5'–A G C T–3' paired with 3'–A G C T–5'
• D. 5'–A G C T–3' paired with 3'–T C G A–5'

Answer: (D)

Verified step by step guidance

1. Recall the Watson-Crick base-pairing rules for DNA: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
2. Identify the sequence of the first strand given as 5'–A G C T–3'. Write down the complementary bases for each nucleotide according to the base-pairing rules.
3. Remember that the complementary strand runs in the opposite direction (antiparallel), so the complementary strand should be written 3' to 5'.
4. Pair each base from the first strand with its complement: A pairs with T, G pairs with C, C pairs with G, and T pairs with A, resulting in the complementary strand 3'–T C G A–5'.
5. Compare the options given to find the one that matches the correctly paired complementary strand 3'–T C G A–5' to the original 5'–A G C T–3' strand.