Which option shows a correctly paired DNA double-stranded segment (assume standard Watson-Crick base pairing and antiparallel strands)?

• A. 5′-A G C T-3′ paired with 5′-T C G A-3′
• 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. Identify the directionality of the given DNA strands. DNA strands are antiparallel, meaning one strand runs 5′ to 3′ and the complementary strand runs 3′ to 5′.
2. Recall the standard Watson-Crick base pairing rules: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
3. Check each option to see if the bases on one strand correctly pair with their complementary bases on the other strand according to these rules.
4. Verify that the strands are antiparallel by ensuring that if one strand is 5′ to 3′, the complementary strand should be oriented 3′ to 5′.
5. Select the option where the bases are correctly paired (A with T, G with C) and the strands are antiparallel, which matches the correct DNA double-stranded segment.