Given below is the DNA coding sequence. Its complementary strand would read as:
5′-GTATTACG-3′
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3′-CATAATGC-5′
The two strands of DNA follow two rules. Base pairing: A pairs with T, and G pairs with C. DNA has thymine (T), never uracil (U). Anti-parallel polarity: if one strand runs 5′ → 3′, the other runs 3′ → 5′.
Write each partner base under the given strand:
5′-G T A T T A C G-3′
3′-C A T A A T G C-5′
Option (1) 5′-CUTUUTGC-3′ — Incorrect. It has uracil, which DNA does not have. Its bases also do not pair with the given strand: under G T A T the partner strand needs C A T A.
Option (2) 3′-CATAATGC-5′ — Correct. Every base is the right partner (G–C, T–A, A–T, C–G), and the strand runs 3′ to 5′, opposite to the given strand.
Option (3) 3′-CUTUUTGC-5′ — Incorrect. The ends are right, but U and T are mixed up and wrongly placed. A pairs with T in DNA, and the given T bases need A as partners, not U or T.
Option (4) 5′-CTUTTUGC-3′ — Incorrect. It also contains uracil, and its polarity runs the same way as the given strand, so it cannot pair with it.