If the coding DNA sequence is ACT, what is the corresponding mRNA codon?

If the coding DNA sequence is ACT, what is the corresponding mRNA codon?

• A. ACC
• B. ACU
• C. ACT
• D. ACG

Answer: (B)

Transcription uses the DNA template to synthesize RNA, and RNA swaps thymine for uracil. The coding (sense) DNA strand sequence matches the mRNA sequence except that T becomes U. So for the coding DNA sequence ACT, the mRNA codon is ACU. Reasoning: A stays A, C stays C, but thymine is replaced by uracil in RNA, giving ACU. The other options would imply either keeping thymine or changing to a different base not produced in RNA (ACC keeps T, ACG changes the last base to G, etc.). A quick check is that the template strand would be TGA, and its transcription yields ACU, consistent with the coding strand rule.

Transcription uses the DNA template to synthesize RNA, and RNA swaps thymine for uracil. The coding (sense) DNA strand sequence matches the mRNA sequence except that T becomes U. So for the coding DNA sequence ACT, the mRNA codon is ACU.

Reasoning: A stays A, C stays C, but thymine is replaced by uracil in RNA, giving ACU. The other options would imply either keeping thymine or changing to a different base not produced in RNA (ACC keeps T, ACG changes the last base to G, etc.). A quick check is that the template strand would be TGA, and its transcription yields ACU, consistent with the coding strand rule.