Premature stop codon shortens polypeptide

Steps:

• Transcribe template DNA to mRNA by base complement: template C→mRNA G, G→C, A→U, T→A; group into 5'-3' codons.
• Original mRNA codons: GGU (Gly), CCG (Pro), AAU (Asn), ACG (Thr); translates to 4 amino acids, no early stop.
• For each option, transcribe mRNA and identify if a stop codon (UAA/UAG/UGA) appears before the fourth codon.
• Option C mRNA: GAA (Glu), UGG (Trp), CAA (Gln), UAG (stop); halts after 3 amino acids.

Why C is correct:

• UAG is a stop codon that terminates translation after the third codon, per the genetic code.

Why the others are wrong:

• A: Missense mutation (CCG→CGU, Pro→Arg); 4 amino acids.
• B: Missense mutation (CCG→UGC, Pro→Cys); 4 amino acids.
• D: Two missense mutations (GGU→GAA Gly→Glu; CCG→CGC Pro→Arg); 4 amino acids.

Final answer: C