An mRNA codon chart is shown below.
Suppose a DNA triplet has the sequence 3′-ATG-5′. Which of these three possible mutations of the triplet will stop translation?
3′-ATC-5′
3′-ACG-5′
3′-ATT-5′
The DNA triplet 3′-ATG-5′ corresponds to the mRNA codon AUG, which codes for the amino acid methionine and serves as the start codon in translation. In order for translation to stop, a stop codon must be present in the mRNA sequence. The three possible mutations of the triplet are 3′-ATC-5′, 3′-ACG-5′, and 3′-ATT-5′.
Out of these three mutations, only the mutation 3′-ATT-5′ will stop translation. The triplet ATT corresponds to the mRNA stop codon UAA. The other two mutations, ATC and ACG, code for different amino acids but do not function as stop codons.