Biology · Introductory biology · Worked example
Translate a gene and test a mutation
The coding strand of a short gene reads 5′-ATG TTC GGA TGG TAA-3′. Write the template strand and the mRNA, and translate the mRNA. A substitution then changes the G at position 12 to A. What happens to the protein, and what kind of mutation is it?
Write the template strand
Pair each base with its complement, and run the strand the other way: 3′-TAC AAG CCT ACC ATT-5′.
Write the mRNA
The mRNA is complementary to the template, so it matches the coding strand with U for T: 5′-AUG UUC GGA UGG UAA-3′.
Translate from AUG
Read three bases at a time from the start codon: AUG is Met, UUC is Phe, GGA is Gly, UGG is Trp, and UAA is a stop codon. Fifteen bases make five codons: four amino acids and a stop.
Apply the mutation
Position 12 is the third base of the fourth codon, TGG. Changing its G to A gives TGA, so the mRNA codon becomes UGA, a stop codon. Translation now ends after glycine.
Name the mutation
A substitution that turns a codon into a stop codon is a nonsense mutation. Here it removes one amino acid; in a real gene, an early stop usually cuts off much of the protein.
Result
Template 3′-TAC AAG CCT ACC ATT-5′; mRNA 5′-AUG UUC GGA UGG UAA-3′; protein Met–Phe–Gly–Trp. The G-to-A change at position 12 turns UGG into the stop codon UGA, a nonsense mutation, and the protein ends after glycine: Met–Phe–Gly.
Your turn
In the original gene, the C at position 6 is replaced by T. What happens to the protein?
Show the answer and explanation
Nothing changes: UUC becomes UUU, which also codes for phenylalanine.
Phenylalanine has two codons, UUU and UUC, which differ only in the third base, so this is a silent, or synonymous, mutation.
Keep exploring
The gene opens in Sequence & protein with the change at base 12 applied; compare the two proteins, then try other changes.
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