Point Mutations

Point Mutations

Silent, Missense, Nonsense

One base changes; re-reading the codon table classifies the effect as silent, missense, or nonsense.

highlighted = computed this step

A point mutation changes one base

A point mutation swaps a single base in a codon. Whether the protein changes is not a guess — it is read straight off the codon table by looking up the new codon. The next three steps each change one base of our worked sequence and re-read the result.

codon→one basenew codon⟶table lookup\text{codon} \xrightarrow{\text{one base}} \text{new codon} \longrightarrow \text{table lookup}

Silent: the amino acid is unchanged

Changing the last base of GCA to GCG still codes for alanine — the table gives the same amino acid, so this change is silent. This is the degeneracy of the code at work. Honesty note: "silent" means no amino-acid change, not necessarily no effect — such changes can still alter splicing, mRNA stability, or translation speed.

GCA→GCG: Ala→Ala\text{GCA} \to \text{GCG}:\ \text{Ala} \to \text{Ala}
A point mutationOne base changes; the amino acid is re-read from the codon table.beforeGCAamino acidAafterGCGamino acidA

Missense: a different amino acid

Changing the last base of UGC to UGG swaps cysteine for tryptophan, so the protein's residue changes. The table makes this a missense change.

UGC→UGG: Cys→Trp\text{UGC} \to \text{UGG}:\ \text{Cys} \to \text{Trp}
A point mutationOne base changes; the amino acid is re-read from the codon table.beforeUGCamino acidCafterUGGamino acidW

Nonsense: a premature stop

Changing the first base of AAA to UAA turns lysine into a stop codon, ending translation early and truncating the protein. The table makes this a nonsense change.

AAA→UAA: Lys→Stop\text{AAA} \to \text{UAA}:\ \text{Lys} \to \text{Stop}
A point mutationOne base changes; the amino acid is re-read from the codon table.beforeAAAamino acidKafterUAAamino acid*