AP® Biology · Unit 6 · Topic 6.7
Point, Frameshift, Nonsense, and Silent Mutations
A mutation changes DNA bases, but its effect on the protein depends on which bases change and whether the reading frame shifts. One swapped base can do nothing, swap one amino acid, create a stop signal, or wreck everything downstream.
The genetic code is read in three-base codons, and that structure explains why mutations differ so wildly in consequence. A point mutation swaps one base for another without changing the total length of the sequence. Because the code is redundant (several codons can specify the same amino acid), swapping a base sometimes lands on a synonymous codon: the amino acid stays the same, and the protein is unaffected. That is a silent mutation. If the swap changes the codon to a different amino acid, it is a missense mutation, and the protein has one amino acid substituted, which may or may not disrupt its shape and function. If the swap changes the codon into a stop codon, it is a nonsense mutation, and translation halts early, producing a truncated, usually nonfunctional protein.
Frameshift mutations work differently because they insert or delete a number of bases that is not a multiple of three. Since ribosomes read mRNA in consecutive, non-overlapping triplets starting from the start codon, removing even one base shifts every codon boundary downstream by one position. From that point on, the ribosome reads a completely different set of triplets, so every amino acid after the mutation site changes, and a premature stop codon often appears in the new frame. This is why frameshifts are usually far more damaging than point mutations: a point mutation affects at most one amino acid, while a frameshift corrupts the entire downstream sequence.
This topic connects forward to how cells regulate and tolerate mutations: not every change reaches a coding region, not every coding change lands on a critical amino acid, and backup codons and repair mechanisms absorb much of the damage before it matters.
Key terms
- point mutation
- A change of a single base in the DNA sequence, which alters at most one codon.
- silent mutation
- A base change that produces a codon coding for the same amino acid, so the protein is unchanged.
- missense mutation
- A base change that causes one codon to specify a different amino acid.
- nonsense mutation
- A base change that converts a codon into a premature stop codon, truncating the protein.
- frameshift mutation
- An insertion or deletion of bases not divisible by three that shifts the triplet reading frame for all downstream codons.
Watch it explained
- How it worksHow does deleting a single base scramble every codon after the deletion?
- How it worksWalk me through a point mutation turning a codon into a premature stop.
- Worked exampleFind what changes in the peptide when GAG becomes GTG in the beta globin gene.
- Worked exampleFind the protein change when one base is deleted from AUG CCU GAA UGG.
- Common mistakeWhy isn't every mutation harmful to the organism carrying it?
Check yourself
End of topic.