6.7 Mutations

Changes in DNA

A mutation is a change in the DNA sequence. Point mutations change a single nucleotide (a substitution): it may be silent (no change to the protein), missense (one amino acid changes), or nonsense (creates an early stop codon). Insertions and deletions add or remove bases.

Frameshifts and effects

Inserting or deleting bases (not in multiples of three) causes a frameshift, shifting how every later codon is read — usually very disruptive. Mutations can be harmful, neutral, or occasionally beneficial. Germ-line mutations (in gametes) are heritable; somatic mutations are not. Mutations are the ultimate source of new genetic variation.

Key terms

Mutation

A change in the DNA sequence; the ultimate source of new genetic variation.

Point mutation (substitution)

A change to a single nucleotide; can be silent, missense, or nonsense.

Frameshift

An insertion or deletion that shifts the reading frame of every later codon.

Silent / missense / nonsense

No change / one amino acid changed / a premature stop codon.

Germ-line vs. somatic

Germ-line mutations are heritable; somatic mutations affect only the individual.

Try an AP-style question

What type of mutation is this, and what is its likely effect?

1. A nonsense mutation that changes only the first codon
2. A missense mutation that changes only one amino acid
3. A silent mutation with no effect on the protein
4. A frameshift that changes every codon after the deletion