A mutation changes DNA sequence. Substitutions replace bases; insertions and deletions change length. A coding insertion or deletion not divisible by three can shift the reading frame. A substitution can be synonymous, change an amino acid or create a stop codon. Larger mutations affect chromosome segments or chromosome number.
Replication errors, ionising radiation, certain chemicals and some viruses can cause or increase mutations. Somatic mutations affect a body-cell lineage; germline mutations can be inherited if an altered gamete forms an offspring. Effects depend on sequence position, gene function and environment. For sequence drills, identify the exact change, preserve the stated frame and use the supplied codon meanings before classifying the consequence.
Example 1
Coding DNA changes from GAA to GAG. Both mRNA codons specify glutamate. What follows?
This substitution is synonymous for the encoded amino acid in the supplied code. It does not necessarily mean every regulatory effect is excluded.
Example 2
One base is deleted near the start of a coding region. Why can many amino acids change?
The downstream triplet grouping shifts. This frameshift can change many codons and may create an early stop.
Watch out: Mutations do not anticipate need. Selection can subsequently change the frequency of variants.