A **mutation** is any change to the DNA base sequence. It can come from errors in **replication or cell division**, or from **mutagens** — ionising radiation, mutagenic chemicals, and some viruses. Whether it matters depends on *what* changes and *where*. ## Point mutations (one base) A **point mutation** swaps a single base. Because the code is read in triplets, the effect varies: - **Silent** — the new codon still codes for the same amino acid (the code is *redundant*), so the protein is unchanged. - **Missense** — the codon codes for a *different* amino acid (sickle-cell anaemia is the classic case). - **Nonsense** — the codon becomes a *stop* codon, cutting the protein short. ## Frameshift mutations (insertions & deletions) Add or remove a base and every codon **after** that point shifts along by one. The whole downstream reading frame is scrambled — usually far more damaging than a point mutation. ## Germline vs somatic — and why it matters - A mutation in a **somatic** (body) cell affects only that individual (e.g. a skin cancer). - A mutation in a **germ** (sex) cell can be **inherited**, changing the characteristics of descendants. Inheritable mutations are the ultimate **source of new alleles** — the raw material that variation and evolution act on. Most are harmful or neutral, but a rare beneficial one can spread through a population.

Mutations