Spontaneous versus induced
A [[spontaneous-mutation|spontaneous mutation]] arises from the cell’s own ordinary workings: a copying slip during DNA replication, a base that chemically rearranges on its own, or a letter that simply falls off the backbone. These happen even in a perfectly clean environment — they are the baseline hum of being alive.
An induced mutation is caused by a [[mutagen|mutagen]] — an external agent that damages DNA or raises the error rate. Common examples include ultraviolet light (which fuses neighboring bases), certain chemicals in tobacco smoke, and ionizing radiation. Mutagens do not create new categories of mutation; they simply make the ordinary kinds happen more often.
The repair crew
Cells are not passive. [[gen-dna-repair|DNA repair]] systems constantly scan the genome and fix the great majority of damage before it ever becomes a permanent mutation. Several specialized pathways handle different kinds of damage. The cleverest exploit the double-stranded design: because the two strands are complementary, the cell can usually rebuild a damaged strand using the intact one as a guide.
- Proofreading: the copying enzyme itself catches most wrong bases as it works and backs up to replace them — the first line of defense during replication.
- Excision repair: patrol proteins cut out a damaged base or a short damaged patch, then resynthesize the gap from the complementary strand.
- Mismatch repair: right after replication, a system spots base pairs that do not match, removes the newly made error, and recopies it correctly.
Double-strand breaks and a fateful choice
The most dangerous damage is a [[double-strand-break|double-strand break]], where both strands snap at once — there is no intact strand left as a template. Cells repair it two ways. [[homologous-recombination|Homologous recombination]] uses an identical sister copy as a guide and is accurate but only available after replication. [[non-homologous-end-joining|Non-homologous end joining]] simply glues the broken ends back together; it works anytime but can lose or add a few bases at the join, sometimes creating a small mutation.