mutagen

A mutagen is any agent from the outside world that raises the rate at which mutations occur — anything that damages DNA or interferes with its copying enough to leave permanent sequence changes behind. Where spontaneous damage is the background hum of chemistry, a mutagen turns up the volume. Mutagens come in two broad kinds: radiation and chemicals.

Radiation mutagens include ultraviolet light from the sun, which fuses neighbouring bases into bulky lesions called thymine dimers, and ionising radiation such as X-rays and gamma rays, which can snap one or both strands of the helix outright. Chemical mutagens work by altering the bases or wedging into the helix. Some, like nitrous acid, deaminate bases; base analogues such as 5-bromouracil masquerade as a normal base but pair sloppily; alkylating agents stick chemical groups onto bases so they mispair; and flat, ring-shaped intercalators like ethidium bromide slide between the stacked base pairs and trick the copying machinery into adding or dropping a base, causing frameshifts.

Mutagens matter because they are the controllable part of the mutation rate. Most are also carcinogens — substances that cause cancer — since cancer is in large part a disease of accumulated mutations, which is why we shield ourselves from UV with sunscreen, limit medical X-ray doses, and screen new chemicals for mutagenic activity. It is worth being precise: a mutagen does not write a chosen mutation; it merely makes random mutations more likely, and the cell's own repair systems still get a chance to undo much of the damage before it becomes permanent.