What is Mutagen? — Biology

Definition 13.5 High School Biology · Chapter 13 — Mutations and Genetic Variation

A mutagen is a physical or chemical agent that raises the rate of mutation by damaging DNA. Ultraviolet light welds neighbouring thymines together on a strand; X-rays and radioactivity break the strands; chemicals such as the benzopyrene of tobacco smoke or the aflatoxin of mouldy grain attach to bases and distort the helix. A cell exposed to a mutagen carries induced mutations in addition to its spontaneous ones — still at random positions, only more of them.

Survival of yeast cells against ultraviolet exposure. Normal cells lose half their number per 10\, s; a strain unable to repair ultraviolet damage loses nine tenths per 10\, s. The difference is the work of repair. — Survival of yeast cells against ultraviolet exposure. Normal cells lose half their number per $10\,\mathrm{s}$ ; a strain unable to repair ultraviolet damage loses nine tenths per $10\,\mathrm{s}$ . The difference is the work of repair.

Two plates seeded with the same bacteria. Left: no exposure, a continuous lawn. Right: after ultraviolet exposure, only the cells whose DNA damage was repaired in time have grown into colonies.

Examples

Example 13.6 (Ultraviolet on yeast)

Yeast cells spread on plates and exposed to ultraviolet light for increasing times: after $10\,\mathrm{s}$ half the cells fail to form a colony, after $20\,\mathrm{s}$ a quarter, after $30\,\mathrm{s}$ an eighth — each dose halves the survivors. Among the survivors, the proportion of colonies with a visibly changed trait (colour, shape, inability to grow on a certain sugar) rises from one in a million to one in a thousand. The lamp kills by damaging DNA beyond repair, and mutates by damaging it just short of that.

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