The mutation rate is increased by mutagenic agents.

The idea

Mutagenic agents (mutagens) increase the mutation rate above the natural spontaneous rate. They include high-energy ionising radiation, such as X-rays and gamma rays, which break DNA strands and disrupt the base sequence. Ultraviolet light causes adjacent thymine bases to bond into dimers, distorting the DNA strand and making replication error-prone. Chemical mutagens act via various mechanisms. For example, base analogues like 5-bromouracil resemble normal bases and are incorporated during replication, but pair incorrectly to cause substitutions. Other chemicals modify bases to alter pairing, while intercalating agents insert between bases, causing additions or deletions that shift the reading frame.

What earns the marks

• Define a mutagenic agent as a factor that increases the rate of mutation above the natural or spontaneous rate.
• Identify forms of ionising radiation (e.g., X-rays, gamma rays) or ultraviolet light as physical mutagens.
• Describe the action of a chemical mutagen, such as a base analogue being incorporated into DNA and pairing incorrectly.
• Clarify that while mutagens increase the frequency of mutations, the specific mutations produced remain random in type and position.
• Link an increased mutation rate in somatic cells to an increased risk of cancer via mutations in tumour suppressor genes or proto-oncogenes.

Common mistakes and how to put them right

• Stating that a mutagen causes one specific, targeted mutation, rather than raising the probability of random changes.

• Using 'mutagen' and 'carcinogen' interchangeably without explaining the link through tumour suppressor genes or proto-oncogenes.

• Saying ultraviolet light 'burns' the DNA rather than describing the formation of thymine dimers or a distorted strand.

• Claiming every mutagen-induced mutation is inherited.

  Put it right: Only mutations in gamete-forming cells can be passed to offspring.

Exam tips

• When explaining how a mutagen increases the mutation rate, focus on the base-level mechanism: incorrect pairing, broken strands, or inserted bases.
• Distinguish somatic mutations from gamete mutations whenever a question mentions inheritance or offspring.
• If the stem introduces an unfamiliar chemical mutagen, describe the mechanism provided in the text rather than naming a learned chemical.