Causes of mutation and randomness

Molecules (Continuity and change) · Mutation and gene editing · note 2 of 3

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Causes of mutation and randomnessSpec D1.3.4, D1.3.5

In short

Gene mutations are caused by mutagens and by errors in DNA replication or repair. Chemical mutagens include benzo[a]pyrene in tobacco smoke; mutagenic radiation includes ultraviolet light, X-rays and gamma rays. Mutation is random: it can occur anywhere in the genome, though some bases mutate more often, and no natural mechanism deliberately changes a base to change a trait.

Causes of gene mutation

A mutagen is an agent that increases the rate of mutation.

Causes of gene mutation
Cause	Examples	How it causes mutation
Chemical mutagens	Benzo[a]pyrene and nitrosamines in tobacco smoke; mustard gas	React with bases or bind to DNA, causing bases to pair incorrectly during replication.
Mutagenic radiation	Ultraviolet (UV) light, X-rays, gamma rays, alpha particles from radioactive isotopes such as radon	UV causes adjacent pyrimidine bases (such as two thymines) to bond to each other; high-energy radiation ionises molecules and can break DNA strands.
Errors in DNA replication	A mismatched nucleotide not corrected by proofreading	A wrong base is copied into the new strand and then into every later copy.
Errors in DNA repair	Damaged DNA repaired incorrectly	The repaired region has a changed base sequence.

Randomness in mutation

Mutations are random. They can occur anywhere in the base sequences of a genome, although some bases have a higher probability of mutating than others (for example, a cytosine with a methyl group added, often where C is followed by G, can lose an amino group and become thymine, so C → T substitutions are especially common at these sites). There is no known natural mechanism for making a deliberate change to a particular base with the purpose of changing a trait. An organism cannot produce a mutation because it would be useful.

Common mistake:

Do not write that organisms mutate in order to adapt. Mutations happen at random; natural selection then acts on the variation they produce.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between a substitution and a frameshift mutation?

A substitution replaces one base with another, so only one codon changes and at most one amino acid is altered; it may have no effect because the genetic code is degenerate. A frameshift is caused by inserting or deleting bases that are not a multiple of three, changing every codon afterwards, so the polypeptide usually stops functioning.

What causes gene mutations?

Gene mutations are caused by mutagens and by errors in DNA replication or repair. Chemical mutagens include benzo[a]pyrene and nitrosamines in tobacco smoke. Mutagenic radiation includes ultraviolet light, X-rays and gamma rays. Mutations occur at random anywhere in the genome; no natural mechanism deliberately changes a particular base to change a trait.

Why are mutations important for evolution?

Mutations are important for evolution because gene mutation is the original source of all genetic variation: it is the only process that creates new alleles. Most mutations are harmful or neutral for an individual, but over long periods some new alleles increase survival and reproduction, so natural selection can act on them and species can evolve.