Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base substitution and base deletion. Because the genetic code is degenerate, not all mutations result in a change to the amino acid sequence. Mutagens increase the rate of mutation, and mutations are one source of genetic diversity within a gene pool.

What this dot point is asking

AQA wants you to define a gene mutation, describe the two named types (substitution and deletion), explain why substitutions are often silent but deletions usually are not, and link mutation (and mutagens) to genetic diversity within a gene pool.

What a gene mutation is

The mutation rate is increased by mutagens - agents such as ultraviolet light, ionising radiation, and chemicals like tar in tobacco smoke.

Types of mutation

Base substitution

One base is replaced by a different base. Only the single codon containing that base is altered. Three outcomes are possible:

• Silent mutation. The new codon still codes for the same amino acid (because the code is degenerate). No change to the polypeptide.
• Missense mutation. The new codon codes for a different amino acid. One amino acid changes, which may or may not affect the protein's shape and function.
• Nonsense mutation. The new codon becomes a stop codon, so the polypeptide is cut short and is usually non-functional.

Base deletion

One base is removed. Because the genetic code is read in non-overlapping triplets from a fixed starting point, removing a base causes a frameshift: every codon after the deletion is shifted and read differently.

Why some mutations have no effect

Three buffering effects explain why mutations are not always harmful:

1. The code is degenerate. A substitution may produce a synonymous codon for the same amino acid, so the amino acid sequence is unchanged.
2. Position matters. A change outside the active site or binding region may not affect function.
3. Introns. A mutation within an intron is removed during splicing and never reaches the polypeptide.

A change to the amino acid sequence may alter the tertiary structure of the protein, because folding depends on the positions of the amino acids and the bonds between their R groups. A changed active site can stop an enzyme working.

Mutation and genetic diversity

Mutation is the ultimate source of all new alleles. Without it, meiosis and sexual reproduction could only reshuffle existing alleles.

• A new allele may be harmful, neutral, or occasionally beneficial.
• A beneficial allele can increase in frequency by natural selection, increasing the genetic diversity of the gene pool and providing the raw material for evolution.

The frequency of an allele in a population, and the range of alleles present, together describe genetic diversity within a gene pool.

Common mistakes

Try this

Q1. Distinguish between a substitution and a deletion mutation. [2 marks]

• Cue. Substitution replaces one base with another (one codon affected); deletion removes a base, causing a frameshift (all later codons affected).

Q2. A spontaneous substitution mutation in a gene had no effect on the organism. Give two reasons why. [2 marks]

• Cue. The new codon coded for the same amino acid (degenerate code), OR the mutation was in an intron and removed by splicing, OR the changed amino acid was not in the active site.

Q3. Explain how mutation increases the genetic diversity of a population. [2 marks]

• Cue. Mutation produces new alleles (new base sequences); a wider range of alleles in the gene pool means greater genetic diversity.