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8.1The Control of Gene Expression (A Level only)

Genetic Mutations (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 34 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

Gene Mutations: Types, Effects and Mutagens — AQA A LEVEL BIOLOGY 7402 notes: Describe the types of gene mutation, relate each to its effect on the polypeptide, and name mutagenic agents.A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result i…

  • Physical mutagens
  • Chemical mutagens
  • Biological mutagens
  • Worked example 1
  • Worked example 2
  • Worked example 3

Part 1 of 5What a gene mutation is

A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide. Mutations occur continuously and spontaneously, and errors in the DNA often occur during DNA replication. They occur randomly, and the rate is estimated at around one mutation per 100 000 genes per generation.

Part 2 of 5Types of gene mutation

TypeWhat happensEffect on polypeptide
SubstitutionOne base replaced by anotherMay change one amino acid (missense), no change because the genetic code is degenerate (silent), or produce a premature stop codon (nonsense)
DeletionA nucleotide (and its base) is randomly deletedChanges the amino acid coded for; causes a frame-shift, changing the groups of three bases further along the sequence, so many amino acids are altered
InsertionAn extra nucleotide is addedLike deletion, changes the amino acid and causes a frame-shift
DuplicationA whole gene or section of a gene is duplicated so two copies appear on the same chromosome; usually occurs during crossing over in meiosisThe original gene remains intact, so the mutation is not harmful; over time the second copy can mutate and develop new functions, so duplications are an important source of evolutionary change
InversionThe DNA of a single gene is cut in two places and the section is reversed and rejoinedThe order of bases in that section is reversed, altering the amino acids it codes for
TranslocationA section of a gene is cut and resealed onto another gene (or chromosome)Can disrupt both genes and alter protein function

You need to be able to relate the nature of a gene mutation to its effect on the encoded polypeptide.

Part 3 of 5Effects of mutations

Mutations occur spontaneously and randomly during DNA replication.

Effects of mutations
Because the DNA base sequence determines the amino acid sequence of a protein, mutations can change the amino acids coded for,
  • and so the tertiary structure and function of the protein.
Most mutations do not alter the polypeptide, or alter it only slightly,
  • so structure and function are unchanged; some are harmful; a few are beneficial.

Part 4 of 5Mutagenic agents

Mutagenic agents are factors that increase the frequency of mutations. Physical mutagens cause mutations by physical means, typically radiation such as ionising radiation (X-rays, gamma rays) and UV. Chemical mutagens interfere chemically with DNA molecules (for example by altering bases). Biological mutagens are organisms or molecules that interfere with DNA, such as viruses (e.g. HPV can lead to cervical cancer).

Part 5 of 5Worked examples

  1. The DNA sequence CAT GGA is changed to CAT GA by a mutation. Name the mutation and explain its effect. [3]3 marks

  2. Explain why a substitution mutation may have no effect on the polypeptide. [2]2 marks

  3. Explain why duplication is important in evolution. [3]3 marks

  4. Name three types of mutagen with an example of each. [3]3 marks

Key terms

TermMeaning
Gene mutationA change in the sequence of base pairs in DNA.
Frame-shiftChange in the reading frame caused by insertion or deletion.
SubstitutionOne base replaced by another.
DuplicationA section of DNA is copied so two copies exist.
TranslocationA section of a gene is moved onto another gene.
Mutagenic agentA factor that increases the mutation rate.

Avoid these mistakes

1
Believing mutations are always harmful.
2
Saying that a substitution always causes a frame-shift.
3
Saying mutagens cause specific mutations — they increase the rate of random mutations.
4
Forgetting that mutations in DNA can only be passed on if they occur in gametes.

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