OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(a) types of gene mutations and their possible effects on protein…
Full specification wording
Cellular control: (a) types of gene mutations and their possible effects on protein production and function
Gene mutations are changes in the base sequence of DNA.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gene mutations are changes in the base sequence of DNA. You need to classify the main types and explain how each can affect the protein produced. Substitution replaces one base with another; this may be silent (because the genetic code is degenerate, the same amino acid is coded), missense (a different amino acid is inserted) or nonsense (a stop codon is created, truncating the polypeptide). Insertion or deletion of bases can cause a frameshift, shifting every subsequent codon and usually producing a very different, often non-functional protein. Mutations may occur in coding or non-coding regions and can affect protein production by altering transcription or splicing.
What earns the marks
- Defines a gene mutation as a change in the base sequence of DNA and distinguishes substitution from insertion and deletion.
- Explains that substitution may be silent because the code is degenerate, or may be missense or nonsense, changing one amino acid or creating a stop codon.
- Explains that insertion or deletion of bases can cause a frameshift, altering all subsequent codons and usually the whole polypeptide.
- Links a change in amino acid sequence to altered tertiary structure and therefore altered or lost protein function.
- Recognises that mutations in non-coding regions can affect protein production, for example by altering transcription or splicing.
Common mistakes and how to put them right
Claiming every substitution changes the protein.
Put it right: The code is degenerate, so some substitutions are silent and code for the same amino acid.
Confusing a nonsense mutation with a missense mutation.
Put it right: Nonsense creates a premature stop codon whereas missense inserts a different amino acid.
Assuming a frameshift changes only one codon.
Put it right: It shifts the reading frame so all downstream codons are altered.
Exam tips
- Name the mutation type precisely (substitution, insertion, deletion) and state whether it is silent, missense or nonsense.
- Always connect the base change to the amino acid sequence, then to tertiary structure, then to protein function.
- Use the degeneracy of the genetic code to explain why some mutations have no effect on the protein.
Try the lesson
The start of the Cellular control: (a) types of gene mutations and their possible effects on protein production and function lesson, made from checked teaching notes for this specification point.
Have a go first
A mutation changes a single DNA base so that one codon becomes a different codon coding for a different amino acid. Which type of mutation is this?
Listen
Gene mutations are changes in the base sequence of DNA. You need to classify the main types and explain how each can affect the protein produced. Substitution replaces one base with another; this may be silent (because the genetic code is degenerate, the same amino acid is coded), missense (a different amino acid is inserted) or nonsense (a stop codon is created, truncating the polypeptide). Insertion or deletion of bases can cause a frameshift, shifting every subsequent codon and usually producing a very different, often non-functional protein. Mutations may occur in coding or non-coding regions and can affect protein production by altering transcription or splicing. A strong answer earns marks for these points. Defines a gene mutation as a change in the base sequence of DNA and distinguishes substitution from insertion and deletion. Explains that substitution may be silent because the code is degenerate, or may be missense or nonsense, changing one amino acid or creating a stop codon. Explains that insertion or deletion of bases can cause a frameshift, altering all subsequent codons and usually the whole polypeptide. Links a change in amino acid sequence to altered tertiary structure and therefore altered or lost protein function. A common slip: Claiming every substitution changes the protein. Instead, the code is degenerate, so some substitutions are silent and code for the same amino acid. Exam tip: Name the mutation type precisely (substitution, insertion, deletion) and state whether it is silent, missense or nonsense.
Key words
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Types of gene mutation
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Degeneracy of the genetic code
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Frameshift and its consequences
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Effect of mutations on protein function and production
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See every definition
- Types of gene mutation
- Gene mutations are changes in the base sequence of DNA, classified as substitution, insertion or deletion, and as silent, missense or nonsense.
- Degeneracy of the genetic code
- Because the genetic code is degenerate, a substitution may be silent and code for the same amino acid.
- Frameshift and its consequences
- Insertion or deletion of bases can cause a frameshift, shifting every subsequent codon and usually producing a very different, often non-functional protein.
- Effect of mutations on protein function and production
- A change in amino acid sequence alters tertiary structure and therefore protein function, while mutations in non-coding regions can affect protein production by altering transcription or splicing.
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