Define gene mutations and their possible effects
A gene mutation changes the sequence of base pairs in DNA and may alter the polypeptide encoded by the gene.
Section 6.2 · Protein synthesis
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What changes in a gene mutation?
What you will learn
- Define a gene mutation as a change in the DNA base-pair sequence
- Explain why a mutation may, but does not always, alter a polypeptide
Mutations change DNA sequence
A gene mutation is a change in the sequence of base pairs in a DNA molecule. This can change the mRNA codons produced during transcription.
Whether the polypeptide changes depends on where the mutation occurs and how the altered codon is translated. A change can be outside a coding region, can produce a synonymous codon, or can alter an amino acid or stop signal.
Effects depend on the code and position
A mutation may result in an altered polypeptide, but this is not inevitable. The genetic code is degenerate, and a base change may leave the specified amino acid unchanged.
If an amino-acid sequence changes, the polypeptide may fold differently and its properties can be affected. Do not claim that every mutation causes disease or a visible trait change.
Pause and explain
Must a DNA base change alter the polypeptide?
Worked example
A single DNA base pair changes, but transcription and translation still produce the same amino acid at that position. Is the DNA change a mutation, and must the polypeptide change?
Show the worked solution
- A base-pair sequence change is a mutation.
- A synonymous codon can leave the amino acid unchanged.
Answer Yes, the DNA change is a mutation. No, the polypeptide does not have to change: the altered codon may still specify the same amino acid.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define a gene mutation and state one possible consequence.
Give me a hint
Give the DNA-level change before the protein effect.
Compare my reasoning
- A mutation changes the base-pair sequence in DNA.
- It may alter a codon and the polypeptide.
A gene mutation is a change in the sequence of base pairs in DNA. It may alter an mRNA codon and result in an altered polypeptide.
Look for these in your work
- DNA change defined
- Possible polypeptide effect stated
Explain why a mutation can occur without changing an amino acid.
Give me a hint
Use the degeneracy of the genetic code.
Compare my reasoning
- A base change may alter a codon.
- Different codons can specify the same amino acid.
A mutation may change an mRNA codon, but the genetic code is degenerate, so the new codon can specify the same amino acid and leave the polypeptide sequence unchanged.
Look for these in your work
- Codon change considered
- Degeneracy linked to unchanged amino acid
A DNA sequence changes outside the coding region of a gene. Can you conclude that the encoded polypeptide changes? Explain.
Give me a hint
Distinguish a DNA mutation from its possible effect.
Compare my reasoning
- The DNA base-pair sequence has changed.
- A polypeptide change is possible but not guaranteed; the changed sequence may not alter a codon.
The DNA change is a mutation, but the information given does not show that the encoded polypeptide changes. A mutation may have no effect on the amino-acid sequence, depending on its location and the codons produced.
Look for these in your work
- Mutation recognised
- Effect not overstated
Common mistakes
- Defining a mutation as a change in amino-acid sequence rather than DNA sequence
- Assuming every mutation changes a polypeptide
- Assuming every altered polypeptide produces a harmful phenotype
What can you explain now?
What changes in a gene mutation?
Compare with the explanation
The sequence of base pairs in DNA
A mutation changes the DNA sequence.
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Make it stick
What defines a gene mutation, and why can its effect on the polypeptide vary?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.