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Gene mutationsEdexcel International A Level Biology: Revision notes

Section 1

What is a gene mutation?

A gene mutation is a change in the sequence of bases in DNA. Mutations arise as errors during DNA replication, for example when DNA polymerase adds a wrong nucleotide or a nucleotide is missed or added. They occur spontaneously at a low frequency.

A change in the base sequence can change the sequence of codons in mRNA and so the sequence of amino acids in a polypeptide.

Key termsgene mutationreading frame

Section 2

Substitution mutations

In a substitution, one base is replaced by a different base. At most one codon is changed, so at most one amino acid is altered. There are three possible outcomes:

  • No change to the polypeptide, because the genetic code is degenerate and the new codon codes for the same amino acid
  • A different amino acid is added, which can change the protein's shape and function
  • A stop codon is made, so translation ends early and the polypeptide is shorter
Key termssubstitution
Exam tip

Always check the new codon. The same amino acid means no effect; a stop codon means a shortened polypeptide.

Section 3

Insertion and deletion mutations

In an insertion an extra base is added. In a deletion a base is lost.

If one or two bases are added or lost, the reading frame shifts. This is a frameshift, and every codon after the mutation is changed. The polypeptide is usually very different and non-functional.

If three bases (or a multiple of three) are added or lost, the reading frame is kept, so only one amino acid is added or lost and the rest of the polypeptide is unchanged.

Example: mRNA AUG GCA UUC GAA. Deleting the first G of GCA gives AUG CAU UCG AA..., so every codon after AUG changes.

Key termsinsertiondeletionframeshift
Common mistake

Do not say that a substitution causes a frameshift. A substitution does not change the number of bases, so the reading frame is kept.

Section 4

Effects of mutations

Many mutations have no observable effect because:

  • the code is degenerate, so the amino acid is unchanged
  • the mutation is in non-coding DNA
  • the changed amino acid has similar properties or is not in a key part of the protein, so the shape and function are unchanged

Some mutations cause genetic disorders. In sickle cell anaemia a substitution changes one codon of the haemoglobin gene, so valine replaces glutamic acid and the haemoglobin has an altered shape. In cystic fibrosis a common cause is the deletion of three bases, which removes one amino acid from a membrane protein.

Some mutations cause cancer. A mutation in a gene that controls cell division can make the protein work wrongly, so the cell divides uncontrollably by mitosis and forms a tumour.

Key termsgenetic disordertumour

Must know

  • A mutation is a change in the base sequence, arising from errors in replication
  • Substitution: one base replaced (at most one amino acid changes)
  • Insertion or deletion: a frameshift if not a multiple of three bases, changing all later codons
  • Many mutations have no effect (degenerate code, non-coding DNA, similar amino acid)
  • Some cause genetic disorders (sickle cell anaemia, cystic fibrosis) or cancer

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Gene mutations

  1. In sickle cell anaemia, a change in the gene for the beta chain of haemoglobin alters one codon in the mRNA from GAG, which codes for glutamic acid, to GUG, which codes for valine. All the other codons are unchanged.
    Explain how this change in one codon can alter the properties of the haemoglobin protein.2 marks
  2. The mRNA of a gene has the codon sequence AUG GCA UUC GAA UAA. A mutation deletes the first base of the second codon, the G of GCA, and the rest of the mRNA is unchanged.
    Explain why deleting three consecutive bases from the middle of a gene may be less harmful than deleting a single base.2 marks
  3. In an mRNA, the codon GAA codes for glutamic acid. The codon GAG also codes for glutamic acid, the codon GAU codes for aspartic acid and the codon UAA is a stop codon.
    Describe and explain the effect on the polypeptide of a substitution that changes the codon GAA to (i) GAG, (ii) GAU and (iii) UAA.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).