Mutations and their effectsAQA A-Level Biology: Revision notes
Section 1
Gene mutations
A gene mutation is a change in the base sequence of DNA. Gene mutations might arise during DNA replication, for example when a wrong base is inserted or a base is missed. There are six types:
- Substitution: one base is replaced by another.
- Addition: one or more extra bases are inserted.
- Deletion: one or more bases are lost.
- Inversion: a group of bases is cut out and rejoined in the reverse order.
- Duplication: one or more bases are repeated.
- Translocation: a group of bases moves from one location to another, in the same chromosome or a different one.
Section 2
Causes of mutation
Gene mutations occur spontaneously, usually through errors in DNA replication, so every organism has a low natural mutation rate.
The rate of mutation is increased by mutagenic agents, such as ultraviolet light, ionising radiation (X-rays, gamma rays), and chemicals such as those in tobacco smoke.
A mutagenic agent does not choose which gene it changes. It simply raises the probability of mutation anywhere in the genome.
Section 3
Mutations and the amino acid sequence
A mutation in a gene can result in a different amino acid sequence in the encoded polypeptide. The order of bases determines the order of amino acids, so a changed base sequence can change the primary structure. A different amino acid can change the bonds in the protein, so the tertiary structure and the function of the protein may change.
The genetic code is degenerate: most amino acids are coded for by more than one triplet. Therefore some mutations change only one triplet but do not change the amino acid (a silent change), for example GAA and GAG both code for glutamic acid.
Some substitutions change one amino acid (missense). A substitution that creates a stop codon causes translation to end early and gives a shorter polypeptide (nonsense).
In an exam, work through the chain: base change, triplet change, amino acid change, tertiary structure change, function change.
Section 4
Frameshift mutations
The genetic code is read in non-overlapping triplets. An addition or deletion of a number of bases that is not a multiple of three changes all the triplets downstream of the mutation. This is a frameshift.
Example (coding strand): ATG ACC GAA TTC codes for Met-Thr-Glu-Phe.
- Adding a G after ATG gives ATG GAC CGA ATT C, which codes for Met-Asp-Arg-Ile. Every amino acid after the addition is changed.
- Deleting the first A of ACC gives ATG CCG AAT TC, which codes for Met-Pro-Asn.
A frameshift usually has a major effect on the polypeptide. If the number of bases added or deleted is a multiple of three, the frame is kept, and only the amino acids coded by those bases are added or lost.
A substitution is not a frameshift. It changes at most one triplet.
Section 5
Relating the nature of a mutation to its effect
Use the type of mutation to predict its effect:
- Substitution: changes one triplet, so it may have no effect (degenerate code), change one amino acid (missense) or create a stop codon (nonsense).
- Addition/deletion (not a multiple of three): frameshift, so most or all downstream amino acids change; the protein is usually non-functional.
- Addition/deletion of a multiple of three: amino acids added or lost, the rest unchanged.
- Inversion: a group of triplets is reversed, so several amino acids change.
- Duplication: extra bases that either shift the frame or add amino acids.
- Translocation: a gene may be moved next to a different regulatory region, or interrupted, so its expression may change.
The effect on the phenotype depends on whether the changed amino acid alters the tertiary structure, for example the shape of an active site or binding site.
Section 6
Exam approach
Typical tasks: name a mutation from a base sequence, state whether a mutation changes the amino acid, and explain the effect on the protein.
- Write out the triplets before and after the mutation.
- Say whether the frame is shifted.
- State which amino acids change, using the genetic code information given.
- Link to tertiary structure and function.
If a stop codon appears, translation ends early and the polypeptide is shorter. Remember that mRNA has the same sequence as the coding strand, with U in place of T.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Mutations and their effects
- A short section of the coding strand of a gene reads ATG GAA CTT AAG GGC and codes for the amino acids methionine, glutamic acid, leucine, lysine and glycine. A mutation changes the third base of the second triplet from A to G, giving GAG, which also codes for glutamic acid. A different mutation changes the same triplet from GAA to TAA.Explain the effect on the polypeptide of the mutation that changes GAA to TAA.2 marks
- The coding strand of part of a gene reads CAT GAT TCC AAG and codes for four amino acids. A mutation deletes the first G of the second triplet.Explain why a deletion of one base usually has a greater effect on a polypeptide than a substitution of one base.2 marks
- A researcher grew a culture of bacteria and counted the number of cells that had mutated to become resistant to an antibiotic. In an untreated culture there were 3 resistant cells per million cells. In a culture exposed to ultraviolet light there were 450 resistant cells per million cells. The resistance was caused by a single-base change in a gene coding for a bacterial protein to which the antibiotic normally binds.Explain the results of the researcher's experiment.3 marks
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).