What are mutations and how do they affect proteins and organisms?
Single gene mutations (substitution, insertion and deletion), the effects of frame-shift and point mutations, chromosome structure mutations (deletion, duplication, translocation and inversion), and how mutations provide the raw material for evolution.
An SQA Higher Biology answer on mutations, covering single gene mutations including substitution, insertion and deletion, the difference between point and frame-shift mutations, the four types of chromosome structure mutation, and the importance of mutations to evolution.
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What this key area is asking
The SQA wants you to describe the types of single gene mutation, explain the difference between point and frame-shift mutations and their effects on the protein, describe the four chromosome structure mutations, and explain why mutations are important as the raw material for evolution.
Single gene mutations
Substitution swaps one base for another. It is a point mutation because it changes at most one codon, so its effect tends to be limited. There are three outcomes:
- Missense - the changed codon codes for a different amino acid, so one amino acid in the protein is altered.
- Nonsense - the changed codon becomes a premature stop codon, so translation ends early and the protein is shortened and usually non-functional.
- Silent - the changed codon still codes for the same amino acid, so the protein is unchanged. This is possible because the genetic code is degenerate: several codons can code for the same amino acid.
Frame-shift mutations
A frame-shift caused by insertion or deletion typically has a far larger impact on the protein than a single substitution. Because the bases are read in non-overlapping triplets, adding or removing a single base pushes all the following bases into new groupings, so almost every amino acid downstream is wrong and the protein rarely works. The only case where a frame-shift has little effect is when the inserted and deleted bases happen to cancel out (for example three bases inserted together, which adds one amino acid without shifting the rest of the frame).
Chromosome structure mutations
These affect a larger section of a chromosome and so usually involve many genes, which is why their effects are often severe:
- Deletion - a section of chromosome is lost, so all the genes in that section are missing.
- Duplication - a section is repeated, giving extra copies of those genes.
- Translocation - a section becomes attached to a different (non-homologous) chromosome.
- Inversion - a section breaks off, turns around and rejoins in reverse order, so the gene order is reversed.
Duplication is especially important in evolution: an extra copy of a gene is free to mutate and take on a new function while the original copy still does its old job.
Why mutations matter
Most mutations are neutral or harmful, but occasionally a mutation is beneficial. Mutations are the only source of new alleles and therefore provide the raw material for evolution, generating the variation on which natural selection acts. The rate of mutation is normally low, but it can be increased by mutagens such as ultraviolet light, other radiation and some chemicals.
Examples in context
Example 1. Cystic fibrosis from a deletion. The most common cause of cystic fibrosis is the deletion of three bases in the CFTR gene, which removes a single amino acid from the CFTR protein. The altered protein cannot fold and transport chloride ions correctly, so thick mucus builds up in the lungs and digestive system. This shows how even a small gene mutation can change a protein's shape and produce a serious genetic condition.
Example 2. Antibiotic resistance as a beneficial mutation. In a population of bacteria, a chance substitution mutation can alter the protein that an antibiotic targets, so the antibiotic no longer binds. In the presence of the antibiotic this mutation is beneficial: the resistant bacteria survive and reproduce, passing on the new allele. This is a clear example of mutation providing the raw material on which natural selection acts.
Try this
Q1. Name the three types of single gene mutation. [1 mark]
- Cue. Substitution, insertion and deletion.
Q2. Explain why an insertion usually has a greater effect on a protein than a substitution. [2 marks]
- Cue. An insertion causes a frame-shift that changes every codon after it, whereas a substitution changes at most one codon.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksExplain the difference between a substitution mutation and a frame-shift mutation, and explain why a frame-shift usually has a greater effect on the protein.Show worked answer →
A 4-mark answer needs both mutation types and a clear reason for the difference in effect.
A substitution swaps one base for another. It is a point mutation, changing at most one codon, so it alters at most one amino acid. It can be missense (different amino acid), nonsense (premature stop codon) or silent (same amino acid).
A frame-shift is caused by an insertion or deletion of a base, which shifts the reading frame so that every codon after the mutation is read differently.
A frame-shift usually has a greater effect because all the amino acids after the mutation are altered, whereas a substitution changes at most one.
Markers reward the definitions of each type, the idea of the reading frame shifting, and the comparison of effects.
SQA Higher 20223 marksName the four types of chromosome structure mutation and state which one moves a section of chromosome onto a different, non-homologous chromosome.Show worked answer →
A 3-mark answer needs the four named types plus the correct identification.
The four chromosome structure mutations are deletion (a section is lost), duplication (a section is repeated), translocation (a section attaches to a different, non-homologous chromosome) and inversion (a section breaks off, turns around and rejoins in reverse order).
The mutation that moves a section onto a different, non-homologous chromosome is translocation.
Markers reward naming all four types and correctly identifying translocation.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)