Mutations - TCE Biology (Tasmania)
“Mutations: models of how mutations in genes and chromosomes result from errors in DNA (point and block mutations, including substitutions, deletions, additions and inversions), errors in cell division (types of chromosomal mutation) and damage by mutagens (physical, UV radiation, ionising radiation, heat and chemical); the impact of mutation on the final gene product, specifically protein”
A mutation is a change in the DNA base sequence. Point mutations change one base or a few bases: substitution (one base swapped for another), deletion (a base removed), addition (a base inserted) and inversion (a section reversed). Chromosomal mutations change the structure or number of chromosomes, often through errors in cell division. Mutagens such as UV radiation, ionising radiation, heat and some chemicals increase the mutation rate. The effect on the protein ranges from none (a silent substitution) to a completely non-functional protein (a frameshift or early stop codon).
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What this dot point is asking
Mutations are examined under criterion 5 (Section B of the TASC exam), with the causes and impacts of mutation named in the external assessment specifications. Mutation also matters for criterion 8, because it is the ultimate source of new alleles.
Point mutations (errors in DNA)
| Type | What happens | Example (original: THE CAT ATE THE RAT) |
|---|---|---|
| Substitution | One base replaced by another | THE CAT ATE THE HAT |
| Deletion | A base is removed; the reading frame shifts | THE ATA TET HER AT |
| Addition (insertion) | A base is added; the reading frame shifts | THE CAT GAT ETH ERA T |
| Inversion | A section of bases is reversed | THE TAC ATE THE RAT |
Effects on the protein
- Silent: a substitution that produces a codon for the same amino acid (the code is degenerate), so the protein is unchanged.
- Missense: a substitution that changes one amino acid. The effect depends on where it is: a change at the active site of an enzyme may stop it working; a change elsewhere may have little effect. Sickle cell anaemia is caused by a single substitution in a haemoglobin gene.
- Nonsense: a substitution that creates a stop codon, producing a shortened, usually non-functional protein.
- Frameshift: a deletion or addition of bases (not in a multiple of three) shifts the reading frame, changing every codon after the mutation. The protein is usually non-functional.
The genetic code is read in non-overlapping triplets from a fixed start point. Adding or removing one base changes every triplet that follows, whereas a substitution changes only one.
Chromosomal mutations (errors in cell division)
A general understanding is needed:
- Deletion: part of a chromosome is lost.
- Duplication: part of a chromosome is copied.
- Inversion: a section breaks off and is reattached the wrong way round.
- Translocation: a section moves to a different, non-homologous chromosome.
- Changes in chromosome number: chromosomes fail to separate properly in meiosis (non-disjunction), giving gametes with an extra or missing chromosome. Down syndrome (trisomy 21), with three copies of chromosome 21, is an example.
Mutagens
A mutagen is anything that increases the rate of mutation.
- Physical: UV radiation (from sunlight), ionising radiation (X-rays, gamma rays, radioactive materials) and heat.
- Chemical: substances such as some chemicals in tobacco smoke and certain industrial chemicals.
Mutations in body cells (somatic mutations) affect only the individual and can lead to cancer if they affect genes that control the cell cycle. Mutations in cells that produce gametes (germline mutations) can be passed on to offspring.
A substitution in a coding sequence
The mRNA codons AUG GCU UGG AAA code for methionine, alanine, tryptophan, lysine. A substitution changes UGG to UGA.
- UGA is a stop codon, so translation ends after alanine.
- The protein is only two amino acids long instead of continuing, so it is almost certainly non-functional.
- This is a nonsense mutation.
Marker's note: name the type of mutation, show the codon change, and describe the effect on the amino acid sequence and function.
- Saying every mutation is harmful
- Many are silent or neutral, and a few are beneficial, which is why mutation drives evolution.
- Saying a substitution causes a frameshift
- Only additions and deletions (not in multiples of three) shift the reading frame.
- Forgetting to link to protein function
- The course asks for the impact on the final protein, so always finish with the effect on shape or function.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksExplain why the deletion of one base near the start of a gene usually has a greater effect on the protein than the substitution of one base.Show worked answer →
Three marks for each mutation type.
Substitution. One base is replaced by another, changing only one codon. Because the code is degenerate, the new codon may code for the same amino acid (no change), a different amino acid (one amino acid changed, which may or may not alter the protein's shape), or a stop codon (a shortened protein).
Deletion. Removing one base shifts the reading frame (a frameshift), so every codon after the deletion is read differently. Most amino acids after that point change, and a premature stop codon is likely, so the protein is usually non-functional. Because the deletion is near the start, almost the whole protein is affected.
Original4 marksName two types of mutagen, give an example of each, and explain how a mutagen increases the risk of cancer.Show worked answer →
Two mutagens (2 marks). Physical: UV radiation from sunlight, or ionising radiation such as X-rays. Chemical: chemicals in tobacco smoke.
Cancer risk (2 marks). Mutagens damage DNA and increase the rate of mutations. If mutations occur in genes that control the cell cycle, cells may divide uncontrollably and form a tumour.