Mutations and mutagens: WACE Year 12 Biology
“Describe the types of gene and chromosomal mutations, their causes, and their effects on phenotype”
A mutation is a change in the DNA sequence. Gene (point) mutations alter single bases by substitution, insertion or deletion; insertions and deletions cause a frameshift that changes every codon after them. Chromosomal mutations change whole sections or numbers of chromosomes. Mutations can be caused by mutagens or replication errors, and only mutations in gametes are passed to offspring and can fuel evolution.
What this dot point is asking
SCSA wants you to classify mutations, explain how each type changes the protein produced, distinguish somatic from germline mutations, and connect mutation to its role as the ultimate source of new genetic variation. A strong answer links the type of change to its effect on phenotype.
What a mutation is
A mutation is any permanent change to the base sequence of DNA. Because the base sequence codes for proteins, a mutation can change the amino acid sequence of a protein, and therefore its shape and function. Mutations are random and arise either spontaneously during DNA replication or because of a mutagen.
Gene (point) mutations
Point mutations affect one or a few bases within a gene.
- Substitution: one base is swapped for another. Because the code is degenerate, this may have no effect (silent), may change one amino acid (missense), or may create a premature stop codon (nonsense).
- Insertion: an extra base is added.
- Deletion: a base is removed.
Insertions and deletions shift the reading frame, because codons are read in fixed groups of three. Every codon after the change is altered, usually destroying the protein. A single substitution changes at most one amino acid, so insertions and deletions are generally far more damaging.
Chromosomal mutations
These affect large sections or whole chromosomes, often during meiosis.
- Deletion of a chromosome segment, duplication of a segment, inversion of a segment, or translocation of a segment to another chromosome.
- Aneuploidy: a change in chromosome number, such as the extra chromosome 21 that causes Down syndrome, usually caused by non-disjunction (failure of chromosomes to separate in meiosis).
Causes: mutagens
A mutagen is any agent that increases the rate of mutation. Examples relevant to Australia include:
- ultraviolet radiation from intense sunlight, a major cause of skin cancers given Australia's high UV index;
- ionising radiation such as X-rays;
- chemical mutagens including those in tobacco smoke.
Many mutations also occur spontaneously as rare uncorrected errors during DNA replication.
Trace the substitution to the phenotype
In the gene for haemoglobin, a single base substitution changes one codon so that the amino acid glutamic acid is replaced by valine. This one amino acid change alters the shape of the haemoglobin protein, making red blood cells sickle under low oxygen. One base change producing a clear phenotype shows how a missense substitution works.
Somatic versus germline mutations
Where a mutation occurs decides whether it is inherited.
- Somatic mutations occur in body cells. They affect only that individual and are not passed on. Some can lead to cancer.
- Germline mutations occur in cells that form gametes. These can be passed to offspring and so enter the gene pool of the population.
Only germline mutations contribute to evolution.
Effects on phenotype
A mutation may be harmful (reducing survival), neutral (no effect, common because of the degenerate code and non-coding DNA), or rarely beneficial (improving survival in a particular environment). Whether a mutation is good or bad depends on the environment: a change that is harmful in one setting may be advantageous in another.
Mutation: a permanent change in the DNA base sequence. Frameshift: a shift in the reading frame caused by an insertion or deletion. Mutagen: an agent that raises the mutation rate. Germline mutation: a mutation in gamete-forming cells that can be inherited.
Why mutation matters for continuity
Mutation is the ultimate source of all new alleles. Without it there would be no new variation for natural selection to act on, and populations could not adapt to changing environments. Most mutations are neutral or harmful, but the rare beneficial ones, accumulated over many generations, drive evolution and underpin the diversity of life.
- Saying all mutations are harmful
- Many are neutral and some are beneficial; the effect depends on the environment.
- Treating a substitution and a frameshift as equally damaging
- A frameshift from an insertion or deletion alters every following codon and is usually far worse than a single substitution.
- Forgetting the somatic versus germline distinction
- Only mutations in gamete-forming cells are inherited and contribute to evolution.
- Saying mutagens cause all mutations
- Many mutations arise spontaneously as replication errors with no mutagen involved.
Exam-style questions
Questions in the style of SCSA 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.
WACE 2022-style6 marksA single base in a gene is deleted. Explain why a deletion (a frameshift mutation) usually has a more serious effect on the protein than a single base substitution.Show worked answer →
A 6 mark answer needs the reading-frame mechanism for both mutation types.
- How codons are read
- The base sequence is read in non-overlapping triplets (codons), each specifying one amino acid, starting from a fixed point.
- Substitution
- A substitution swaps one base for another. It changes at most one codon, so it affects at most one amino acid. Because the code is degenerate it may even be silent (no change to the amino acid).
- Deletion (frameshift)
- Removing a base shifts the reading frame for every codon downstream of the deletion. All subsequent codons are read incorrectly, so the entire amino acid sequence after the deletion is wrong, and a premature stop codon often appears, truncating the protein.
- Conclusion
- Because a frameshift alters many codons while a substitution alters at most one, the deletion usually has a far more serious effect on protein structure and function.
Markers reward the triplet reading frame, one-codon effect of substitution and the downstream frameshift effect of deletion.
WACE 2024-style5 marksDistinguish between somatic and germline mutations, and explain why only one of these can contribute to the evolution of a population. Use mutagens relevant to Australia as examples of causes.Show worked answer →
A 5 mark answer needs the distinction, the heritability point and Australian mutagens.
- Somatic mutations
- Occur in body cells. They affect only that individual (and can cause cancer) but are not passed to offspring.
- Germline mutations
- Occur in gamete-forming cells, so they can be passed to offspring and enter the population's gene pool.
- Why only germline matters for evolution
- Evolution is a change in allele frequencies across generations. Only germline mutations are inherited, so only they add new alleles to the gene pool that selection can act on over generations.
- Australian mutagens
- Ultraviolet radiation from intense sunlight (a major cause of skin-cell mutations and skin cancer), ionising radiation such as X-rays, and chemical mutagens in tobacco smoke.
Markers reward the body-cell versus gamete-cell distinction, the heritability link to evolution and at least one relevant mutagen.