How do changes to the DNA sequence arise and what are their consequences?
Describe the types of gene and chromosomal mutations, their causes, and their effects on phenotype
A focused answer to the WACE Year 12 Biology dot point on mutations. Covers point mutations including substitution, insertion and deletion, frameshift effects, chromosomal mutations, mutagens, and why only mutations in gametes are heritable.
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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.
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.
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.
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.