Causes and Consequences of Mutations
What this note covers
- What Is a Mutation?
- Mutagens: Physical, Chemical and Biological
- Consequences: Beneficial, Neutral and Harmful Mutations
- Mutations and Cancer
- Mutations and Heritable Disease
- DNA Repair and Why Mutations Persist
- HSC Exam Application and Extended Response Strategy
7 sections · 14 key terms & formulas · 6 common mistakes
What Is a Mutation?
A mutation is any heritable change in the nucleotide sequence of DNA or in the structure/number of chromosomes. Mutations occur at two scales:
- Gene (point) mutations — changes to one or a few nucleotide bases within a single gene. Subtypes include substitution (one base swapped for another), insertion (extra base/s added) and deletion (base/s removed).
- Chromosomal mutations — large-scale alterations: deletion, duplication, inversion or translocation of chromosome segments, or changes in whole-chromosome number (aneuploidy, e.g. trisomy) or whole genome number (polyploidy).
It is essential to distinguish a somatic mutation (occurring in body cells — not passed to offspring, but can lead to cancer) from a germline mutation (occurring in gametes or cells that will form gametes — heritable and the basis of inherited disease and evolution).
Mutations arise either spontaneously (errors during DNA replication, roughly 1 in 109 base pairs per replication cycle before repair) or are induced by external agents called mutagens.
Mutagens: Physical, Chemical and Biological
A mutagen is any agent that increases the rate of mutation above the spontaneous baseline. The NSW HSC syllabus requires you to classify mutagens into three categories.
| Category | Examples | Mechanism of DNA Damage |
|---|---|---|
| Physical | Ultraviolet (UV) radiation, X-rays and gamma rays (ionising radiation), alpha and beta particles | UV causes adjacent thymine bases to bond together, forming thymine dimers that block normal replication. Ionising radiation breaks phosphodiester bonds, causing single- or double-strand breaks and generating reactive free radicals. |
| Chemical | Benzene (cigarette smoke), formaldehyde, aflatoxin B1 (mould on peanuts), nitrosamines, alkylating agents (e.g. ethyl methanesulfonate), base analogues (e.g. 5-bromouracil) | Chemicals interact with bases directly (alkylation alters pairing), intercalate between bases (causing insertion/deletion errors) or mimic normal bases and mispair during replication. |
| Biological | Human papillomavirus (HPV), Epstein–Barr virus (EBV), Helicobacter pylori, hepatitis B and C viruses | Viral genomes integrate into host DNA, disrupting gene sequences or inserting viral oncogenes. Some bacteria produce toxins or chronic inflammation that increases reactive oxygen species, damaging DNA. |
Worked example — UV and thymine dimers: A skin cell's DNA is exposed to UV-B radiation. UV energy causes two adjacent thymine (T) nucleotides on the same strand to form a covalent bond, creating a T–T dimer. During the next round of replication the polymerase cannot read the dimer accurately and may insert incorrect bases opposite it. If the cell's nucleotide excision repair (NER) system fails to remove the dimer first, a point mutation is incorporated into one daughter cell. This is the molecular explanation for why chronic UV exposure causes most non-melanoma skin cancers (squamous and basal cell carcinomas).
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