# Mutations

> What mutations are, what causes them, point and frameshift mutations, chromosomal mutations, somatic versus germline mutations, and how mutations change gene pools.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/mutation/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The sun and the skin
Australia has one of the highest rates of skin cancer in the world. Many Australians have fair skin from ancestors who lived in cloudy northern Europe, and they now live under one of the strongest suns on Earth.

Ultraviolet (UV) light from the sun can damage DNA in skin cells. Usually the cell repairs it. But if a mistake slips through in a gene that controls cell division, that cell may start dividing out of control. That is why "slip, slop, slap, seek and slide" (shirt, sunscreen, hat, shade and sunglasses) has been taught to Australian children since the 1980s. It is a campaign against mutations.

**In short:** A mutation is a permanent change in the DNA sequence, and it is the source of all new alleles. Mutations can be caused by mutagens such as UV radiation and some chemicals, and only mutations in gametes (germline mutations) can be passed on to offspring.

## Starting point: what you already know

- DNA bases are read three at a time as codons (see From DNA to Protein).
- Meiosis makes gametes. Mitosis makes body cells (see DNA, Chromosomes and Cell Division).
- New alleles are a source of variation for natural selection.

## What is a mutation?

A **mutation** is a permanent change in the DNA sequence. It can be a change to one base, to a gene, or to a whole chromosome. Mutations are the **only** source of brand new alleles.

Mutations can be

- **harmful** (causing disease, like cystic fibrosis),
- **neutral** (no effect, the most common kind), or
- **helpful** (like the mutation that lets some adults digest milk sugar).

## Causes of mutation

Mutations can happen **spontaneously**, as copying errors when DNA replicates. Their rate rises with **mutagens**: things that damage DNA.

| Type of mutagen | Examples |
|---|---|
| **Physical (radiation)** | UV light, X-rays, gamma rays from radioactive substances |
| **Chemical** | Chemicals in cigarette smoke, asbestos, some pesticides, mustard gas |
| **Biological** | Some viruses insert their DNA into ours (HPV can cause cervical cancer). Transposons ("jumping genes") move within the genome |

## Point mutations

A **point mutation** changes **one base**.

### Worked example: Three outcomes of a substitution
Original mRNA: **AUG GAG CUU** → methionine, glutamic acid, leucine

- **Silent**: GAG → GA**A**. Both code for glutamic acid. No change, because the code is degenerate.
- **Missense**: GAG → G**U**G. Now valine instead of glutamic acid. This is the exact change that causes **sickle cell anaemia**: red blood cells become curved and stiff.
- **Nonsense**: a codon changes into a **stop** codon. The protein is cut short and usually does not work.

**Frameshift mutations** are the insertion or deletion of a base. Every codon after it is read in the wrong groups of three, like a sentence with one letter removed:

THE FAT CAT ATE THE RAT → delete F → THE ATC ATA TET HER AT

Frameshifts usually wreck the protein. The most common cystic fibrosis mutation deletes three bases (one whole codon), losing one amino acid from the CFTR protein.

## Chromosomal mutations

These change the **structure** or **number** of chromosomes.

| Type | What happens |
|---|---|
| Deletion | Part of a chromosome is lost |
| Duplication | Part is copied twice |
| Inversion | Part is flipped the wrong way round |
| Translocation | Part breaks off and joins a different chromosome |
| **Aneuploidy** | One chromosome too many or too few, from **non-disjunction** (chromosomes failing to separate in meiosis). **Down syndrome** is three copies of chromosome 21 (trisomy 21) |
| **Polyploidy** | Whole extra sets of chromosomes. Common in plants: bread wheat has six sets |

## Somatic and germline mutations

### Key idea: Which cells does it happen in?
- **Somatic** mutations happen in **body cells**. They can affect that person (for example, causing cancer) but are **not inherited** by children.
- **Germline** mutations happen in cells that make **gametes**. They can be **passed to offspring**, and so can change the gene pool.

**Mind map**

```text
Mutations
  What
    Permanent DNA change
    Only source of new alleles
    Harmful, neutral or helpful
  Causes
    Copying errors
    Radiation: UV, X-rays
    Chemicals: smoke
    Viruses
  Point
    Silent
    Missense: sickle cell
    Nonsense: early stop
    Frameshift
  Chromosomal
    Deletion, duplication
    Inversion, translocation
    Non-disjunction: Down syndrome
  Somatic or germline
    Somatic: not inherited
    Germline: inherited
```

### Year 11: Coding and non-coding DNA
Only about 1.5% of human DNA codes for proteins. The rest is **non-coding DNA**, including **introns**, **promoters** and other **regulatory** sequences that control when genes are switched on.

- A mutation in **coding** DNA can change a protein's amino acid sequence.
- A mutation in **non-coding** regulatory DNA can change **how much** of a protein is made, or **when**. For example, a mutation in a regulatory region near the lactase gene keeps it switched on in adults, which is why many adults can digest milk.

Non-coding DNA was once called "junk DNA". Scientists now know much of it has important jobs.

**How UV damages DNA.** UV light can join two neighbouring thymine bases on the same strand (a **thymine dimer**). This kinks the DNA, and mistakes are made when it is copied.

**Least likely to matter.** A substitution in an intron, or in the third base of a codon (often silent), is least likely to change the protein. An insertion near the start of a gene is the most damaging, because it shifts every codon after it.

**Reading chromosome changes.** ABCDEFG becoming ABCFEDG means the DEF section was flipped: an **inversion**. A karyotype of 45 chromosomes with one X and no Y is **Turner syndrome**, another result of non-disjunction.

**Changes with no effect.** A dolphin with two chromosomes fused end to end has the same genes, just packaged differently, so it looks normal.

### Year 12: Mutations and the gene pool
A new allele only changes a population if it is in the **germline** and is passed on. Then four forces decide its fate:

- **Natural selection**: helpful alleles become more common, harmful ones rarer.
- **Gene flow**: alleles enter or leave with migrating individuals. Gene flow makes populations more alike.
- **Genetic drift**: chance changes, strongest in small populations. In a **bottleneck** (a population suddenly shrinks) or a **founder effect** (a few individuals start a new population), rare alleles can become common or be lost by luck.

**Sickle cell** shows a trade-off: people with one sickle allele resist malaria, so the allele is common where malaria is common, even though two copies cause disease.

**Mutations and cancer.** **Tumour suppressor** genes, such as BRCA1, normally stop damaged cells dividing or repair DNA. A somatic mutation that switches one off removes this brake, so the cell may divide out of control.

**Why flu vaccines change.** Influenza viruses mutate quickly. Small changes in their surface proteins mean antibodies made against last year's virus no longer fit well, so a new vaccine is made most years.

**Mutations drive evolution.** Germline mutations add new alleles to the gene pool. Selection, drift and gene flow then change how common they become over many generations.

## Quick summary

- A mutation is a permanent change in DNA, and the source of all new alleles.
- Mutagens include radiation (UV, X-rays), chemicals (cigarette smoke) and some viruses.
- Point mutations can be silent, missense or nonsense. Insertions and deletions cause frameshifts.
- Chromosomal mutations change structure or number. Non-disjunction causes Down syndrome.
- Somatic mutations are not inherited. Germline mutations can be.

## Quiz

Try these questions from the chapter quiz.

1. A mutation is
   - A. a permanent change in the DNA sequence
   - B. any change in the environment
   - C. a change in diet
   - D. a type of protein
   - Answer: A. a permanent change in the DNA sequence. Mutations change DNA.

2. Which is a physical mutagen?
   - A. Cigarette smoke
   - B. UV light
   - C. Asbestos
   - D. HPV
   - Answer: B. UV light. Radiation is a physical mutagen.

3. A base substitution that changes one amino acid is a
   - A. silent mutation
   - B. missense mutation
   - C. frameshift mutation
   - D. nonsense mutation
   - Answer: B. missense mutation. One amino acid is swapped for another.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/mutation/#quiz