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.
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
- Permanent DNA change
- Only source of new alleles
- Harmful, neutral or helpful
- Copying errors
- Radiation: UV, X-rays
- Chemicals: smoke
- Viruses
- Silent
- Missense: sickle cell
- Nonsense: early stop
- Frameshift
- Deletion, duplication
- Inversion, translocation
- Non-disjunction: Down syndrome
- Somatic: not inherited
- Germline: inherited
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
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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
Show answer
A: a permanent change in the DNA sequence
Mutations change DNA.
Which is a physical mutagen?
- A Cigarette smoke
- B UV light
- C Asbestos
- D HPV
Show answer
B: UV light
Radiation is a physical mutagen.
A base substitution that changes one amino acid is a
- A silent mutation
- B missense mutation
- C frameshift mutation
- D nonsense mutation
Show answer
B: missense mutation
One amino acid is swapped for another.