What is a Mutation?
A mutation is a sudden change in the genetic material (DNA) of a cell. It changes the instructions carried by a gene or the structure or number of chromosomes. Mutations may be inherited if they occur in sex cells (gametes), or they may affect only body cells. They can be harmful, harmless (neutral) or, occasionally, beneficial.
Two Main Types
- Gene mutation — a change in a single gene, for example the substitution of one DNA base for another. Sickle-cell anaemia results from a gene mutation.
- Chromosome mutation — a change in the structure or number of whole chromosomes. Down syndrome is caused by having an extra copy of chromosome 21 (three copies instead of two).
Key idea
A gene mutation affects a small part of the DNA sequence; a chromosome mutation affects large blocks or whole chromosomes. Both can change the phenotype of an organism.
Causes of Mutation (Mutagens)
Factors that increase the rate of mutation are called mutagens. They include:
- Radiation such as ultraviolet (UV) rays, X-rays and gamma rays;
- Chemicals such as those in cigarette smoke and certain pesticides;
- Some viruses.
Example
Long exposure to strong UV light from the sun can cause gene mutations in skin cells and lead to skin cancer. This is why sunscreen is recommended even in sunny Malaysia.
A mutation may arise spontaneously, through a mistake made while the cell copies its DNA, or it may be induced by a mutagen from outside. The natural rate of mutation is usually very low because cells have repair mechanisms that correct most errors, but heavy exposure to mutagens raises the rate sharply and overwhelms these repairs.
Remember
Not all mutations are bad. A rare beneficial mutation can give an organism an advantage and is the raw material for evolution and for producing new crop varieties.