IGCSE Biology / Chromosomes, genes and DNA
Mutations and what causes them revision notes
Edexcel International GCSE Biology (4BI1), chapter 16: Chromosomes, genes and DNA, specification point 3.34, 3.35B, 3.36B, 3.37B. Free lesson.
Watch the video lesson · IGCSE Biology past papers
A mutation is a change in the sequence of bases in DNA. Most mutations have no effect, but some affect proteins and characteristics, while rare beneficial mutations provide new alleles for natural selection.
What a mutation can do
The effect depends on which base changes and where it is in the DNA. A changed base may leave the protein unchanged, alter one amino acid, or affect how well a protein works. No effect: Several different triplets can code for the same amino acid, so a base change may still produce exactly the same protein. Most mutations are like this. One amino acid changed: The protein is nearly the same and often still works. Sometimes it does not, and this can cause a genetic disorder. Active site changed: If an enzyme's active site changes shape, it may no longer be complementary to its…
How a mutation can change a characteristic
When a mutation affects a protein, the change can pass from the DNA base sequence through to the characteristic that the protein controls.
What increases the mutation rate
Mutations happen by chance, but mutagens make them happen more often. Some damage DNA through ionising radiation, while chemical mutagens react with DNA. Gamma rays come from radioactive materials and are also used in radiotherapy. X-rays are used in medical imaging. Scans are kept to what is necessary, and radiographers stand behind a screen. Ultraviolet rays come from sunlight and sunbeds. Exposure is linked to skin cancer. Chemical mutagens include chemicals in tobacco smoke, which are linked to lung cancer.
Why mutations matter for evolution
It is not true that mutations are always harmful. Most have no effect because several triplets can code for the same amino acid. Some mutations are harmful and a very few are beneficial. Mutation is the only source of new alleles, so it supplies the variation that natural selection can act on.