Mutation
- A gene mutation is a change in the sequence of base pairs in a DNA molecule; this may result in a new allele
- Mutations occur all the time and at random
- There are certain points in the cell cycle when mutations are more likely to occur, for example, copying errors when DNA is being replicated (S phase of interphase)
- As the DNA base sequence determines the sequence of amino acids that make up a polypeptide, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for
- Most mutations are harmful or neutral (have no effect) but some can be beneficial
- Inheritance of mutations:
- Mutations present in normal body cells are not inherited, they are eliminated from the population once those cells die
- Mutations within gametes are inherited by offspring, possibly causing genetic disease
Substitution mutations
- A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base is known as a substitution mutation
- A substitution mutation will only change the amino acid for the triplet (group of three consecutive bases) where the mutation occurs; it will not have a knock-on effect further along the gene/polypeptide
An example of a substitution mutation altering the sequence of amino acids in the polypeptide
Examiner Tip
You don't need to know about deletions, insertions and frameshift mutations - just substitution mutations!