Gene Mutations
- 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 nucleotide 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
Substitution mutation diagram
An example of a substitution mutation altering the sequence of amino acids in the polypeptide
Insertion mutations
- A mutation that occurs when a nucleotide (with a new base) is randomly inserted into the DNA sequence is known as an insertion mutation
- An insertion mutation changes the amino acid that would have been coded for by the original base triplet, as it creates a new, different triplet of bases
- Remember – every group of three bases in a DNA sequence codes for an amino acid
- An insertion mutation also has a knock-on effect by changing the triplets (groups of three bases) further on in the DNA sequence
- This is sometimes known as a frameshift mutation
- This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
Insertion mutation diagram
An example of an insertion mutation
Deletion mutations
- A mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence
- Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for
- Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence
- Like an insertion mutation, this is sometimes known as a frameshift mutation
- This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function