Nature of Mutations
- A gene mutation is a change in the sequence of bases in a DNA molecule
- Mutations may result in an altered polypeptide; as the DNA base sequence of a gene 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
- Mutations occur spontaneously during DNA replication
- There are different ways that a mutation in the DNA base sequence can occur, e.g.
- Substitution
- Insertion
- Deletion
- Substitution, insertion, and deletion mutations are all examples of point mutation; mutations that involve a change in the DNA base sequence at a single location
- Other types of mutation can affect entire genes or entire chromosomes
- Genes can be replicated or lost
- Chromosomes can be divided unequally during meiosis, resulting in cells with extra or missing chromosomes
Substitution of nucleotides
- A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base
- A substitution mutation will only change the amino acid for the triplet in which the mutation occurs, and will have no impact on triplets located elsewhere in the gene
- Substitution mutations include
- Silent mutations
- The mutation does not alter the amino acid sequence of the polypeptide; this is due to the degenerate nature of the genetic code
- Missense mutations
- The mutation alters a single amino acid in the polypeptide chain, e.g. sickle cell anaemia is caused by a single substitution mutation changing a single amino acid in the haemoglobin protein
- Nonsense mutations
- The mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function, e.g. cystic fibrosis can be caused by a nonsense mutation
- Note that a stop codon provides a signal for the cell to stop translation of the mRNA molecule into an amino acid sequence
- The mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function, e.g. cystic fibrosis can be caused by a nonsense mutation
- Silent mutations
Substitution mutations involve swapping one nucleotide for another
Insertion of nucleotides
- A mutation that occurs when a nucleotide 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 that every group of three bases in a DNA sequence codes for an amino acid
- An insertion mutation also has a knock-on effect on other base triplets by changing the triplets further on in the DNA sequence
- This means that insertion mutations cause what is known as a frameshift mutation; they don't only change the triplet where the insertion has occurred, but every triplet downstream of the insertion
- This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function
Insertion mutations occur when a new nucleotide is added into a base sequence
Deletion of nucleotides
- A mutation that occurs when a nucleotide is randomly deleted from the DNA sequence
- Like an insertion mutation, a deletion mutation changes the triplet in which the deletion has occurred, and also changes every group of three bases further on in the DNA sequence
- This is 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