Biology (Year 12) - Variation and Mutation
Image: Mackenzie Angell, Graphic Designer
Substitutions occur when one nucleotide is replaced by another nucleotide. Differences of a single nucleotide in a sequence are also called single nucleotide polymorphisms (SNPs), as when the codon in the sequence that codes for amino acids is altered, the mutation may have a subtle or dramatic effect on its structure and function.
There are three subcategories of substitutions.
1. Synonymous mutations (silent): These occur when the substituted base results in a codon that codes for the same amino acid as the original codon.
2. Missense mutations: This arises when an SNP changes the amino acid. For example, a substitution of an AGA codon (arginine) to an AGC codon results in a serine amino acid being produced.
3. Nonsense mutations: This arises when an SNP creates a new stop codon, leading to early termination of translation of the transcribed gene sequence.
Image: Mackenzie Angell, Graphic Designer An insertion mutation is the addition of either one or more nucleotides at a particular place in the original sequence. A deletion mutation is the loss of one or more nucleotides at a particular place in the original sequence. The effect of this type of mutation is called a frameshift effect, as the whole sequence has been moved. In other words, the reading frame for the amino acids has been moved slightly which has drastic effects on all the codons for the rest of the sequence.