Mutation: Advanced (Edexcel IGCSE Biology)

• A change in DNA can affect the phenotype of an individual by altering the sequence of amino acids in a protein
• There are three main ways that the sequence of DNA can be changed (which will alter the sequence of amino acids in the protein)
  • Insertion of a new base into the DNA sequence
  • Deletion of a base within the DNA sequence
  • Substitution of a base within the DNA sequence

Insertions

• A new base is randomly inserted into the DNA sequence
• An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs
  • Remember – every group of three bases (codon) in a DNA sequence codes for an amino acid
• An insertion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence

An example of an insertion mutation

Deletions

• A 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 by the group of three bases in which the mutation occurs
• 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

Substitutions

• A base in the DNA sequence is randomly swapped for a different base
• Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the group of three bases in which the mutation occurs; it will not have a knock-on effect

An example of a substitution mutation

• Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• Therefore there is no effect on the phenotype
• Some mutations can have a small effect on the protein or organism
• However, a small number of mutations code for a significantly altered protein with a different shape
• This may affect the ability of the protein to perform its function. For example:
  • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
  • A structural protein (like collagen) may lose its strength if its shape changes
• On rare occasions mutations lead to the development of new alleles and so new phenotypes
• Occasionally, the new allele (and its resulting phenotype) gives the individual a survival advantage over other members of the species
• For example:
  • A bird develops a mutation leading to a change in feather colours
  • This makes it more attractive to birds of the opposite sex
  • Therefore the bird will breed more frequently and have more chances of passing on the mutated phenotype to the next generation
• Mutations can also lead to harmful changes that can have dramatic effects on the body - for example the red blood cell disorder, sickle cell anaemia in humans

• Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
  • Gamma rays, x-rays and ultraviolet rays - all types of ionising radiation which can damage bonds and cause changes in base sequences
  • Chemical mutagens - for example, chemicals such as tar in tobacco
• Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer