Mutations: Advanced (Edexcel IGCSE Biology (Modular)): Revision Note

Changes in DNA can Affect the Phenotype

• 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

The Effects of Mutations

• Most genetic mutations do not alter the protein, or only alter it slightly, so that its function is unaffected

  • Therefore, most mutations have no effect on the phenotype

• Some genetic mutations can slightly alter the shape of a protein, causing a minor effect on the phenotype

• Rarely do mutations have a significant effect on the phenotype

  • If a mutation produces a protein with a different shape, this may affect its function (affecting the phenotype)

  • For example, if the active site of an enzyme changes shape, the substrate may no longer bind, preventing enzyme-substrate complexes from forming

• Occasionally, mutations create new alleles, resulting in new phenotypes

• If the new phenotype provides a survival or reproductive advantage, it can be passed on to future generations

  • For example, if a mutation changes a bird’s feather colour, If the new colour makes it more attractive to mates, the mutation is more likely to be passed on

• Mutations can also be harmful, causing significant health problems

  • For example: sickle cell anaemia, caused by a mutation in the gene coding for haemoglobin

The Causes of Mutations

• 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