Mutations (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Mutation & Genetic Variation

• There is usually extensive genetic variation within a population of a species

• All genetic variants arise from mutations

  • Mutations are rare, random changes that occur in the sequence of DNA bases in a gene or a chromosome

  • Mutations occur continuously

  • As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for

The effects of mutations

• Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed

• 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

  • Which causes the bird to 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, sickle cell anaemia in humans

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

  • Certain types of chemicals - 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