The Effect of Genetic Mutations (AQA A Level Biology): Revision Note

The effect of genetic mutations

• Mutations occur spontaneously and randomly during DNA replication

• 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 amino acid sequence coded for by the gene

• Most mutations do not alter the polypeptide or only alter it slightly, so that its structure or function is not changed

• As the genetic code is degenerate, some mutations will not cause a change in the amino acid sequence

  • For example, substitution mutations are the mutations that usually have a smaller effect on the resultant polypeptide

• Some gene mutations change all base triplets downstream from (after) the mutation, which will result in a non-functional polypeptide

  • For example, insertion and deletion mutations result in a frameshift

 The effect of gene mutations on polypeptides 

• Most mutations do not alter the polypeptide 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 polypeptide 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

The effect of gene mutations on phenotype

• Polypeptides affect the phenotype of an organism via specific cellular mechanisms

• If a mutation causes a major alteration in a polypeptide, then cellular mechanisms could be affected, which may impact the phenotype of the organism

• For example, a mutation in the TYR gene in humans affects the structure of an enzyme that is needed for the production of the pigment melanin

  • The phenotype of humans is affected by the lack of melanin

  • Individuals with the mutation have albinism, characterised by very pale skin, and often white hair

Causes of mutations

• The rate at which mutations occur can be estimated as around one mutation per 100,000 genes per generation

• Mutagenic agents are factors that increase the frequency of mutations in DNA; they include

  • physical mutagens, which cause mutations through physical means, typically involving radiation such as ionising radiation (e.g. X-rays, gamma rays)

  • chemical mutagens, which interfere with the DNA molecules chemically

  • biological mutagens, which are organisms or biological molecules that interfere with DNA, such as viruses (e.g. HPV can lead to cervical cancer)

• Exposure to mutagenic agents can increase the rate of mutation