8.1.2 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 (more than one triplet code codes for the same amino acid) some mutations will not cause a change in the amino acid sequence
  • 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, this will result in a non-functional polypeptide
  • Insertion and deletion mutations result in a frameshift

Causes of mutations

• The rate that mutations occur can be estimated as around one mutation per 100 000 genes per generation
• Exposure to mutagenic agents can increase the rate of mutation, they include
  • High energy ionising radiation, such as alpha, beta or gamma radiation
  • Chemicals, such as nitrogen dioxide or benzopyrene from tobacco smoke

 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 / proteins 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 the human is affected by the lack of melanin
  • Individuals with the mutation have albinism; very pale skin and hair