AQA A Level Biology

Revision Notes

8.1.2 The Effect of Genetic Mutations

The Effect of Genetic Mutations

  • Mutations occur spontaneously
  • 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

 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

Exam Tip

In the exam, you will be expected to relate the nature of a gene mutation to its effect on the encoded polypeptide.

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While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.
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