3.8.1 Gene mutations

• • Gene mutations might arise during DNA replication.
  • They include addition, deletion, substitution, inversion, duplication and translocation of bases.
• • Gene mutations occur spontaneously.
  • The mutation rate is increased by mutagenic agents.
  • Mutations can result in a different amino acid sequence in the encoded polypeptide.
• • Some gene mutations change only one triplet code.

  Due to the degenerate nature of the genetic code, not all such mutations result in a change to the encoded amino acid.

  •Some gene mutations change the nature of all base triplets downstream from the mutation, ie result in a frame shift.
• Not all mutations result in a change to the amino acid sequence of the encoded polypeptide. Explain why [2 marks]

  • Genetic code is degenerate
  • Mutation in intron - non-coding DNA
• Inversion

  • A sequence of bases / nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order
• Translocation

  • A sequence of DNA bases / nucleotides detaches and is inserted at a different location within the same or a different chromosome
• Explain why a change in amino acid sequence is not always harmful

  ● May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)

  ● May positively change the properties of the protein, giving the organism a selective advantage