Mutations

• Mutation


  An alteration to the DNA base sequence. Often arise spontaneously during DNA replication
• Mutagenic agent


  Factors that increase the rate of gene mutation
• Causes of mutations

  • Chemical mutagens (include alcohol, tar in tobacco)
  • Ionising radiation (alpha & beta, UV and X-ray)
  • Spontaneous errors in DNA replication
• Addition


  Where one or more nucleotides (bases) are inserted from the DNA sequence
• Deletion


  Where one or more nucleotides (bases) are deleted from the DNA sequence
• Substitution


  Where one nucleotide (base) in the DNA sequence is replaced by another
• Duplication


  Where one or more nucleotides (bases) duplicate and repeat
• Inversion


  Where a group of nucleotides (bases) become separated from the DNA sequence, then rejoin in the reverse order i.e. they have flipped
• Translocation


  Where a group of nucleotides (bases) become separated from the DNA sequence, and are then inserted into the DNA of a different chromosome
• Mutations can result in a different amino acid sequence in the encoded polypeptide
• Mutation types and their impact


  • Addition, deletion, duplication, translocation are most likely to have a significant impact as they produce a frameshift, meaning the entire amino acid sequence produced will be different
  • Substitution & inversion are most likely to have a smaller impact as they only alter 1 or very few triplets, the amino acid sequence might not be affected due to the degenerate nature of the genetic code
• A mutation resulting in a change to the amino acid sequence may not always be harmful as it may be neutral if the resulting change in protein has no effect on the organism. E.g, mutation on an intron, or recessive allele
• Mutations may be beneficial - basis for evolution & natural selection