1.4

• Gene


  Section of a chromosome which codes for a protein
• Homologous chromosomes


  Pair of chromosomes (one maternal and one paternal)
• Mutations


  A random, rare and spontaneous change in the structure of a gene, chromosome or number of chromosomes
• DNA replication


  Carefully regulated to ensure conservation of the genetic code
• Mutations to the genetic code can occur
• Results of mutations


  • No protein is expressed
  • An altered protein is expressed
• Categories of mutation


  • Single gene mutation
  • Chromosome structure mutation
• Single gene mutation


  Affects only a couple of bases
• Chromosome structure mutation


  Affects the structure of one or more chromosomes
• Types of single gene mutation

  • Nucleotide substitutions
  • Nucleotide insertions or deletions
• Nucleotide substitutions


  Substitutions result in a change that only affect one codon
• Types of nucleotide substitutions

  • Missense mutations
  • Nonsense mutations
  • Splice-site mutations
• Missense mutations


  Result in one amino acid being changed for another, may result in a non functional protein or have little effect
• Nonsense mutations


  Result in a premature stop codon being produced which results in a shorter protein
• Splice-site mutations


  Result in some introns being retained and/or some exons not being included in the mature transcript
• Frame-shift mutations


  Cause all of the codons and all of the amino acids after the mutation to be changed, having a major effect on the structure of the protein produced
• Causes of mutations


  • Ionising radiation (gamma rays, X-rays, ultraviolet rays)
  • Chemical mutagens (tar from cigarette smoke)
• The greater the dose of radiation a cell gets, the greater the chance of a mutation
• Mutations could cause different genes to be switched on/off
• Types of chromosome structure mutations

  • Duplication
  • Deletion
  • Inversion
  • Translocation
• The substantial changes in chromosome mutations often make them lethal