Chapter 9

• Mutation


  An alteration to the DNA base sequence
• Mutations often arise spontaneously during DNA replication
• Substitution mutation


  When a nucleotide in the DNA sequence is replaced by another. This is more likely to be a quiet mutation, meaning no change occurs in the amino acid sequence.
• Deletion mutation


  When a nucleotide in the DNA sequence is lost. This is more likely to be harmful and significant, as it leads to a frame shift which means the entire amino acid sequence will be different.
• Mutagenic agent


  Factors that increase the rate of gene mutation. Examples: X-rays, UV light, gamma rays, certain chemicals e.g. in alcohol and tobacco.
• Polyploidy chromosome mutation


  Where an individual has three or more sets of chromosomes instead of two.
• Chromosome non-disjunction


  When chromosomes fail to separate correctly in meiosis, resulting in gametes with one more or less chromosome than normal.
• Meiosis


  A form of cell division that produces four genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes.
• Meiosis vs Mitosis

  • Meiosis produces four genetically different cells with half the number of chromosomes as the parent cells. Mitosis produces two genetically identical cells with the same number of chromosomes as the parent cells.
• Meiosis I

  1. Homologous chromosomes pair to form bivalents.

  2. Crossing over (exchange of sections of genetic material) occurs at chiasmata.

  3. Cell divides into two. Homologous chromosomes separate randomly. Each cell contains either maternal or paternal copy.
• Meiosis II

  1. Independent segregation of sister chromatids.

  2. Each cell divides again, producing 4 haploid cells.
• Why a mutation may not lead to change in amino acid sequence: Genetic code is degenerate so mutation may end up coding for same amino acid as the original triplet, or mutation may occur in intron.
• Two ways meiosis produces genetic variation

  • Crossing over during meiosis I
  • Independent assortment (random segregation) of homologous chromosomes & sister chromatids