8.1

• Meiosis

  A reduction division. The main role of meiosis is the production of haploid gametes as cells produced by meiosis have half the number of chromosomes of normal body (somatic) cells.
• Meiosis introduces variation via

  1. Crossing over

  2. Independent assortment
• Genetic variation

  Differences in the genetic makeup of individuals within a species
• Factors that increase genetic variation

  • Mutations
  • Random assortment and crossing over during meiosis
  • Random fertilisation during sexual reproduction
• Mutation

  An alteration to the DNA base sequence, often arising spontaneously during DNA replication
• Addition mutation

  Where one or more nucleotides (bases) are inserted into the DNA sequence, more likely to be harmful and significant as it leads to a frame shift
• Deletion mutation

  Where one or more nucleotides (bases) are deleted from the DNA sequence, more likely to be harmful and significant as it leads to a frame shift
• Substitution mutation

  When a nucleotide in the DNA sequence is replaced by another, more likely to be a quiet mutation with no change in the amino acid sequence
• How meiosis creates new combinations of alleles

  1. Random arrangement of chromosomes during lining up

  2. Crossing over of chromatids before the first division
• Random fertilisation

  Gametes are haploid cells, so every gamete contains different DNA, allowing the same two individuals to produce genetically different offspring
• Why might a mutation not lead to a change in the amino acid sequence?

  A mutation may not lead to a change in the amino acid sequence because the genetic code is degenerate, meaning the mutation may end up coding for the same amino acid as the original triplet unless a frame shift occurs