Meiosis

• Meiosis

  • Nuclear division
  • In reproductive organs to produce gametes
  • Involves halving chromosome number so daughter cells are haploid
  • Halving chromosome number is important so diploid number can be restored in fertilisation
  • Each parent cell divides to form 4 genetically different gametes
  • Source of genetic variation
  • Haploid: (n) One set of chromosomes per cell
  • Diploid: (2n) Two sets of chromosomes per cell
  • Homologous Chromosomes: Chromosome pairs determine same characteristics
  • Gene: Short DNA section that codes for specific polypeptide; location on its chromosome is Gene locus
• Meiosis Stages

  • Interphase I:
  • Chromosome + Centriole replicate
  • Prophase I
  • Metaphase I:
  • Shortest
  • Bivalents align at equator
  • Anaphase I:
  • Homologous chromosomes separate to poles
  • Sister chromatids attached at centromere
  • Telophase I
  • Cytokinesis I:
  • Cytoplasm division makes 2 haploid cells
  • 1 of each homologous pair
  • In most plant cells, goes Anaphase I to Prophase II
  • Meiosis II:
  • No interphase II/DNA replication
  • Prophase to telophase II same as mitosis
  • Cytokinesis II:
  • Cytoplasm division occurs in both cells
  • Produces 4 haploid gametes; tetrad
  • 1 of each homologous pair
• Prophase I:

  • Longest
  • Most Complex
  • 90% of Meiosis
  • Early Prophase:
  • Chromosomes condense by supercoiling
  • Bivalent is two chromosomes or four chromatids (sister and non-sister)
  • Late Prophase:
  • Crossing over
  • Non-sister chromatid sections break and reattach to other
  • Occurs at chiasmata
  • ~2-3 cross-over events on each pair
• Variation in Meiosis

  • Crossing over (prophase I)
  • Independent assortment (metaphase I): Orientation of pairs to poles is random
  • Random fertilisation:
  • Nuclei of two haploid gametes must fuse to restore diploid number of zygote
  • Which gametes fuse is completely random
  • Random mating
  • Mutations
  • DNA mutation may occur in DNA replication before mitosis and meiosis
  • Chromosome mutations during mitosis or meiosis
  • Mutated gamete in fertilisation will be present in every body cell