Meiosis

 
 
6.3 Meiosis
 
  1. A process of nuclear division that reduces the number of chromosomes into half.
  2. Produces haploid gametes.
  3. Provides genetic variation from one generation to another through the process of crossing-over, independent assortment and random fertilisation.
  4. Occurs in the gonads; testes and ova (humans); anther and ovary (flowering plants).
  5. Consists of two separate nuclear divisions; meiosis I and meiosis II.
  6. The cell undergoes interphase before entering meiosis.
 
Meiosis I:
 
Phase Key event
Prophase I
  • Chromosomes condensed and thickened.
  • Homologous chromosomes paired up to form bivalents through synapsis.
  • Non-sister chromatids of homologous chromosomes exchange the DNA segments through crossing-over.
  • It leads to genetic recombination.
  • Centrioles migrate to opposite poles and form spindle fibres.
  • Nucleolus and nuclear membrane disappear.
Metaphase I
  • Spindle fibres pull the homologous chromosomes to the middle cell and aligned at the metaphase plate side by side.
  • The homologous chromosomes are arranged independently.
Anaphase I
  • Spindle fibres pull the homologous chromosomes to the opposite poles.
  • Each chromosome still consists of two sister chromatids.
Telophase I and cytokinesis
  • The chromosomes arrived at both poles.
  • Each pole now has a haploid daughter nucleus because it contains one set of chromosome.
  • Spindle fibres disappear, nucleolus and nuclear membrane reappear.
  • Produce two haploid daughter cells.
  • Each daughter cell receives one chromosome from the homologous pair.
 
Meiosis II:
 
  1. The process is similar to mitosis.
  2. Produce four haploid cells which are genetically different from one another and parent cell.
 
Effects of Uncontrolled Meiosis:
 
  1. Abnormal meiosis leads to non-disjunction of chromosomes.
  2. Non-disjunction chromosomes lead to Turner's syndrome, Klinefelter's syndrome, XYY males and XXX females.