6.3 - Meiosis

Definition and Need for Meiosis

• Meiosis: Cell division in reproductive organs that produces gametes with half the chromosome number (haploid) of the parent cell (diploid).
• Occurs in the testes and ovaries of animals and humans.
• Necessity of meiosis:
  • Forms gametes through gametogenesis.
  • Maintains the diploid chromosome number from one generation to the next in sexually reproducing organisms.
  • Produces genetic variation within the same species.

Stages of Meiosis

• Consists of two successive cell divisions: meiosis I and meiosis II.

Meiosis I

• Prophase I:
  • Chromatin shortens and thickens to form visible chromosomes.
  • Homologous chromosomes pair through synapsis, forming a bivalent or tetrad of four chromatids.
  • Crossing over exchanges genetic material between non-identical chromatids and produces new gene combinations.
  • The crossing point is called a chiasma.
  • The nuclear membrane and nucleolus begin to disappear.
  • Centrioles move to opposite poles; spindle fibres form.
• Metaphase I:
  • Homologous chromosomes arrange at the equatorial plane.
  • One chromosome of each homologous pair attaches to spindle fibres from one pole; its homologue attaches to spindle fibres from the opposite pole.
  • Sister chromatids remain joined because the centromeres have not separated.
• Anaphase I:
  • Spindle fibres contract; homologous chromosomes separate and move to opposite poles.
  • Each chromosome still consists of two sister chromatids joined at the centromere and moves as one unit.
• Telophase I and cytokinesis:
  • Chromosomes reach opposite poles; each pole contains one haploid set of chromosomes.
  • Spindle fibres disappear; nucleoli and nuclear membranes reform.
  • Cytokinesis produces two haploid daughter cells.
  • The interphase before meiosis II is usually short; DNA does not replicate.

Meiosis II

• Prophase II:
  • Nucleoli and nuclear membranes disappear.
  • Each chromosome consists of sister chromatids joined at the centromere.
  • Spindle fibres form in both daughter cells.
• Metaphase II:
  • Chromosomes arrange randomly at the equatorial plane of each daughter cell.
  • Each chromatid attaches to a spindle fibre at the centromere.
  • Metaphase II ends when centromeres separate.
• Anaphase II:
  • Centromeres separate; sister chromatids move to opposite poles, centromeres first.
  • Each chromatid is now called a chromosome.
• Telophase II and cytokinesis:
  • Chromosomes reach the poles.
  • Spindle fibres disappear; nuclear membranes and nucleoli reform.
  • Cytokinesis produces four haploid daughter cells.
  • Each cell has half the parent cell’s chromosome number, differs genetically from the diploid parent cell, and develops into a gamete.

Comparison between Mitosis and Meiosis

Similarities

• Both involve nuclear division and cytokinesis.
• Both use spindle fibres to separate chromosomes or chromatids.

Differences

• Site: Mitosis occurs in somatic cells; meiosis occurs in cells of reproductive organs.
• Number of nuclear divisions: One in mitosis; two in meiosis.
• Synapsis and crossing over: Absent in mitosis; occur during prophase I of meiosis.
• Metaphase arrangement: Chromosomes arrange singly in mitosis; homologous chromosomes arrange in pairs during metaphase I.
• Separation: Sister chromatids separate during mitotic anaphase; homologous chromosomes separate during anaphase I and sister chromatids separate during anaphase II.
• Daughter cells: Mitosis produces two diploid, genetically identical daughter cells; meiosis produces four haploid daughter cells with genetic variation.