Meiosis and Genetic Variation

Overview of Meiosis

• Meiosis: A process where a diploid cell produces four haploid gametes, each with individual chromosomes.
• Two rounds of nuclear division:
  • Meiosis I: Separation of homologous chromosomes.
  • Meiosis II: Separation of sister chromatids.

Preparatory Phase

• Interphase: Chromosomes and organelles are copied, but chromosomes are not visible as distinct structures.

Stages of Meiosis I

1. Prophase I

   • Chromosomes condense and become visible.
   • Homologous chromosomes link through chiasmata, forming bivalents.
   • Crossing over may occur, exchanging alleles.
   • Nuclear membrane breaks down; spindle apparatus forms as centrioles move to opposite poles.

2. Metaphase I

   • Homologous chromosome pairs align on the equator of the spindle apparatus.

3. Anaphase I

   • Spindle fibers shorten, homologous chromosomes move to opposite poles.
   • Chiasmata break between homologous chromosomes.

4. Telophase I

   • Chromosomes reach poles; nuclear membranes reform.
   • Chromosomes uncoil to chromatin state; cytokinesis occurs.
   • Result: Two haploid cells.

Stages of Meiosis II

1. Prophase II

   • Chromosomes condense; nuclear membrane breaks down.
   • Spindle fibres develop.

2. Metaphase II

   • Chromosomes align on equator of spindle apparatus.

3. Anaphase II

   • Centromeres divide; spindle fibers shorten.
   • Chromatids pulled to opposite poles.

4. Telophase II

   • Chromatids (now chromosomes) reach poles.
   • Nuclear membranes reform; chromosomes uncoil to chromatin state.
   • Cytokinesis produces two haploid cells.
   • Result: Four genetically unique haploid cells.

Genetic Variation in Meiosis

• Reduction Division: Meiosis reduces chromosome number from diploid to haploid.
• Genetic Variation Sources:
  • Crossing Over: Exchange of alleles during Prophase I.
  • Independent Assortment: Random alignment of homologous chromosomes during Metaphase I.
    • Example: With 23 chromosome pairs, 2^23 (over 8 million) potential gametes.

Additional Genetic Variation Factors

• Random Fusion of Gametes:
  • Male and female gametes fuse randomly during fertilization, adding genetic variability.

Conclusion

• Meiosis involves complex stages leading to genetic diversity.
• Key mechanisms for genetic variation include crossing over, independent assortment, and random gamete fusion.