4.3.3 Meiosis

Key Features of Meiosis

1. Production of Haploid Gametes:

• Meiosis reduces the chromosome number by half, creating haploid gametes (sperm and egg cells in animals).
• This ensures that the diploid chromosome number is restored during fertilisation.

2. Genetic Variation:

• Meiosis introduces genetic diversity in offspring due to:
• Independent assortment of chromosomes: Homologous chromosomes are randomly distributed into daughter cells.
• Crossing over: Exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I.

Stages of Meiosis

Meiosis consists of two divisions:

1. Meiosis I (Reduction Division):

• Prophase I:
  • Chromosomes condense and become visible.
  • Homologous chromosomes pair up to form bivalents.
  • Crossing over occurs at points called chiasmata.
• Metaphase I: Bivalents align along the equator of the cell.
• Anaphase I: Homologous chromosomes are separated and pulled to opposite poles.
• Telophase I: Two haploid cells are formed, each containing half the chromosome number.

2. Meiosis II (Similar to Mitosis):

• Prophase II: Chromosomes condense again in each haploid cell.
• Metaphase II: Chromosomes align at the equator.
• Anaphase II: Sister chromatids are separated and pulled to opposite poles.
• Telophase II: Four haploid daughter cells are formed, each genetically different.

Key Outcomes of Meiosis

1. Produces four haploid cells, each containing half the chromosome number of the parent cell.
2. Introduces genetic variation through crossing over and independent assortment.