3.3 Meiosis Notes

Introduction

Meiosis is a specialized form of cell division that reduces the chromosome number by half, resulting in the production of haploid cells from diploid cells. This process is essential for sexual reproduction in eukaryotic organisms, as it ensures genetic diversity through recombination and independent assortment. Meiosis occurs in two sequential stages: Meiosis I and Meiosis II, each with its own sub-stages.

Overview of Meiosis

Key Points:

• Meiosis results in four haploid cells from one diploid parent cell.
• It involves two rounds of cell division: Meiosis I and Meiosis II.
• Each division has four stages: Prophase, Metaphase, Anaphase, and Telophase.
• Meiosis introduces genetic variation through crossing over and independent assortment.

Occurrence:

• In animals, meiosis occurs in the testes (sperm production) and ovaries (egg production).
• In plants, meiosis occurs in the anthers (producing pollen) and ovaries (producing ovules).

Stages of Meiosis I

Meiosis I is known as reduction division because it reduces the chromosome number by half. This stage is more complex and introduces genetic variation.

Prophase I

• Chromosome Condensation: Chromosomes condense and become visible as two sister chromatids joined at the centromere.
• Formation of Bivalents: Homologous chromosomes pair up to form bivalents (also called tetrads).
• Crossing Over: Non-sister chromatids exchange genetic material at points called chiasmata, introducing genetic variation.
• Spindle Formation: Centrioles move to opposite poles and spindle fibers form.
• Nuclear Envelope Breakdown: The nuclear envelope disintegrates.

Metaphase I

• Alignment at Equator: Bivalents align at the equator of the spindle.
• Independent Assortment: The orientation of each bivalent is random, leading to independent assortment of chromosomes.

Anaphase I

• Separation of Homologous Chromosomes: Microtubules pull homologous chromosomes to opposite poles.
• Centromeres Do Not Split: Unlike mitosis, the centromeres remain intact.

Telophase I

• Chromosomes Reach Poles: Chromosomes arrive at opposite poles.
• Nuclear Envelope Reformation: Nuclear envelopes may form around the two sets of chromosomes.
• Cytokinesis: The cell divides into two haploid cells.

Stages of Meiosis II