Introduction

Meiosis is a type of cell division that reduces the chromosome number by half, producing four haploid daughter cells from a single diploid parent cell. It occurs in sexually reproducing organisms to generate gametes (sperm and egg cells in animals, pollen and ovules in plants). Unlike mitosis, which produces identical daughter cells, meiosis introduces genetic variation through recombination and independent assortment.

Phases of Meiosis

Meiosis consists of two consecutive divisions: Meiosis I (reductional division) and Meiosis II (equational division). Each division has four phases: prophase, metaphase, anaphase, and telophase.

Meiosis I (Reductional Division)

This division reduces the chromosome number from diploid (2n) to haploid (n).

1. Prophase I

• Chromosomes condense and pair up with their homologous counterparts in a process called synapsis.
• Crossing over occurs at chiasmata, where homologous chromosomes exchange genetic material, increasing genetic variation.
• The nuclear membrane dissolves, and spindle fibers begin to form.

2. Metaphase I

• Homologous chromosome pairs align at the cell’s equator.
• Independent assortment occurs, randomly distributing maternal and paternal chromosomes to daughter cells.

3. Anaphase I

• Homologous chromosomes separate and move to opposite poles, reducing the chromosome number.
• Sister chromatids remain attached at the centromere.

4. Telophase I and Cytokinesis

• Chromosomes reach the poles, and a new nuclear membrane may form temporarily.
• Cytokinesis follows, dividing the cytoplasm to produce two haploid daughter cells.

Meiosis II (Equational Division)

This division resembles mitosis and separates sister chromatids.

1. Prophase II

• Chromosomes condense again, and spindle fibers reappear.

2. Metaphase II

• Chromosomes align at the equator, similar to mitosis.

3. Anaphase II

• Sister chromatids separate and move to opposite poles.

4. Telophase II and Cytokinesis

• Chromosomes reach the poles, nuclear membranes reform, and cytokinesis occurs.
• Four haploid daughter cells are produced, each with a unique genetic combination.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Formation of gametes for sexual reproduction
Number of Divisions	One	Two
Number of Daughter Cells	Two	Four
Genetic Variation	None (identical cells)	Present (crossing over and independent assortment)
Chromosome Number	Diploid (2n) → Diploid (2n)	Diploid (2n) → Haploid (n)

Significance of Meiosis

• Genetic Variation: Crossing over and independent assortment create genetic diversity.
• Stable Chromosome Number: Prevents chromosome doubling in sexually reproducing organisms.
• Gamete Formation: Produces haploid cells necessary for fertilization.

Conclusion

Meiosis is essential for sexual reproduction and genetic diversity. Through two consecutive divisions, it ensures that offspring inherit a mix of parental traits while maintaining the correct chromosome number across generations.