How does the first division of meiosis reduce chromosome number through separation of homologous chromosomes?

Sourav PanNovember 8, 2024

Answered

The first division of meiosis, known as meiosis I, is crucial for reducing the chromosome number from diploid (2n) to haploid (n) through the separation of homologous chromosomes. Here’s a detailed explanation of how this process occurs:

Overview of Meiosis I

Meiosis I consists of several stages: prophase I, metaphase I, anaphase I, and telophase I. The key events during these stages facilitate the reduction in chromosome number.

1. Prophase I

• Chromosome Pairing: Homologous chromosomes pair up and form structures called bivalents or tetrads. Each bivalent consists of four chromatids (two from each homolog) and undergoes a process called crossing over, where segments of DNA are exchanged between non-sister chromatids. This genetic recombination increases genetic diversity.
• Formation of Chiasmata: The points where crossing over occurs are called chiasmata, which help hold the homologous chromosomes together until they are separated later in meiosis.

2. Metaphase I

• Alignment at the Metaphase Plate: The bivalents align at the metaphase plate, with each homologous chromosome facing opposite poles of the cell. This arrangement is critical for the next phase as it sets up the random orientation of chromosomes.

3. Anaphase I

• Separation of Homologous Chromosomes: During anaphase I, the homologous chromosomes are pulled apart by spindle fibers attached to their kinetochores. Each member of a homologous pair moves to opposite poles of the cell. Importantly, sister chromatids remain attached at their centromeres during this phase, ensuring that each daughter cell will still contain two copies of each chromosome.
• Reduction in Chromosome Number: As a result of this separation, one complete set of chromosomes (each still composed of two sister chromatids) moves toward each pole, effectively halving the chromosome number in preparation for the next division.

4. Telophase I and Cytokinesis

• Reformation of Nuclear Envelope: After the chromosomes reach the poles, the nuclear envelope may reform around each set of chromosomes.
• Cytokinesis: This is followed by cytokinesis, where the cell divides into two daughter cells, each containing half the original number of chromosomes (haploid).

Summary

In summary, meiosis I reduces chromosome number through:

• The pairing and alignment of homologous chromosomes.
• The separation of these homologs during anaphase I while keeping sister chromatids together.
• The end result is two haploid daughter cells, each containing one set of chromosomes that are still in duplicated form (two sister chromatids per chromosome). This reduction is essential for maintaining genetic stability across generations during sexual reproduction.