Meiosis Cell Division: Stages and Purpose

Meiosis: Cell Division for Reproduction

Meiosis is a type of cell division involved in sexual reproduction. Its purpose is to produce reproductive cells called gametes, which have half the number of chromosomes as normal somatic cells (they are haploid). Meiosis consists of two consecutive divisions. Starting with a single diploid cell, four haploid daughter cells are obtained. These daughter cells are genetically different due to the exchange of information between homologous chromosomes. Meiosis occurs in the reproductive organs or gonads: ovaries and testes in animals, and ovaries and anthers in plants.

Meiosis I

Prophase I

Meiosis begins with steps similar to mitosis. The main difference is that homologous chromosomes pair up and become fully compacted. They then exchange genetic information (crossing over), transferring fragments from one chromosome to another. The number and location of chiasmata (the points where crossing over occurs) may differ in each meiosis.

Metaphase I

Homologous chromosome pairs remain together and align at the metaphase plate. They bind to the mitotic spindle fibers as pairs.

Anaphase I

The spindle fibers contract and pull the homologous chromosomes towards opposite poles of the cell. Unlike mitosis, entire chromosomes (each still consisting of two chromatids) are separated, not individual chromatids. One chromosome from each homologous pair moves to a different pole.

Telophase I

The chromosomes may not fully decondense, and a nuclear membrane may not fully reform. The nucleolus and Golgi apparatus typically do not reappear. This is often because the second meiotic division follows immediately, making these processes unnecessary. Cytokinesis usually occurs, dividing the cell into two haploid daughter cells.

Meiosis II

Prophase II

The second meiotic division begins. If a nuclear membrane formed in Telophase I, it disappears. Centrioles (if present) duplicate and move to opposite poles. Chromosomes remain condensed.

Metaphase II

Chromosomes align individually at the metaphase plate. They bind to the spindle fibers at their centromeres.

Anaphase II

The centromeres divide, separating the sister chromatids. Each chromatid is now considered a chromosome. Spindle fibers pull these individual chromosomes towards opposite poles of the cell.

Telophase II

Chromosomes reach the poles and begin to decondense into chromatin. Nuclear membranes reform around the chromosomes at each pole. The nucleolus reappears. Cytokinesis occurs, dividing each of the two cells from Meiosis I into two daughter cells, resulting in a total of four haploid daughter cells.

At the end of meiosis, four haploid cells are obtained. These cells are genetically different from one another and from the original diploid mother cell.