Different Stages in the Process of Meiosis

Meiosis is a type of cell division that occurs only in eukaryotes (organisms with membrane-bound cell organelles). The process of meiosis is exhibited by higher forms of organisms, which reproduce sexually. In plants, meiosis is observed after spore production; whereas, in animals, meiosis takes place during gamete (sperm and egg) formation. Like other cell division, meiosis produces daughter cells.

However, there is a significant difference between meiosis and other types of cell division like mitosis or binary fission. In meiosis, the parent cell divides and produces four gametes that are not capable of further division; whereas, in other types of cell division, the parent cell produces identical daughter cells, which can undergo division on their own. Let's take a look at the different stages of meiosis.

Different Stages in Meiosis

There are two stages of meiosis, namely, meiosis I and meiosis II. The parent cell or the dividing cell undergoes a preparatory phase, known as interphase, before entering the two stages of meiosis. In the interphase, the parent cell synthesizes more DNA (deoxyribonucleic acid) and proteins, increasing the overall size and mass of the cell. As a part of the preparatory phase, the dividing cell duplicates or doubles its chromosomes. With these major changes, the parent cell enters the first stage of meiosis. Following is a brief discussion about the two stages and their phases.

Meiosis I: Meiosis I is divided into four phases, which are as follows:

Prophase I

• The chromosomes condense and migrate towards the nuclear envelope.
• Each chromosome is made up of two identical chromatids, known as sister chromatids.
• Formation of spindle fibers.
• Synapsis or pairing of homologous chromosomes takes place.
• The homologous chromosomes interchange equivalent sections of chromatids, which is a process known as crossing over.
• The chromosomes undergo thickening and move away from the nuclear envelope.
• The nuclear envelope and nucleoli dissolves.

Metaphase I

• Pairing of bivalents or homologous chromosomes take place in the equatorial plane, that lies in the center of the cell.
• The centromeres, a region in the chromosome where the chromatids are held together, are located in the opposite poles.

Anaphase I

• The chromosomes migrate to the opposite poles of the cell.
• The sister chromatids are not separated, but remain together.

Telophase I

• The chromosomes continue to migrate towards the poles.
• Both the poles have haploid number of chromosomes.
• Condensation of the chromosomes and cytokinesis (division of cytoplasm) take place.
• Nuclear envelope starts forming.
• Two daughter cells with haploid chromosome number are formed.

Meiosis II: Meiosis II comprises the following four stages:

Prophase II

• The nuclei and nuclear membrane are separated.
• The chromosomes start moving towards the equatorial plane.
• The two sister chromatids are still held by the centromere.

Metaphase II

• The chromosomes are aligned in the equator.
• The centromeres are oriented towards the opposite poles.

Anaphase II

• The sister chromatids held at the centromere are separated by the spindle fibers.

Telophase II

• Four nuclei (two each in a daughter cell) are formed, along with the process of cytokinesis.
• Each of the four nuclei develops a nuclear envelope.
• Four daughter cells or gametes are formed.

The resulting four gametes are haploid, meaning that they contain half the number of chromosomes. This is the reason as to why meiosis cell division is also referred to as reduction division. During fertilization, two gametes, one from the mother and another from the father, fuse, thus resulting in doubling of chromosome number. The fusion of gametes leads to the production of a zygote that has the same chromosome number of the parents. Variation occurs in the resulting zygote due to the process of meiosis and fertilization of gametes. Zygote after attaining maturity, is capable of dividing into daughter cells.