Meiosis: Definition and Phases

The term meiosis means lessening since it is a reductional division. The meiosis is a special type of cell division of germ cells in sexually reproducing organisms used to produce the gametes, such as sperm or egg cells.

Meiosis is a cell division process that describes the division of germ cells, which comprises two nucleus fissions. The nucleus's two fissions, meiosis 1 and meiosis 2, produce four sex cells known as gametes. The number of chromosomes in each sex cell is half that of the parent cell. This kind of cell division happens during reproduction, when one cell divides into four haploid daughter cells. The nucleus splits into four daughter cells (nuclei).

Read More: Stages of the cell cycle

Keyterms: Meiosis, Germ cell, Cell division, Sexually reproducing organisms, Gametes, Sperm, Egg cell, Nuclei, Nucleus, Meiosis 1, Meiosis 2, Haploid daughter cell

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What is Meiosis?

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It is a process in which a single cell, a diploid cell, undergoes division twice to produce four haploid daughter cells, and the cells produced are known as the sex cells or gametes (sperms in males and egg in females). It contains half of the original amount of genetic information. The haploids only have half the number of chromosomes of the parent cell.

Meiosis

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Phases of Meiosis

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Meiosis can be divided into nine stages. The process is usually divided into two parts. The first time a cell divides (meiosis I) and the second time it divides (meiosis II). The phases are as follows –

Read More: Mitosis and Meiosis

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Meiosis I

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• Interphase – The DNA present in the parent cell is copied resulting in the formation of two identical full sets of chromosomes. Each of the two centrosomes present outside of the nucleus contains a pair of centrioles, these structures are critical for the process of cell division. During this phase, the microtubules extend from these centrosomes.
• Prophase I – In this phase, the chromosomes begin to condense into X-shaped structures that can be easily seen under a microscope. Two sister chromatids containing identical genetic information are present in each chromosome. The chromosomes then pair up. Both the copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on with the other chromosomes. In a process called recombination or crossing over, the pairs of chromosomes may then exchange bits of DNA. The meiotic spindle which consists of microtubules and other proteins is found to be extended across the cell between the centrioles. At the end of this phase, the membrane around the nucleus in the cell dissolves away and the chromosomes are released. Prophase I may be divided into five different stages – Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

Meiosis I

• Metaphase I – The homologous pairs of chromosomes line up next to each other along the centre (equatorial plate) of the cell. The centrioles now shift at the opposite poles of the cell with the meiotic spindles found to be extended from them. In each pair, the meiotic spindle fibres remain attached to one chromosome.
• Anaphase I – The homologous pairs of chromosomes are pulled apart by the meiotic spindle on the opposite poles. The sister chromatids in this phase remain attached to each other which is different from what happens in mitosis and meiosis II.
• Telophase I and Cytokinesis I – In this phase, the chromosomes completely move to the opposite poles of the cell and at each of the poles a full set of chromosomes gather together. The nuclear membrane reforms around each set of chromosomes to create two new nuclei and the chromosomes also decondense. Two separate haploid daughter cells are formed from the single-cell when the latter pinches in the middle to initiate division. These daughter cells contain a full set of chromosomes within a nucleus.

Telophase I and Cytokinesis I

Read More: Mitosis Stages

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Meiosis II

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• Prophase II – There are now two daughter cells, each with 23 chromosomes (23 pairs of chromatids). The chromosomes present in each of the two daughter cells condenses again into visible X-shaped structures that can be easily seen under a microscope and the nuclear envelope in each daughter cell dissolves away releasing the chromosomes. The centrioles also duplicate and the meiotic spindle forms again.
• Metaphase II - The non-homologous chromosomes (pair of sister chromatids) in each of the two daughter cells line up end-to-end along the equator (equatorial plate) of the cell. The centrioles present in each of the daughter cells are now at opposites poles and the meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.
• Anaphase II – The sister chromatids move to opposite poles due to the action of the meiotic spindle and the separated chromatids now become individual chromosomes after they are pulled apart.
• Telophase II and Cytokinesis II – In this phase, the chromosomes completely move to the opposite poles of the cell. At each of the poles of the cell, a full set of chromosomes gather together. The nuclear membrane reforms around each set of chromosomes to create two new nuclei and the chromosomes also decondense and the nuclear membrane reforms. Thus the cell finally divides into four haploid daughter cells. Though this is the last phase of meiosis, the cell division is not complete without another round of cytokinesis.

Meiosis II

At the end of the cytokinesis and the process of meiosis, there are four granddaughter cells, each with half a set of chromosomes (haploid). In the case of males, all these four cells are called sperm cells while in the case of females one of the cells is an egg cell while the other three are polar bodies i.e. small cells that do not develop into eggs.

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Things to Remember

• Meiosis is a process in which a single cell divides twice to generate four cells with half the amount of genetic material as the original cell. These cells are called sperm and eggs, respectively, in men and females.
• Meiosis is divided into two phases.
• Haploids are organisms or gametes that have just one copy of each gene in each cell.
• If each cell has two copies of each gene, then it is known as Diploid.