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Meiosis is defined as a process in which a single cell divides twice to produce four cells consisting of half the original amount of genomic information. During meiosis each cell divides twice to form four daughter cells. These cells possess half the number of chromosomes of the parent cells i.e. they are haploid. Meiosis produces sex cells.
Cell Division
A single parent cell divides into two daughter cells, then this is known as cell division. The two daughter cells are similar to one another and to the parent cell as well.
Cell division occurs when a parent cell splits into two or more daughter cells, which are termed as daughter cells. It usually happens as part of a bigger cell cycle. Each parent cell divides into two daughter cells, resulting in the reproduction of the entire cell.
Phases of cell Cycle
There are two phases of cell cycles which are given below.
Interphase
Mitosis
Meiosis
Before the beginning of meiosis, in the S phase of the cell cycle DNA present in each chromosome is replicated so that it contains two genetically identical sister chromatids which are held together via sister chromatids cohesion. This S phase is known as “premeiotic S phase” or “meiotic S phase”. Immediately after DNA replication meiotic cells enter into a G2 like stage by the name meiotic prophase. At this time homologous chromosomes pair with each other and undergo a programmed process known as genetic recombination, where the DNA may be cut and is repaired so that it can exchange some of its genetic information. A set of recombination events results in crossovers that lead to the formation of physical links referred to as chiasmata between homologous chromosomes. In various organisms these crosslinks help each pair of homologous chromosomes to segregate away from each other during Meiosis I and thus, resulting in the formation of two haploid cells that contain half the number of chromosomes as that of the parents.
During meiosis II the cohesion that exists between two sister chromatids segregates from one another just like in mitosis. Whereas sometimes all four of the meiotic products result in the formation of gametes like sperms, spores or pollens. As the number of chromosomes is halved during meiosis the resulting gametes can fertilize to form a diploid zygote containing two copies of each chromosome, one from each of the parents. Thus sexual reproduction is enabled by the alternating cycles of meiosis and fertilization, resulting in successive generations which maintains the same number of chromosomes. Like a diploid human cell comprises 23 pairs of chromosomes along with 1 pair of sex chromosomes which is half of maternal origin and half of paternal origin. Meiosis produces only haploid gametes (i.e. sperms and ovum) having one set of 23 chromosomes. Meiosis can be seen in all sexually reproducing single celled or multicellular organisms including plants animals and fungi.
Phases of Meiosis
Meiosis is divided into two meiotic phases meiosis I and meiosis II and further it is divided into Karyokinesis I and Cytokinesis I followed by Karyokinesis II and Cytokinesis II. Only the preparatory steps that lead to meiosis are similar to the interphase of the mitotic cell cycle. Interphase is followed by meiosis I and meiosis II.
Meiosis I
It refers to the segregation of homologous chromosomes which are joined as tetrads resulting in two haploid cells. Since the ploidy level decreases from diploids to haploids then meiosis I is referred to as a reductional division. Whereas meiosis II is an equational division and is analogous to mitosis where the sister chromatids segregate to form four haploid daughter cells.
Prophase I
The DNA present in the cell was copied, leading to the formation of two identical sets of chromosomes. These copied chromosomes condense to form X shaped structures that are easily visible under a microscope. Each of the chromosomes possess two sister chromatids that contain identical genetic information. Chromosomes pair up so that both the copies of the chromosomes are together. These pairs of chromosomes exchange DNA via a process known as recombination or crossing over. During the end of Prophase I the membrane around the nucleus in the cell dissolves away, thereby releasing chromosomes. The meiotic spindle containing microtubules and some other proteins extends across the cells between the centrioles. Prophase I is further divided into five steps namely:- Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
Metaphase I
The chromosome pairs line up next to each other along the entire length of the equator of the cell. The centrioles now lie at the opposite poles of the cell along with meiotic spindles extending from them. These meiotic spindle fibres attach to one chromosome of each pair.
Anaphase I
The pairs of chromosomes move apart via the meiotic spindle that pulls one chromosome towards one pole of the cell and the other chromosome towards the opposite poles. In meiosis I the sister chromatids remain together, this is different from mitosis and meiosis II.
Telophase I and Cytokinesis I
The chromosomes are present at the opposite poles of the cell. At each pole of the cell a whole set of chromosomes gathers there. A membrane is formed around each set of chromosomes which creates two new nuclei. The cell then makes a furrow in the middle to form two separate daughter cells each comprising a full set of chromosomes inside a nucleus. This process is known as Cytokinesis.
Meiosis II
This second meiotic division generally involves equational segregation or separation of sister chromatids. Though the genetic results are fundamentally different, the process of meiosis II is quite similar to mitosis. This results in the formation of four haploid cells from two of the haploid cells that were produced in meiosis I. Furthermore meiosis II is divided into different phases.
Prophase II
This phase is marked by the disappearance of nucleoli and nuclear envelope along with the shortening and thickening of the chromatids. The centrioles move towards the polar regions and arrange the spindle fibres for the second meiotic division.
Metaphase II
A membrane is formed around each set of chromosomes which creates two new nuclei. The cell then makes a furrow in the middle to form two separate daughter cells each comprising a full set of chromosomes inside a nucleus. This process is called Cytokinesis.
Anaphase II
Metaphase II is followed by anaphase II. During anaphase II the sister chromatids are pulled towards the opposite poles via the action of the meiotic spindle. These separated chromatids are now referred to as individual chromosomes.
Telophase II and Cytokinesis II
At each pole of the cell a full set of chromosomes are gathered together. A membrane is formed around each set of chromosomes which creates two new nuclei. This represents the last phase of meiosis, whereas cell division is not complete without another round of division of cytoplasm i.e. the cytokinesis. As cytokinesis is completed there are four sets of chromosomes each with a half set of chromosomes. In males all these four cells form the sperms, whereas in females one cell is an egg cell while the other three are polar bodies.
Importance of Meiosis
It plays a vital role in formation of sex cells or gametes which is responsible for sexual reproduction.
It is involved in activating the genetic information for the development of sex cells and deactivates sporophytic information.
Independent assortment of maternal and paternal chromosomes can be seen during meiosis.
It also results in genetic mutation due to irregularities in cell division via meiosis. The mutation is carried via natural selection.
Crossing over results in a new combination of traits and variation.
Conclusion
Meiosis is very essential in life as it produces sex cells for continuation of life processes on earth. This process may take about minutes or hours depending upon the kinds of cells. It is influenced by temperature and chemicals.
To conclude we can say that meiosis is divided into two phases- meiosis I and meiosis II, which further possess subdivisions like prophase, metaphase, anaphase, telophase and cytokinesis. We hope that this article was helpful in clearing all your doubts regarding meiosis.
