Meiosis | Definition, Process, Diagram & Significance

The term mitosis was coined by farmer and Moore in 1905. The division was studied by Van Benedin Winiwarter and Strassburger. Meiosis or meio is “to lessen”, hence meiosis is a double division in which a diploid cell form for haploid cell, each having half the number of chromosomes. In this division, a single parent cell (diploid) forms four daughter cell (haploid). Meiosis involves two sequential cycles of nuclear division but only one cycle of DNA replication due to which for haploid daughter cells are formed.

Occurrence: only the cells of sexually reproducing organisms undergo meiosis, and only special cells in the multicellular organism switch from mitosis to meiosis at specific time in the lifecycle.

An offspring produced by sexual reproduction involves fusion of two haploid gametes. These haploid gametes are formed by meiosis. Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organism whereas fertilization restores the diploid phase.

Meiosis consists of two divisions, i.e., Meiosis I and Meiosis II, which occurs in a sequential manner. Meiosis I is called reductional division because during this division, the chromosome number is reduced to half. Meiosis II is called equational division because during this division, the number of chromosome remains the same as produced at the end of meiosis I. Meiotic events can be group under following phases:

Meiosis I	Meiosis II
1. Prophase I 2. Metaphase I 3. Anaphase I 4. Telophase I	Prophase II Metaphase II Anaphase II Telophase II

A Meiosis I

It is the reductional division in which the number of chromosomes is reduced to half. It is studied in four stages:

1. Prophase I :

Prophase I is more elaborate, prolonged, complex than prophase of mitosis. The events occurring during prophase I are also slightly different than prophase of mitosis. The long and complicated prophase I is further subdivided into five stages, viz., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. We would study the details of these subdivisions.

i. Leptotene: Condensation and coiling of chromatin fibres begins during leptotene. The chromatin material condenses it to form distinct chromosomes which gradually become visible under the light microscope.

ii. Zygotene: Zygotene is the second stage of prophase I. It occurs after leptotene. A diploid cell contains two sets of chromosomes. The two chromosomes which are similar in form, size, structure are called homologous chromosomes. One of the homologous chromosomes is paternal chromosome and the other is maternal chromosome.

During zygotene, these homologous chromosomes start pairing together. These homologous chromosomes come to lie side by side in pairs and this pairing is known as synapsis. The complex formed by a pair of synapsed chromosome is called bivalent. Bivalent because two homologous chromosome form a pair.

Electron microscopic studies of this stage shows that chromosomal synapses is accompanied by a structure called Synaptonemal Complex which is thought to stabilize the two homologous chromosomes till the crossing over is completed.

iii. Pachytene: It is the third stage of prophase I. It occurs after zygotene and the following events occur during pachytene:

1. The synapsed chromosomes continue to become thick and short. The chromatids of the homologous chromosomes now become clearly visible as tetrad. The two chromatids of the same chromosome are called sister chromatids and two chromatids of the two different homologous chromosomes are called non-sister chromatids.
2. During pachytene, crossing over occurs between the non-sister chromatids of the homologous chromosomes. The exchange of genetic material (DNA) between the non-sister chromatids of the homologous chromosomes is known as crossing over.
3. Crossing over leads to recombination of genetic material which involves a mutual exchange of the corresponding segments of non-sister chromatids of homologous chromosomes. It takes place by breakage and reunion of chromatid segment.
4. The site where crossing over occurs forms a recombination nodule. The recombination is an enzyme-mediated process. An enzyme called recombinase is involved during this process.
5. By the end of pachytene, the recombination between the homologous chromosome is complete and the two chromatids are linked at the site of crossing over.

iv. Diplotene: Diplotene is the fourth stage of prophase I. During this stage, the crossing over is completed and the two homologous chromosome begin to separate from each other.

The following events occur during this phase:

1. The synaptonemal complex formed during the zygotene dissolves during diplotene. Therefore, the homologous chromosome separate except in the region of crossing over.
2. The point of attachment between the homologous chromosomes after dissolution of the synaptonemal complex is called chiasmata. It becomes visible during diplotene stage. The two homologous chromosomes begin to separate from each other but remain attached at the chiasmata. Chiasmata marks the sites where crossing over occurred during pachytene. The homologous chromosomes do not separate at chiasmata, and hence, they are seen as X-shaped structure.
3. In oocytes of some vertebrates, diplotene lasts for month or years. It is called dictyotene stage.

Diakinesis: It is the final stage of prophase I. It represents transition to metaphase I. The following events occur during diakinesis:

1. Spindle assembles to prepare homologous chromosomes for separation. It assembles at the poles, so that it separates the homologous chromosomes into two cells.
2. Terminalisation of chiasmata: Due to tight condensation of chromosomes, the chiasmata disappear from the chromosomes by slipping off or sliding from the tip of the chromosomes. The chiasmata moves or shift to the tip of the chromosome and from there it either slip off or remains at the tip.
3. Nucleolus disappears and the nuclear envelope disintegrates.

2. Metaphase I :

After completing prophase I, chromosomes enter metaphase I. The nuclear envelope disintegrates, hence the chromosomes move into the cytoplasm. Following events occur during metaphase:

1. The bivalent chromosomes align themselves on the equatorial plate. The centromere of the two chromosomes are arranged in two rows at the equator. So double metaphasic plate is formed.
2. The distributions of the bivalent chromosomes are at random. The two chromosomes can face either of the poles. There is no fixed direction in which paternal or maternal chromosome would face.
3. The microtubules of the spindle fibres from opposite poles attached to the centromere of the chromosomes facing towards it.

3. Anaphase:

Following events occur during anaphase I:

1. The homologous chromosomes separate from each other. They start moving towards the poles.
2. The intact chromosome for univalent containing two chromatids held together by a centromere separate and move towards the opposite poles due to spindle fibre attached to it. In this phase division at centromere does not occur.
3. Since only one chromosome out of a pair reaches the pole, the number of chromosome becomes half in the daughter cells. The reduction in the number of chromosomes occurs during anaphase I.

4. Telophase:

It is the final stage of reductional division, i.e., meiosis I. It is characterised by following events:

1. The chromosomes reach the poles. The spindle fibres completely disappear.
2. The nucleolus and nuclear membrane reappear.
3. The chromosomes and uncoil and elongate but remain straight in this phase. They do not reach the extremely extended state of the interphase nucleus.
4. It produces two daughter cells each containing a single nucleus. The nucleus of the daughter cell receives only one chromosome from each homologous pair and thus, it has half the number of chromosome but double the amount of nuclear DNA as both the chromatid move together to a single pole. The separation of these two chromatids occur during meiosis II.

Cytokinesis: It generally follows the first nuclear division, so two daughter cells are formed which are haploid.

Interkinesis or Intrameiotic Interphase

Interkinasis or intrameiotic interphase is a metabolic stage between telophase I and prophase II. It is a gap which exists between meiosis I and meiosis II. During this phase, the chromosomes are elongated but do not form chromatin fibres. There is no replication of DNA during this phase, but centrioles pairs replicate in animal cell. The RNA and protein required during meiosis II are synthesized during this phase.

B Meiosis II

Meiosis II is similar to mitosis, i.e., equational division, but not an exact copy of mitosis because mitosis occurs in diploid somatic cell but meiosis II always occurs in haploid germ cell. Mitosis is always followed by DNA replication, but meiosis II is not followed by DNA replication. The daughter cell formed after meiosis II are neither similar to each other nor similar to the parent cell.

The main event which occurs during meiosis II is the separation of the chromatid of the univalent chromosomes present in the daughter cell formed after meiosis I. The chromatids present in the univalent chromosome differ from each other due to crossing over. Meiosis II is divided into four phases, namely – prophase II, metaphase II, Anaphase II, Telophase II.

1. Prophase II : Prophase II is not long and complicated as prophase It is a short phase where the chromatids of the univalent condenses. The chromatin material again becomes compact. The nucleolus and the nuclear envelope disintegrate and disappear.
2. Metaphase II : The univalents, i.e., chromosomes align themselves at the equator, (on the equatorial plane) in the metaphase II. The microtubules from the opposite poles extend towards the equator and attaches at the kinetochore of the chromatid.
3. Anaphase II : it is the third phase of meiosis II. During this Phase, the centromere holding the two chromatids splits and allow the separation and movement of the two chromatids. Chromatids move to the opposite poles.
4. Telophase II : this is the last stage of meiosis II. During this phase, the chromatids reach the poles and start uncoiling. They decondense and become thin. The spindle fibres degenerate. The nuclear membrane and nucleolus reappear and four haploid nuclei are formed. The telophase II is now followed by cytokinesis which divides the cytoplasm and forms four individual haploid cells.

Cytokinesis

The cytoplasmic divide by forming a furrow in the animal cell and cell plate in the plant cell. After cytokinesis a single cell divides into two daughter cells. The daughter cells contain a single nucleus and their own cytoplasm. Daughter cells of meiosis I divides to form daughter cells and hence at the end of meiosis, i.e., after meiosis I and meiosis II, a diploid cell gives rise to four haploid daughter cells.

Significance of Mitosis

1. Formation of gametes : Meiosis produces gametes for sexual reproduction. Gametes are essential for sexual reproduction because in sexual reproduction an organism is formed by the fusion of two gametes.
2. Maintenance of chromosome number : Meiosis reduces the chromosome number to half in the gametes, so that fertilization restores the original diploid number in the zygote.
3. Introduction of variations : Meiosis provides a chance for the formation of new combinations of chromosomes. This brings out variations.