Meiosis

Unlike mitosis, meiosis does not occur in the somatic cells. Instead, it occurs in the gametes. The doubling of the gametic chromosomes occurs due to meiosis. It occurs in two stages. The first meiotic stage leads to the reduction of the chromosome number from the diploid to haploid. The second meiotic division is the same as the mitotic division. A single chromosomal duplication precedes the two divisions. Meiosis also consists of stages such as prophase-I, metaphase-I, anaphase-I, and telophase-I in its meiosis-I stage. The meiosis-II stage also consists of prophase-II, metaphase-II, anaphase-II, and telophase-II. Let us discuss each of the phases in details.

Meiosis-I		Meiosis-II
Prophase-I	Leptonema or leptotene	Prophase-II
	Zygonema or zygotene
	Pachynema or pachytene
	Diplonema or diplotene
	Diakinesis
Metaphase-I		Metaphase-II
Anaphase-I		Anaphase-II
Telophase-I		Telophase-II

Table: Different stages involved in Meiosis-I and Meiosis-II

Meiosis-I:

Following are the subphases of meiosis-I:

Image: Different stages of Meiosis-I and Meiosis-II

Prophase-I:

It shows a similarity with the mitotic prophase. However, a slight difference in the substages makes the prophase-I different from the mitotic prophase. The meiotic prophase-I consists of five substages such as leptotene, zygotene, pachytene, diplotene, and diakinesis. The leptotene stage involves the coiling of the chromosomes. It helps in committing the cell to enter the meiosis. The chromosomes look like threads in leptotene. They get an orientation of the bouquet. It is known as the bouquet configuration. Each chromosome in the leptotene looks like a single chromosome. The pachytene stage reveals the two chromosomes. The DNA replication occurs well before the leptotene stage.

The zygonema stage or the zygotene is the early mid-phase of the prophase-I. The chromosomes get shortened in this stage. The chromosomes in the stage are known as homologous chromosomes. The pairing of these chromosomes occurs only in the meiosis. It does not occur during mitosis. Mitotic recombination occurs very rarely.  The homologous chromosomes are a pair of essentially identical chromosomes. They, later on, involve synapsis. The genes in the homologous chromosomes belong to the common ancestor. Hence, they get retained. Synapsis involves a point-by-point pairing of the homologous chromosomes. It occurs mainly during the zygonema stage. Mainly the dipteran tissues such as the Drosophila salivary glands undergo synapsis. Parallel, dense elements showing lateral position surround the medial complex. These elements form a ribbon-like tripartite structure. This structure is known as a synaptonemal complex. It occurs in the central axes of the paired homologous chromosomes present in a pachytene bivalent. Synaptonemal complex helps in maintaining the parallel configuration of the lateral elements.

Synapses also play a crucial role in forming the zipper-like structures along the length of the chromatids. It also reduces the chromosomal threads into the half. Their appearance becomes like bivalents instead of a single chromosomal look. The bouquet-like arrangement also involves the role of telomeres (chromosomal ends). The pachytene stage, each paired chromosome separates into two sister chromatids. Exceptions include the centromeric regions. The division occurs longitudinally. It forms chromatin tetrads. Meaning, four chromatids occur due to the longitudinal division. Next step involves a localized breakage. It exposes the non-sister chromatids. These non-sister chromatids exchange the genetic material with each other.

The process of exchange is known as the crossing over. It plays a crucial role in giving a different set of alleles to the progeny, slightly differing from the parental alleles. Hence, it gives rise to the new genetic material (recombinants). The disassembly of the synaptonemal complex commits the cell to the diplotene stage. Each pair of sister-chromatids in a tetrad start separating in this phase (except the places of exchange). The cross-shaped structure arising due to the overlapping chromatids is known as chiasmata. The process of terminalisation brings the chiasma towards the end of the tetrads. Diakinesis involves tight coiling of the chromosomes. The nucleolus and the nuclear envelope disappear. The first meiotic division produces two secondary gametocytes containing the dyads.

 Metaphase-I:

It involves a complete breakdown of the nuclear envelope. It shows an alignment of the bivalents on the equatorial plane. It gives rise to spindle formation followed by the microtubule attachment.

Anaphase-I, Telophase-I, and cytokinesis:

In the anaphase, the chromosomal disjoining occurs. The chromosomes migrate to the opposite poles, thereby moving the centromeres. However, the centromeres do not get separated from their sister chromatids. They remain attached. It leads to the formation of a new nuclear envelope. Next, the cell divides into two through a process known as cytokinesis.

Meiosis-II:

The second meiotic division mimics the mitotic division. The chromosomes start getting condensed in the prophase-II. The chromosomes get aligned on the equator in the metaphase-II. The centromeres split, moving the chromatids apart on the opposite poles. The anaphase-II plays a crucial role in proper centromere splitting. After the telophase-II, the microscopic observations reveal well-defined chromosomes.

Gene segregation:

Half the numbers of chromosomes occur in a haploid cell. Thus, it is a result of meiosis. A diploid cell enters the meiotic phase and results in haploid cells. Two meiotic divisions occur after the DNA replication which occurs only once (in the S phase). However, the haploid nuclei fuse together if the diploid nuclei are required. Thus, meiosis helps in maintaining the chromosome number. The chromosomes aligning at the equatorial plane may either be paternally derived ones or maternally derived ones. There involves no restriction in the alignment of the chromosomes. Thus, nuclei derived out of meiosis contain a combination of both the paternal and the maternal chromosomes. The process of variation leads to variation. It gives rise to recombinants.

Meiotic drive is a condition in which a meiotic division gives rise to an unequal recovery of the gametes produced by a heterozygote. It involves an intragenomic conflict.

References:

[1] The Cell, Bruce Alberts

[2] All About Mitosis and Meiosis, Elizabeth Cregan

[3] Mitosis and Meiosis, Part 1

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