Reproductive cells or germ cells divide by meiosis to produce reproductive units called gametes and spores. Meiosis allows the chromosome number of the parent cell to get reduced to half in gametes and spores.
As there is reduction in the chromosome number due to meiotic division, the division is also called as reductional division.
Like mitosis, the nuclear division (karyokinesis) of meiosis precedes the division of the cytoplasm (cytokinesis). The nuclear division of meiosis is comparatively an elaborate phenomenon and requires more time and precision. Meiosis comprises of two successive divisions. The first division is reductional, known as meiosis I and the second division is equational, known as meiosis II.
Like mitosis, meiosis I is divided into prophase I, metaphase I, anaphase I and telophase I.
Unlike prophase of mitosis, many complex events take place in prophase I of meiosis. Depending on the behaviour of chromosomes, this phase is divided into five sub-stages), which are described in the following section.
(a) Leptotene (Leptonema): Thin thread stage
Progressive condensation of chromatin is marked due to coiling of chromatin fibres. Chromatins in this stage are seen as thin threads so named as leptonema. Nuclear membrane is distinct.
(b) Zygotene (Zygonema): Pairing stage
A diploid cell has a nucleus having pairs of similar chromosomes known as homologous chromosomes. During this sub stage such homologous chromosomes come together and pair through a cementing substance known as synaptonemal complex. These are called bivalents. Such pairing produces yoked thread, so the stage is known as pairing stage or yoked thread stage. The phenomenon of pairing is known as synapsis.
(c) Pachytene (Pachynema): Thick thread stage
The paired chromosomes continue to condense. They become thicker. The end point of chromosomes can be traced. The paired chromosomes are called bivalents.
Two sister chromatids of each homologue become visible and thus each bivalent is comprised of four chromatids forming a tetrad. Breakage and reunion of segments of non-sister chromatids occur allowing the exchange of genetic material. This process is known as crossing-over.
The region of crossing over is known as chiasma (chiasmata-plural). Pachynema stage is also known as thick thread stage.
(d) Diplotene (Diplonema):
In this stage, there is further condensation of the homologous pairs. Chiasmata become more distinct. The two chromosomes of each bivalent move away from each other and chiasmata start moving towards the termini. The two chromosomes form large loop by the way of being separated farther and farther. This process is known as terminalisation. Each bivalent thus is marked as two threads. Therefore, this stage is also known as double thread stage.
The nucleolus and nuclear membrane disappear. Chromosomes become well spread in the total volume of the cytoplasm and spindle fibres start organising at two poles or centrioles.
Spindle formation is completed in metaphase I. The bivalents have arranged themselves at the equatorial plate of the spindle. The imaginary plate is also known as metaphase plate the centrosome of each chromosome of the bivalent orients.towards opposite poles. The spindle fibres of respective poles are attached to the nearest centromeres of the chromosome.
The homologous chromosomes of each bivalent separate from each other and move towards poles. The two chromatids of moving chromosomes are found to remain joined with each other at the centromere. Thus, one chromosome of each homologous pair moves and reaches the pole. Each pole thus contains half the number of chromosomes present in the parent cell. If one considers the chromosome number of the cell as diploid, the chromosome set at the pole is haploid.
In this phase the spindle fibres disappear and the nuclear membrane is formed around the chromosomes at each pole. Thus, pair of daughter nuclei is formed within the cell.
Meiosis I may or may not be followed by cytokinesis. In animals, cleavage occurs after telophase forming two daughter cells. In some plants, cell wall formation is marked after telophase I.
Interphase after meiosis I is marked in animal cells. Chromatids usually uncoil to certain extent. But in this phase there is no S phase. In many plants, there is no distinct telophase after meiosis I. Rather, the cell passes from late anaphase I to prophase of meiosis II.
Meiosis II is equational and is divided into prophase II, metaphase II, anaphase II and telophase II.
The chromatids begin to condense. The nuclear membrane and nucleolus disappear. Spindle fibres start to orient at poles. In case of cells having centriole, the centrioles, if any in the cell, divide into two pairs and then each pair moves towards each pole. The spindle fibres are formed and each is atached to the centromere of a chromosome. The length of this phase is invariably proportional to the length of telophase I.
Condensed chromosomes (sister chromatids) are arranged at the equatorial region of the spindle. Each chromosome is with two sister chromatids held together at the centromere. The centromere divides and structurally becomes double to which spindle fibres are attached.
The structurally double centromeres separate at this stage. The sister chromatids start moving towards the poles due to the contraction of the spindle fibres. Now each sister chromatid can be recognised as a daughter chromosome.
Significance of Meiosis:
Maintenance of constant chromosome number:
Meiosis leads to the formation of four haploid cells from a diploid cell. Such haploid cells give rise to gametes. In sexual reproduction haploid male and female gametes fuse to form diploid zygote which gives rise to the embryo and then the organism. Thus diploid level of the organism is maintained.
In meiosis, there is a phenomenon of crossing over which results in the exchange of chromosome segments. This leads to genetic variation, which eventually leads to natural selection and origin and evolution of new species.