This type of division is restricted to only the sex organs. In plants meiosis occurs in the flowers in the anthers (male meiosis) and in the ovary (females’ meiosis) and in animals such divisions occur in male and female gonads.
Meiosis is relatively long (sometimes very long) and complex where single diploid (2n) cells are reduced to form haploid (n) cells or gametes. For example, if the somatic cells of Alliums cape have 16 chromosomes in the nucleus, the meiotic products shall have 4 cells with 8 chromosomes in each. All the sexually reproducing plants and animals have this type of cell division to produce gametes.
The gametes of opposite sex (male and female) unite to produce a zygotic cell, which forms the first cell of a sporophyte.
A plant or an animal has a fixed number of chromosomes in its nucleus and this number is maintained for generations through the process of meiosis, which reduce the number of chromosomes of the uniting gametes to half before their union.
The process of meiosis is completed through two major phase’s viz. Meiosis – I and Meiosis-II. Meiosis-I consumes a lot of time and major events occur in this phase.
This has been divided into prophase-I, metaphase-I, anaphase-I and telophase-I with prophase-I taking the longest duration among them. This rather elaborate pro-phase- I is sub-divided for convenience to Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis, based on the appearance of chromosomes during the progress of meiosis.
Each of the sub-stages has characteristic features of chromosomes, which are normally clearly visible under microscope. Prophase-I is quickly followed by metaphase, anaphase and telophase in succession to wind up the process of Meiosis-I resulting in two cells. After a short gap, the cells enter the second phase of division, the Meiosis-II that is more or less similar to mitosis.
The end result is formation of 4 haploid cells from every meiocyte. Like mitosis meiosis -I is initiated only after S phase where the parental chromosomes are replicated to produce identical sister chromatids. But the pattern of chromosome segregation in meiosis-I is dramatically different from that in mitosis. The sequence is as follows:
Leptotene (“Thin thread”)
This is marked by an increase in nuclear volume. The chromosomes start to appear clearly as a result of condensation and coiling of chromatin fibers.
The appearance of chromosomes here are like thin threads and hence the name. In some cases, there are certain deeply stained regions along the length of the chromosome and such regions are called chromomeres’. During this stage the nucleolus enlarges as RXA synthesis continues.
Zygotene (“Yoked thread)
During this stage, the homologous chromosomes (one paternal and one maternal), which are of identical size and structure pair in such a way that there is exact alignment of portions of chromosomes including the genes.
A cementing substance made of proteins help binding the pair of chromosomes tightly. This pairing is called ‘synapsis’ and the cementing material is called ‘synaptonemal complex’. The chromosomal pairs are called ‘bivalents’ whose number corresponds to the haploid chromosome number of any species.
Pachytene (Thick thread)
In this stage, the bivalents become more condensed and therefore thicker. In addition, each chromosome becomes doubled and the bivalent turns to a ‘tetravalent’ with two paternal sister chromatids and two maternal sister chromatids. Also in this stage, exchange of genetic material occurs.
This is made possible by the formation of a few breaks at identical site in the chromatids. The broken segments then reunite with or without exchanges of chromatid segments. The process of exchanges of segments between non-sister chromatids is of great genetic consequence since it results in reshuffling of parental genes which are present in the chromatids. This process is also called ‘crossing over’.
Diplotene (double thread)
With the condensation process still continuing, the tetravalent appear thicker. Now, the pairing homologous units undergo a process of repulsion and start moving apart. However, two non-sister chromatids are held united at the points of exchanges. Such points are called chiasmata. It gives a characteristic chromosomal configuration as given in the diagram.
Diakinesis (double ending)
This stage is the last of pairing process and the end of prophase I. Here the chromosomes appear very thick and they move away from each other and spread towards periphery.
The chiasmata found at the points of crossing over between chromatids undergo a process called terminalization whereby some departing chromosomes are found held up only terminally as shown in the diagram.
This stage is also marked by the breakdown of the nucleolus and nuclear membrane and organization of spindle at the poles.
Here the bivalent chromosomes align at the equatorial plate vegion of the spindle. In contrast to mitosis, the centromeres of sister chromatids are adjacent to each other and oriented in the same direction, while the centromeres of homologous chromosomes are pointed towards opposite spindle poles.
So, the microtubules from the same pole the spindle attach to sister chromatids, while microtubules from opposite poles attach to homologous chromosomes.
This phase is initiated after complete disruption of the chiasmata at which homolog chromosomes are joined. The homologous chromosomes then separate while sister chromatids remain joined at the centromeres.
Telophase – I
This marks the completion of meiosis and formation of two daughter cells that ha acquired one number of each homologous pair consisting of two sister chromatids. Meiosis-II initiates immediately after cytokinesis (in some cases no cytokinesis occur at this stage), before chromosomes have fully descended. In contrast to meiosis-I, this is just another mitosis with prophase-II, metaphase-II, Anaphase-II and Telophase II.
At metaphase-II, the chromosomes align on the spindle with microtubules fro opposite poles of the spindle attached to the centromeres of sister chromatids.
The link between the centromeres of sister chromatids is broken in anaphase-II and sister chromatids segregate to opposite poles. Cytokinesis occurs after telophase-II and no four haploid daughter cells are formed.