Meiosis is a type of cell division that is vital for sexual reproduction. Meiosis takes place in the reproductive organs. It results in the formation of gametes with half the normal chromosome number. Therefore, haploid sperms are made in the testes and haploid eggs are made in the ovaries. In flowering plants, haploid gametes are made in the anthers and ovules. Meiosis involves two divisions of the cell. These two divisions are termed meiosis I and meiosis II.
Each one includes prophase, metaphase, anaphase and telophase. In the first meiotic division, the members of each homologous pair of chromosomes separate and is distributed into separate cells. In the second meiotic division, the chromatids that make up each chromosome separate and are distributed to the daughter cells. Thus, the number of chromosomes and the amount of DNA per cell are eventually reduced by half. The meiotic division takes place at the end of the G2 phase of the interphase, as in the case of mitotic cell division.
The important stages that take place during meiosis are: (i) Two successive divisions without any DNA replication occurring between them. (ii) Formation of chiasmata and crossing over. (iii) Segregation of homologous chromosomes. (iv) Separation of sister chromatids.
Meiosis- I
Prophase I
Prophase I is a long and complex stage. For convenience, the first meiotic prophase is divided into the following five sub-stages: Leptotene (Leptonema), Zygotene (Zygonema), Pachytene (Pachynema), Diplotene (Diplonema), and Diakinesis.
Leptotene
The chromatin fibres of interphase nucleus shorten and elongated chromosomes become clear. Each chromosome is attached at both of its end to the nuclear envelope via a specialised structure called attachment plate. Although each chromosome has replicated and consists of two sister chromatids, these chromatids are very close to each other and as a result appear to be single.
Zygotene
The homologous chromosomes (one paternal and one maternal) pair together by a process known as synapsis or zygotene pairing. The paired chromosomes are known as bivalents. Synapsis starts when the homologous ends of the two chromosomes are brought together on the nuclear envelope. The pairing is completed in three different ways as follows: 1. Proterminal pairing: The two homologous chromosomes start pairing at the terminals, which gradually progresses towards the centromere. 2. Procentric pairing: The pairing starts at the centromere and proceeds towards the end. 3. Random or intermediate pairing: The pairing may be at many points towards the ends. As a result of synapsis, the two homologous chromosomes are brought together through a characteristic ladder-like structure, called synaptonemal complex. Each of the homologous chromosomes consists of two closely apposed sister chromatids, thus each bivalent contains four chromatids, and is also called tetrad.
Pachytene
Pachytene is defined as the phase at which large recombination nodules appear at intervals on the synaptonemal complex. These recombination nodules intervene for chromosomal recombination. The nonsister chromatids twist around and exchange segments with each other.
Diplotene
The beginning of diplotene stage is manifested by the commencement of separation of the paired homologous chromosomes, and the tight pairing is relaxed. But the separation of homologous chromosomes is not completed. They remain attached at one or more points where crossing over has occurred. These points of attachment are called chiasmata. Lampbrush chromosomes are transitory structures that exist during an extended diplotene of the first meiotic division in oocytes of amphibians and some other organisms. It is at this stage that the chromosomes decondense and engage in RNA synthesis. Lateral loops are extended from the main axis of the chromosome. These loops are sites of active gene transcription. Towards late oogenesis, the loops retract back towards the main axis and the chromosomes become highly condensed again.
Diakinesis
The fifth and last stage of prophase I of meiosis, during which the chromosomes undergo terminalisation of chiasmata, i.e. the chiasmata tend to lose their original position and move toward the ends of the chromosomes. Also, during diakinesis RNA synthesis stops and the chromosomes condense, thicken, and become attached to the nuclear envelope. Each pair of sister chromatids is attached at their centromeres, whereas non-sister chromatids of homologous chromosomes are in contact with each other at or near their telomeres.
Metaphase I
The bivalents become arranged in the plane of the equator forming equatorial plate. The centromere of each chromosome is directed towards the opposite poles and the arms of chromosomes face the equatorial plate.
Anaphase I
During anaphase I the two members of each bivalent seem to repel each other and move towards the opposite poles. As a result each pole receives half the number of chromosomes or the haploid set of the chromosomes. Hence, actual reduction in number of chromosomes occurs. The movement of chromosomes is brought by the spindle fibres, similar to the chromosomal movement during mitosis.
Telophase I
During telophase I, nuclear membranes are formed by the endoplasmic reticulum around the groups of daughter chromosomes with the appearance of one nucleolus in each nucleus. It results in the formation of two daughter cells each with haploid number of chromosomes. Intrameiotic interphase This is the stage between the telophase of the first meiotic division and the prophase of the second meiotic division. During intrameiotic interphase, the chromosomes do not synthesize new DNA and there is no duplication of chromosomes. This is vital for reduction in the DNA complement in the daughter cells.
Second Meiotic Division This second meiotic division is very similar to a mitotic division. It divides each haploid meiotic cell into two daughter haploid cells. Similar to mitotic division it can be explained under four phases:
Meosis- II
Prophase II
Prophase II does not show the complex nuclear behaviour of prophase I and conforms to the characteristics of mitotic prophase. In prophase II a new spindle is formed at right angles to the first one and the nuclear membrane disappear.
Metaphase II
The chromosomes become arranged on the metaphase plate, much as the chromosomes do in mitosis, and are attached to the now fully formed spindle.
Anaphase II
The centromeres separate and the sister chromatids—now individual chromosomes—move toward the opposite poles of the cell.
Telophase II
At this stage, the four groups of chromosomes become organised into four haploid nuclei. The chromosomes return to the interphase condition. A nuclear envelope forms around each set of chromosomes and the nucleolus reappears. Each nucleus at this stage contains the haploid number of chromosomes and forms four cells.
Significance of Meiosis
In all sexually reproducing organisms, meiosis provides a way to keep the chromosomal number constant generation after generation. Not only is the chromosomal number halved precisely, each daughter cell receives a copy of each kind of chromosome. This ensures that each daughter cell receives one of each kind of gene. By crossing over, the meiosis provides a possibility for the exchange of genes and, thus, causes genetic variation within the species. The variation serves as the raw material for the evolutionary process.
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