Meiosis:

Meiosis involves the reduction of the chromosome number to half, thus producing haploid daughter cells. Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication. Recombination during meiosis leads to the generation of genetic variations between members of the same species.

Meiotic events can be grouped under the following phases:

Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Stages of meiosis I (Reductional division):

Prophase I: Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

a. Leptotene: The compaction of chromosomes continues throughout leptotene and the chromosomes become gradually visible under the light microscope.

b. Zygotene: Chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex.

c. Pachytene: clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing overRecombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.

d. Diplotene: The recombined homologous chromosomes of the bivalent tend to separate from each other except at the site of crossovers. The dissolution of synaptonemal complex is seen. These sites (X-shaped structures), are called chiasmata. . In oocytes of some vertebrates, diplotene can last for months or years

e. Diakinesis: The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase

Metaphase I: The bivalent chromosomes align on the equatorial plate (Figure 10.3). The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes.

Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres

Telophase I: The nuclear membrane and nucleolus reappear

Cytokinesis I: haploid dyad of cells are formed after cytokinesis I.

Interkinesis : This is a short lived stage between the two meiotic divisions and is generally short lived. There is no replication of DNA during interkinesis.

Stages of Meiosis II (equational division):

Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated and resembles a normal mitosis. Meiosis II is subdivided into following stages:

Prophase II: It is much simpler than Prophase I. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.

Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids

Anaphase II: Centromere of each chromosome splits, allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.

Telophase II: Meiosis ends with telophase II. The two groups of chromosomes once again get enclosed by a nuclear envelope.

Cytokinesis II: Tetrad of cells i.e, four haploid daughter cells are formed after cytokinesis II.

Significance of meiosis:

1. Meiosis leads to the conservation of specific chromosome number.
2. It helps in the generation of haploid spores and gametes.
3. It is the source of genetic variability in the population of organisms from one generation to the next.
4. These variations are very important for the process of evolution.

Difference between mitosis and meiosis