Meiosis

        MEIOSIS

Meiosis is a peculiar type of cell division that takes place at some point in the life cycle of any sexually reproducing Eukaryotic species.

Meiosis Definition: 

Meiosis is a complex cell division process in which the diploid number of chromosomes of the mother cell is reduced to the haploid number of chromosomes in the four daughter cells.

In animals, it typically occurs during the maturation of sex cells ( gametes). Gametes are formed from specialised diploid cells.

Note: "*" know to understand the topic.

* ( This is a specialised kind of cell division. Sexual reproduction in animals involves the union of two gametes, each bearing one set of chromosomes. The resultant cell i.e. usually a fertilized egg or zygote, is diploid, with two sets of chromosomes.)

Meiosis consists of a single duplication division. In animals and lower plants, meiosis is terminal or gametic, i.e. it occurs before the formation of gametes.

 In the male, four haploid sperms are produced. In females, one ovum and three polar bodies are produced. Cells in meiosis are called meiocytes.

In humans, the normal chromosome number of 46 ( 23 homologous pairs) per cell is reduced meiotically to just 23 in the sperm and egg. Fertilization then brings together the two homologous pairs, restoring the diploid state in the zygote.

* Meiosis is a kind of division that ensures the production of the haploid phase in the life cycle of sexually reproducing organisms whereas fertilization restores the diploid phase.

Meiosis involves two sequential cycles of nuclear and cell division called Meiosis I and Meiosis II but only a single cycle of DNA replication.

Meiosis I: Diagram 

*Meiosis I is initiated after the parental chromosome has replicated to produce identical sister chromatids at the S phase.

* Meiosis involves the pairing of homologous chromosomes and recombination ( crossing over) between them.

* Four haploid cells are formed at the end of meiosis II.

 Meiotic division can be grouped under Meiosis I and Meiosis II

Meiosis I:

The first meiotic division is called heterotypic division which reduces the chromosome number from diploid into two haploid daughter cells 

Just like mitosis, the actual process of meiosis starts in the interphase and DNA replication also takes place in the pre-meiotic S-phase. But at the G2 Phase, some changes occur that direct the cell towards meiosis.

The first meiotic division has the following divisional stages:

Prophase I:

Prophase I of meiosis is longer compared to that of mitosis so it is further divided into

1. Leptotene(Leptonema):

• a. Chromatin fibres condense and form chromosomes which can be seen under a light microscope.
• b. This compaction continuous throughout Leptotene.

2. Zygotene ( Zygonema): 

• a.During this stage chromosome starts pairing. Such paired chromosomes are called homologous chromosomes and this process of association is called synapse/ syndesis. This occurs to form bivalents.
• b.Two members of each chromosome pair behave remarkably. They come together and zip up so that each allele ( different forms of a gene that are found at the same place on a chromosome) lies in intimate contact with its allele in the homologous chromosome forming a synapse.
• Electron micrograph studies show that this process is accompanied by the formation of a complex structure called Synaptonemal complex (SC). This Complex formed by a pair of synapsed homologous chromosomes is called bivalent or a tetrad.

3. Pachytene ( Pachynema):

• a.During this stage the bivalent chromosomes appear as tetrads (bundles of four strands) with shorter and thicker chromosomes.
• b. This stage is characterized by the appearance of recombination nodules or bars. i.e. sites at which crossing over ( crossing over is the exchange of genetic material between two homologous chromosomes) occurs between non-sister chromatids of the homologous chromosomes at the molecular level.
• Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.
• c. By the end of pachytene, The recombination between homologous chromosomes is completed leaving them linked at the site of crossing over.

4. Diplotene ( Diplonema):

The onset of diplotene is characterized by the dissolution of SC and the tendency of the recombined homologous chromosome of the bivalents to separate from each other except at the site of crossing over, i.e. chiasmata,  the X-shaped structure(chiasmata was first seen by Johanssen, 1909)

* crossing over, chiasmata or points of interchange chiasmata or nucleoprotein attaching points.

* There is at least one chiasma per bivalent chromosome.

At each Chiasma, there is a piece of SC that ultimately disappears and is replaced by a chromatin bridge.

* Diplotene lasts for months or years. e.g. in Oocytes( egg cells) of some vertebrates

Diakinesis:

 It is the final stage of meiotic prophase I, representing the transition to metaphase.

It is marked by,

• a. The shifting of chaismata towards the chromosome end i.e. terminalisation of chaismata.
• b. The chromosomes are fully condensed and the meiotic spindles begin to develop with centrioles in animals and without centrioles in plants.
• c. There is a reduction in the number of chiasmata and further contraction.
• d. The nucleolus disappears and the nuclear envelope breaks in places.

Metaphase I:

1. The bivalent chromosomes align on the equatorial plate
2.  A bipolar fibrous spindle appears in the area of the nucleus, having esters at the two poles i.e. amphiaster in animal cells and without esters in plant cells.
3. The bivalent chromosomes shift to the equator forming a metaphasic plate with the help of their centromere.
4. Each chromosome gets attached to the spindle poles of its side by chromosome fibre.

Anaphase I: 

1. Chaismata disappears completely and the homologous chromosomes separate. This process is called disjunction.
2.  The separated chromosomes ( univalents) show divergent chromatids and are called diads.
3. Diads move towards spindle poles to form two groups of haploid chromosomes.

Telophase I:

1. Chromosomes elongate.
2.  The nucleolus, nucleoplasm and nuclear envelope appear over each chromosome group forming nuclei.

During the interphase between the two meiotic divisions, no replication of chromosomes occurs and division II is very similar to Mitosis resulting in a haploid number of chromosomes with single chromatids in the nucleus.

Meiosis II:

Meiosis II is an equational division ( homotypic) maintaining the haploid number and separating the two chromatids of a chromosome which have become different due to crossing over. Replication of DNA is absent.

Meiosis II: Diagram 

The stages can be divided into:

1. Prophase II:

• a.The chromatin fibre shortens to form chromosomes.
• b. Breakdown of the nuclear envelope and nucleolus.
• c. Formation of the spindle.

* In some organisms both telophase I and prophase II are omitted and anaphase I directly leads to metaphase II  e.g. Trillium.

2. Metaphase II:

• a. Chromosomes come to lie at the equator of the spindle forming a single metaphasic plate.
• b. The centromere of each chromosome gets attached by both its surfaces to the spindle poles of their sides by distinct chromosome fibres.

3. Anaphase II: 

• a. centromere of each chromosome divides into two. This separates the two chromatids of a chromosome into two independent daughter chromosomes.
• b. Each daughter chromosome is attached to the spindle pole of its side by a chromosome fibre.
• c. Chromosomes move towards the spindle poles forming two groups. Since they were two spindles, in total four groups were formed.

4. Telophase II:

The four groups of chromosomes organise themselves into four haploid nuclei.

Cytokinesis: 

Cytokinesis may occur after each successive division or simultaneously at the end of meiosis. It is generally through cleavage. 

In the case of plants, wall material is deposited in the furrows.

Finally, cytokinesis gives rise to four haploid cells.

 Significance of Meiosis:

In all organisms, "Meiosis" takes place at the time of gamete formation. 4 haploid cells are produced as a result of meiosis in a diploid cell. The product of meiosis gives rise to gametes after metamorphosis, some of them may be viable or nonviable.

The significance of meiosis (reduction division) is as follows.

1. To keep the chromosome number the same in daughter cells.
2. To keep the parental characters in daughter cells.
3. This division enables the cells to pass from one generation to another.
4. Fertilization duplicates the chromosome number but meiosis halves it to maintain the chromosomal balance in all organisms.
5. At the time of chiasmata formation, the exchange of chromatid material causes a combination of new characters. Thus meiosis helps in bringing about variation in population.

In plants, in the sporophytic generation, the number of chromosomes is 2x i.e. diploid and passes into gametophytic generation. Reduction in several chromosomes takes place so that each of the gametophytic generation has only X number of chromosomes. So this phenomenon is essential for completing the life history of a plant.

Meiosis may last for 50 years in human females as the number of oocytes in a newborn female is about 1 million. By the age of 7 years, there are some 300,000 oocytes left while only 400 reach maturity between 12 & 15 years. Thus meiosis may last up to 50 years. This may explain the increased incidence of chromosome aberration with the increased age of the mother. In human males, meiosis starts after puberty.

Need to know about the difference between Mitosis and Meiosis