Metaphase

Every organism is made up of cells. The process makes new cells of cell division both in prokaryotes and eukaryotes. However, the process of cell division greatly varies between the two. The process of cell division in eukaryotes is generally called the cell cycle. The cell cycle has three main components: a replication of DNA, division of the nucleus (karyokinesis), and, ultimately, the division of cytoplasm (cytokinesis). Metaphase comes from the Greek word meaning after. It arises in the cell cycle after the prophase and before the anaphase. This phace takes around 1 per cent of the total cell cycle time.

The Two Types of Cell Division

 Cell division can take place by two processes. 

1. Flemming gave this term when no new genetic recombination occurs in a cell division or mitosis. Strasburger first discovered it in plants. Walter and Flemming discovered it in animal cells. The process generally takes 30 minutes to 3 hours to complete. It occurs during embryonic development as it is the main division of growth.
2. When genetic recombination occurs in a cell and new cells produced are markedly different from the parent cell, such division is called meiosis. This process was first-time demonstrated by Von benden.

Cell Cycle

The cell cycle is divided into four stages: G1, S, G2, and M.

Strasburger divided the M phase into four stages called

1. Prophase 
2. Metaphase
3. Anaphase 
4. Telophase.

Each stage has a characteristic feature and plays an important part in cell division.

Here we will discuss Metaphase in detail.  

What is Metaphase?

• In the cell cycle, it arises after the prophase stage and before the anaphase stage.

• It is characterised by the complete formation of the nuclear spindle apparatus. In metaphase, the chromosomes arrange themselves on the equator of the nuclear spindle apparatus. The Equator is the broadest part of the nuclear spindle apparatus. 
• At metaphase, chromosomes show maximum condensation. So there is no genetic activity expressed in this phase. 
• Metaphase gives the best count of chromosomes.

Prometaphase

• A Prometaphase is observed before metaphase and after the prophase. The time gap is present between the disappearance of the nuclear envelope and the formation of the nuclear spindle apparatus. 
• At the time of the nuclear spindle apparatus formation, both centrioles become antipodal in position. Microtubules are involved in the formation of the nucleus spindle apparatus. 
• Those microtubules involved in the formation of spindle fibres of nuclear spindle apparatus are charged, and their negative are towards the pole and positive and are towards the equator.

Metaphase in Mitosis

• The centrosomes are now at opposite poles of the cell.

• The chromosomes have all arrived at the metaphase plate, an equidistant plane between the spindle’s two poles. The chromosome’s centromeres lie at the metaphase plate.

• The kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles for each chromosome.

Metaphase in Meiosis

Metaphase appears two times in meiosis. 

• In metaphase 1  of meiosis, the homologous pairs of chromosomes show congression movement by which two homologous chromosomes arrange themselves equidistant from the equator facing each other. Spindle fibre gets attached to the centromere of the chromosome. Each chromosome is attached to the spindle fibre of only one side (the pole where the centrosome is facing). The centromeres slightly project towards the periphery.  Due to two centromeres, a bivalent metaphasic plate is observed. Some interzonal fibre develops between the centromere of homologous chromosomes. These inter zonal spindle fibres push the chromosome to the opposite poles, called pushing bodies.
• The metaphase 2 of meiosis is much similar to mitosis. Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical. The kinetochores of sister chromatids are attached to microtubules extending from opposite poles.

Mitotic poisons

Chemicals that seriously affect me tosses and even stop them are mitotic poisons. For example, mustard gas causes the fragmentation of chromosomes.

Colchicine affects the metaphase of mitosis as It breaks the nuclear spindle apparatus, preventing chromosomal separation. This brings endo duplication of chromosomal numbers leading to polyploidy. The mitosis which is affected by colchicine is called Mitosis-C. Colchicine is obtained from the seeds of the Colchicum luteum plant, which belongs to the family Liliaceae.

Conclusion

Therefore, metaphase is an important phase in cell division. It further leads to splitting centromeres of each chromosome, which ultimately divide into two, leading to the formation of new cells. The formation of the metaphysic or equatorial plate is a characteristic feature of metaphase. The number and study of the morphology of chromosomes are done at this stage.