MEIOSIS
DEFINITION:-
Meiosis is a  division of a germ cell involving two fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell.
Meiosis is a special type of cell division of germ cells and apicomplexans in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilization, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome again.
Meiosis is the process in which a single cell divides twice to form four haploid daughter cells. These cells are the gametes – sperms in males and egg in females. The process of meiosis is divided into 2 stages. Each stage is subdivided into several phases.
Meiosis I:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Cytokinesis I
Meiosis II:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
- Cytokinesis II
Stages of Meiosis:
Meiosis cell division takes place in the following stages:
Meiosis I
Prophase I:
- The nuclear envelope disintegrates.
- Chromosomes begin to condense.
- Spindle fibres appear.
Prometaphase II:
Spindle fibres attach to the chromosomes at the centromere.
Metaphase I:
The homologous chromosomes align at the equatorial plate ensuring genetic diversity among offspring.
Anaphase I:
The homologous chromosomes are pulled towards the opposite poles.
Telophase I:
- Spindle fibres disappear.
- Nuclear envelope is reformed.
Cytokinesis I:
The cytoplasm and the cell division result in 2 non-identical haploid daughter cells.
Meiosis II:
Prophase II
- The chromatin condenses into chromosomes.
- Nuclear envelope disintegrates.
- Centrosomes migrate to either poles.
- Spindle fibres are reformed.
Metaphase II
The chromosomes align along the equatorial plate. On the contrary, the chromosomes in metaphase I were in homologous pairs.
Anaphase II
Sister chromatids are pulled to the opposite poles.
Telophase II
Nuclear envelope redevelops and the spindle fibres disappear.
Cytokinesis II
The cytoplasm and cell divide producing 4 non-identical haploid daughter cells.
Signification of meiosis:
Meiosis is the process in which the parent cell divides twice into four daughter cells containing half the original amount of genetic information, i.e., the daughter cells are haploid. The gametes are produced by meiosis.
Features of Meiosis
- It results in the formation of four daughter cells in each cycle of cell division.
- The daughter cells are identical to the mother cell in shape and size but different in chromosome number.
- The daughter cells are haploid.
- Recombination and segregation take place in meiosis.
- The process occurs in the reproductive organs and results in the formation of gametes.
- The process is divided into two types-Meiosis-IÂ reduces the chromosome number to half and is known as reductional division. Meiosis-II is just like the mitotic division.
Significance
- Meiosis is responsible for the formation of sex cells or gametes that are responsible for sexual reproduction.
- It activates the genetic information for the development of sex cells and deactivates the sporophytic information.
- It maintains the constant number of chromosomes by halving the same. This is important because the chromosome number doubles after fertilization.
- In this process independent assortment of maternal and paternal chromosomes takes place. Thus the chromosomes and the traits controlled by them are reshuffled.
- The genetic mutation occurs due to irregularities in cell division by meiosis. The mutations that are beneficial are carried on by natural selection.
- Crossing over produces a new combination of traits and variations.
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