When a single cell divides into two or more cells, we say that cell division has occurred. The single cell that divides itself is known as a parent cell or parental cell. The resulting cells from the parent cell are known asdaughter cells. A cell cycle can be defined as the sequence of events in which a cell synthesises the other constituents of cells by duplicating itself and eventually dividing itself into daughter cells.
This diploid cell forms 4 haploid daughter cells, each having only half the number of chromosomes.
It takes place in 2 divisions:
Meiosis - I: In this chromosome number is reduced to half, which is why this division is known as reductional division.
Meiosis - II: In this chromosome numbers remain the same till the end of the division, which is why this division is known as equational division.
Meiosis I
In this chromosome number is reduced to half, which is why this division is known as reductional division.
Prophase I
It is further divided into five stages that are:
Leptotene: This word is made up of two words that are "leptos" and "tene", and their meanings are "thin" and "thread". In this stage, you will observe that a thin thread of chromatin fibres will appear, and they will get condensed by dehydration. Here, the nuclear membrane remains undamaged, and chromosomes appear thread-like and are made up of two sister chromatids.
Zygotene: During this chromosomes will get paired, then this whole process of association is known as synapsis. Now, there will be the formation of a bivalent (a pair of synapsed chromosomes forms a complex called a bivalent).
Pachytene: Crossing of two non-sister chromatids of homologous chromosomes takes place in this stage. While this crossing there will be the exchange of genetic material; this is an enzyme-mediated process, and the enzyme involved in this process is known as recombinase. By the end of this stage, the recombination of chromosomes will be completed.
Diplotene: In this stage, nuclei and the nuclear membrane will start disappearing. Homologous chromosomes will separate in this stage. Chiasmata(X-shaped structure) will be formed.
Diakinesis: chiasmata will terminate.
Metaphase- I
Pairs of chromosomes align themselves on the equator of the cell.
The spindle fibres at each pole of the cell are attached to each chromosome of each pair.
Centrioles are now at the opposite poles of the cell.
Anaphase- I
A pair of chromosomes will be appressed by the spindle fibres.
Now one chromosome will be at one pole of the cell, and another chromosome will be at the opposite pole.
In this stage, sister chromatids will stay together.
Telophase- I
Chromosomes have completed their whole moment and reach the poles of the cell.
Around each set of chromosomes, there will be a formation of a membrane to create two nuclei.
The nuclear membrane and nucleolus will reappear.
Cytokinesis
Each parent cell will now divide itself to form haploid daughter cells.
Interkinesis
The stage between the telophase-I and prophase-II.
No DNA replication will take place.
Chromosomes will elongate in this stage
Meiosis II
In this chromosome numbers remain the same till the end of the division, which is why this division is known as equational division.
Prophase- II
Two daughter cells are formed will have condensed chromosomes that are visible in an X-shaped structure, which can be seen under a microscope.
Duplication of centrioles takes place.
Spindle fibres will be formed again
Metaphase- II
Pairs of chromosomes align themselves on the equator of the cell.
Centrioles are now at the opposite poles of the cell.
The spindle fibres at each pole of the cell are attached to each chromosome of each pair.
Anaphase- II
The pair of chromatids is now pulled to opposite poles of the cell due to the action of the spindle.
Now, these chromatids will become individual chromosomes.
Telophase- II
Chromatids will reach to poles and decondense themselves.
The nuclear membrane and nucleolus will reappear.
Nuclei of four haploid cells are formed.
Cytokinesis
Each daughter cell at meiosis I will divide itself to form new daughter cells. Basically, two haploid daughter cells will convert themselves into four haploid cells.
