Meiosis and its phases

Meiosis is a method of cell division, as a result of which four haploid daughter cells are formed from one diploid mother cell.

Cell preparation for meiosis occurs in the interphase: DNA doubles, ATP accumulates, fission spindle proteins are synthesized.

Meiosis includes two consecutive divisions.

The first division of meiosis (meiosis I) leads to a decrease in the chromosome set and is called reduction. It includes four phases.

Prophase I

Twisting of DNA molecules and the formation of chromosomes occurs. Each chromosome consists of two homologous chromatids – 2n4c.
Homologous (paired) chromosomes converge and twist, that is, chromosomes are conjugated.
Then the homologous chromosomes begin to diverge.
In this case, intersections are formed and crossing over occurs – an exchange of areas between homologous chromosomes.
The nuclear envelope dissolves.
The nucleoli are destroyed.
A fission spindle is formed.

Metaphase I

Spiralization of chromosomes reaches its maximum.
Pairs of homologous chromosomes (four chromatids) line up along the equator of the cell.
A metaphase plate is formed.
Each chromosome is connected to the filaments of the fission spindle.
The chromosome set of the cell is 2n4c.

Anaphase 1

Homologous chromosomes, consisting of two chromatids, branch off from each other.
The spindle filaments stretch the chromosomes towards the poles of the cell.
From each pair of homologous chromosomes, only one gets to the poles.
There is a reduction – a decrease in the number of chromosomes by half.
At the poles of the cell there are haploid sets of chromosomes, consisting of two chromatids.
Chromosome set by the end of anaphase: at the poles – 1n2c, in the cell – 2n4c.

Telophase I

Nuclei are formed.
The cytoplasm is divided.
Two cells are formed with a haploid set of chromosomes.
Each chromosome consists of two chromatids.
The chromosome set of each of the formed cells is 1n2c.

After a short period of time, the second division of meiosis begins. At this time, DNA duplication does not occur. Two haploid cells, which were formed as a result of the first division, divide.

Prophase II

Nuclear shells collapse.
Chromosomes are arranged randomly in the cytoplasm.
A fission spindle is formed.
The chromosome set of the cell is 1n2c.

Metaphase II

Chromosomes are located in the equatorial plane.
Each chromosome consists of two chromatids.
Fission spindle threads are attached to each chromatid.
The chromosome set of the cell is 1n2c.

Anaphase II

Fission spindle threads pull sister chromatids to the poles.
Chromatids become independent chromosomes.
Daughter chromosomes are directed towards the poles of the cell.
The chromosome set at each pole is 1n1c (in the cell – 2n2c).

Telophase II

Nuclei are forming.
The cytoplasm is divided.
Four haploid cells are formed – 1n1c.
The chromosome sets of the formed cells are not identical.

The value of meiosis

The cells formed as a result of meiosis differ in their chromosome sets, which provides a variety of living organisms.
During meiosis, the number of chromosomes is halved, which is necessary for sexual reproduction. The fertilization process again restores the diploid set of chromosomes in the zygote.