Meiotic cell division ( Meiosis ) or reduction division
Meiosis produce the male gametes and the female gametes to complete the sexual reproduction , It occurs only in the reproductive cells , Where it occurs in the testis to produce the male gametes ( the sperms ) and in the ovary to produce the female gametes ( ova ) in the humans and the animals .
Meiosis occurs in the anther to produce the male gametes ( the pollen grains ) and in the ovary to produce the female gametes ( the eggs ) in the flowering plants .
Phases of Meiosis
Before starting Meiosis , the cell passes through Interphase where the chromosomes are doubled , Meiotic division takes place in two stages which are the first meiotic division , And the second meiotic division .
First Meiotic division
It produces two cells , Each of them contains half the number of chromosomes , The phases of first Meiotic division are Prophase I , Metaphase I , Anaphase I , And Telophase I .
Prophase I : It is very important to know that in this phase , the chromatin reticulum condenses ( intensifies ) and appears in the form of distinct chromosomes , The chromosomes are arranged in homologous pairs , Each pair consists of 4 chromatids which are called a tetrad .
The crossing over phenomenon occurs , The nuclear membrane and the nucleolus disappear , Each two homologous chromosomes ( in the tetrad ) move away from each other , The spindle fibres appear and connect to the chromosomes at the centromere .
Metaphase I : It is very important to know that in this phase , The chromosomes pairs arrange at the cell equator .
Anaphase I : You should know that in this phase , The spindle fibers shrink , so every two homologous chromosomes move away from each other , One of the two chromosomes migrates towards the cell pole and the other migrates towards the other pole , Therefore , Each pole contains half the number of the parent cell chromosomes .
In Telophase I , The spindle fibers disappear , The nuclear membrane is formed around the chromosomes , at each of the cell’s poles leading to the formation of two nuclei , Each nucleus contains half the original number of parent cell chromosomes , Each cell contains ( N ) chromosomes .
The crossing over phenomenon
The traits are different between the members of the same species due to the crossing over phenomenon which is a phenomenon that takes place at the end of prophase I and in which some parts of the two inner chromatids of each tetrad are exchanged to produce new genetic arrangements .
The crossing over phenomenon works on the variations of genetic traits among the members of the same species , where it contributes in the genes ( that carry the genetic traits ) exchanging between the two homologous chromosomes’s chromatids and distributing them randomly in the gametes .
Second Meiotic division
The second meiotic division aims to increase the number of the produced cells from the first meiotic division , Prophase II , Metaphase II , Anaphase II , And Telophase II are the second meiotic division phases .
Each cell of the two cells resulted from the first meiotic cell division is divided in a way similar to the mitotic cell division , In the final phase ( Telophase II ) of this division , four cells ( four gametes ) are produced and each of them contains half the number of the chromosomes of the parent cell .
Meiotic cell division is called by reduction division because the produced cells contain half the number of chromosomes of the parent cell .