What is Meiosis?
Meiosis is the specialized process of cell division in which a cell divides twice to produce four daughter cells. Daughter cells produced at the end of two divisions are haploid (n), containing half of the chromosomes than the parent cell. The two meiotic divisions are called meiosis I and meiosis II. The only purpose of meiosis is to produce sperms in males and eggs in females. When the sperm and egg cells meet, fertilization occurs making a diploid cell called zygote. Thus zygote has a complete set of genetic information.
Meiosis is divided into nine stages between first meiotic division (meiosis I) and second meiotic division (meiosis II). As the cell divides twice during mutation, thus it creates four gametes.
During Interphase, the DNA of the cell copies itself, as a result, two identical set of chromosomes are created. The microtubules extend from the two centrosomes present outside the nucleus.
Prophase is the longest phase of meiosis. During this phase, homologous chromosomes line up next to each other and condense into X-shaped structures, in a process known as synapsis. These structures are clearly visible under a light microscope. Thus it creates a tetrad of chromatids . This tetrad contains two homologous chromosomes, every chromosome has its sister chromatid. This pairing up of homologous chromosomes initiates crossing over. Crossing over is a form of genetic recombination and is a source of genetic variation.
Each homologous pair of chromosomes lines up side-by-side in the center of the cell. The centrioles are present at the opposite poles of the cell. And the meiotic spindle fibers expand from them. These spindle fibers bond with one chromosome of every pair.
During anaphase I, lined up chromosomes pull apart from each other toward the opposite poles of the cell. However, unlike mitosis and meiosis II, sister chromatids stay together.
Telophase I and Cytokinesis
Telophase I is similar to telophase of mitosis. The chromosomes are completely moved to the poles. At each pole, there is a complete set of chromosomes present. A membrane starts to form around each set of chromosomes and creates two nuclei. The cell then starts to pinch from the center. The contractile ring cleaves the cell from the middle to create two identical daughter cells containing full sets of chromosomes during cytokinesis.
Meiosis II looks just like mitosis except the number of chromosomes is reduced by half. Four stages of meiosis are described below.
During prophase II there are two daughter cells each with a diploid number of chromosomes. The homologous chromosomes line up next to each other and condense into X-shaped structures once again. The nuclear membrane starts to dissolve and membranous chromosomes disperse. The centrioles are duplicated and meiotic spindle form once again.
In each daughter cell, the chromosomes line up next to each other in the center of the cell. The centrioles are present at the opposite poles of the cell. And the meiotic spindle fibers expand from them. Meiotic spindles attach to the sister chromatids from their centromere and line up in the center of the cell.
The chromatids start pulling out towards the opposite poles of the cell. The sister chromatids are now separated and they are individual chromosomes at this stage.
Telophase II and Cytokinesis
The chromosomes complete their movement to the poles of the cell. At each pole, there is a complete set of chromosomes present. A membrane starts to form around each set of chromosomes and creates two new cell nuclei. It is the last phase of meiosis but cell division is incomplete without cytokinesis. Cytokinesis splits the cell body into two. Thus at the end of meiosis II there are four daughter cells, or better called as granddaughter cells, each with half the number of chromosomes than original parent cell.