Meiosis Called Reduction Division

By Author: Pierce Brosnan
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Introduction on meiosis called reduction division:

Meiosis can occur in all kinds eukaryotic cells. Meiosis called the reduction division because the cells divide to form new cells that have half of the original chromosome number; hence the number of chromosomes is reduced to half. For example when a cell with 8 chromosomes gets divided by meiosis, it will produce only 4 chromosomes in each of the new daughter cells. The main stages of division in meiosis are known as stages of meiosis division 1 and 2.

Explanation to Meiosis 1 or first meiotic division:

Meiosis 1 or first meiotic division: In the first meiotic division, the two homologous chromosomes, of each pair separate from each other and go to separate daughter cells. This reduces the number of chromosomes from diploid to haploid conditions. Meiosis 1 is therefore known as heterotypic division. The four phases of this division are called prophase-1, metaphase -1, anaphase -1, and telophase -1.

Prophase -I: Prophase -1 is the longest phase of meiosis-1 and is subdivided into five stages namely Leptotene, Zygotene, Pachytene, Diplotene ...
... and Diakinesis. At prophase-1, chromosomes inherit from each parent form pair. This pairing is synopsis and is done only by homologous chromosomes. Two chromosomes of a pair are called homologous chromosomes since they are genetically identical; one of them comes from father through his sperm and the other from mother through her egg. Each homologous chromosome of a pair has two chromatids called sister chromatids, both of which are attached with a centromere. Then by an important process called crossing over or recombination.

Metaphase- I: At metaphase -1, nuclear membrane be is lost. In the cell a spindle – like structure is formed by help of micro tubules. Chromosomes at the equator of the spindle. Certain spindle fibres get attached with the centromeres of chromosomes.

Anaphase- I: At this stage one homologous chromosome with its two chromatids moves to one pole of the cell and the other homologous chromosome go to the other pole of cell.

Telophase- I: At this stage the dividing cell forms two nuclei, each with only half the number of chromosomes. Each of these nuclei is haploid containing one chromosome from each homologous pair.

Explanation to Meiosis 2 or second meiotic division

Meiosis 2 or second meiotic division: It is similar to mitosis. In this division, the two chromatid of each chromosome separate from each other and go to separate daughter cells. With the result the number of chromosomes remains the same as produced by meiosis 1. Meiosis 2 is therefore known as homotypic division. The four stages of this division are

Prophase -II: when there is no interkinesis, the telophase -1 spindle, as stated above is replaced by two new spindles and the centrioles and asters, if present, duplicate and one copy of each comes to lie at each pole of new spindle. Chromosomes, each consisting of two chromatids, appear in the nucleus. They are set free in the cytoplasm by breakdown of nuclear envelop. Nucleolus disappears.

Metaphase -II: The chromosomes get arranged at the equator of the spindle as a meta phase plate. The chromatids of each chromosome are joined at their kinetochores by chromosomal micro tubules extending from the opposite poles of the spindle as in mitosis.

Anaphase- II: the two chromatids of each chromosome separate and move to the opposite poles of the spindles. Here they are called chromosomes. Each pole has haploid number of chromosomes and haploid amount of DNA. This amount is ¼ th of DNA present in the original cell which entered meiosis.

Telophase II: The chromosome at each poles re condense and nuclear envelope develops around them. This produces two nuclei. Nucleolus is formed in each nucleus. Spindle and asters disappears.

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