Meiosis : The basis of sexual reproduction

Meiosis is a type of cell division in which number chromosome reduced to one half in the daughter cell. Sexual reproduction requires a genetic ontribution from two different sex cells. Egg and sperm cells are specialized sex ells. They are called gametes. A male gamete (sperm) unites with a female gamete (egg) during fertilization to form a zygote. The fusion of gametes is ailed syngamy. The zygote is the first cell of the new animal. Each of the two gametes contributes half of the genetic information to the zygote.

he sexually reproducing animals produce gametes by meiosis. The gametes have half chrormosomcs. It maintains constant number of chromosomes in the next generation. Au of the somatic cells in the bodies of most animals have the diploid (2N) number of chromosomes.

Meiosis occurs in specialized cells of the ovaries and testes. It reduces the number of chromosomes to the haploid (1N) number. The nuclei of the two gametes combine during fertilization and restore the diploid number.

Meiosis begins after the G2 phase in the cell cycle after DNA replication. Two nuclear divisions take place during meiosis. These are called meiosis I and meiosis II. The two nuclear divisions of meiosis result in four daughter cells. Each cell has half of the number of chromosomes of the parent cell. These daughter cells are not genetically identical.. Meiosis is a continuous process. It is divided into two phases:

THE FIRST MEIOTIC DIVISION

Prophase I

Following process take place.during prophase I:

  1. Leptotene

The folding of chromatin material takes place in this stage. Thus chromosomes become visible under a light Microscope. The cell contains the diploid number of chromosomes. It has a copy of each type of chromosome from each original parent cell. The chromosomes are present in homologous pairs. The homologous chromosomes carry genes for the same traits, have same length, and have a similar staining pattern. It makes them identifiable as matching pairs.

  1. Zygotene

The homologous chromosomes line up side by side. This process is called synapsis. It forms a tetrad of chromatids. It is also called a bivalent. The tetrad contains the two homologous chromosomes. Each homologous chromosome contains its copy in the form of chromatid. A network of protein and RNA is present between the sister chromatids of the two homologous chromosomes. This network holds the sister chromatids. Thus there is an exact matching of genes. Therefore, each gene is directly across with its sister gene on the opposite homologous chromosome.

  1. Pachytene

Synapsis also initiates crossing over. The non sister chromatids of the homologous chromosomes exchange DNA segments during crossing over. This process redistributes genetic information among the paired homologous chromosomes. Crossing over produces new combination of genes on the various chromatids in homologous pairs. Thus, each chromatid ends with new combinations of traits. Crossing over is a form of genetic recombination and is a major source of genetic variation in a population of a given species.

  1. Diplotene

The homologous chromosomes repel each other. Thus the tetrad opens lengthwise.

  1. Diakinesis

The chaismata between the homologous chromosomes are dissolved. Now the homologous chromosomes spread in the entire cell.

Metaphase I

The microtubules form a spindle apparatus. Each pair of homologues lines up in the center of the cell. Their centromeres are on each side of the equator of the spindle.


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Anaphase I

The homologous chromosomes separate. They begin to move toward each pole:

The rientation of each pair of homologous chromosomes is random. Therefore, eac pole receives specific chromosomes from each pair of homologues.

Tel phase I                             •

It is similar to mitotic telophase. The chromosome uncoils. The nuclear me brane around each set of chromosomes appears.

Int rkinesis

The transition to the second nuclear division is called intrerkinesis. The DNA doe not replicate during intrerkinesis. After a varying time period, meiosis II occ

TH SECOND MEIOTIC DIVISION

The second meiotic division resembles an ordinary mitotic division. The number of ahromosomes has been reduced by half. The phases are prophase II, met:phase II, anaphase II, and telophase II. The final products of these divisions of eiosis are four haploid cells. These cells finally form gametes (sex cell).

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SPERMATOGENESIS AND 00GENESIS

T e meiosis produces sperm and egg cells in most animals.

Spermatogenesis

The formation of sperm in the testes is called spermatogenesis. It follows the sequence of meiosis and produces mature sperm cells. All four products of meiosis acquire a.flagellum for locomotion. They also get a cap like structure. This cap helps in the penetration of the egg

Oogenesis

The formation of ova or egg in the ovaries is called oogenesis. It produces a mature ovum or egg. Only one of the four meiotic products develops into the functional gamete. So it differs from spermatogenesis. The other three products of meiosis are called polar bodies. These polar bodies disintegrate. In some animals the mature egg is the product of the first meiotic division. It completes meiosis only if it is fertilized.

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