Meiosis : The basis of sexual reproduction

Meiosis

The basis of sexual reproduction

Meiosis is a type of cell division in which chromosomal number lessen to one half in the daughter cell.

Sexual reproduction requires a genetic contribution from two non-identical or different sex cells or gametes cells. Egg and sperm cells are specialized sex cells. They are called gametes. A male sex cell or gamete (sperm) unites with a female sex cell or gamete (egg) during fertilization to form or develop a zygote. The fusion of sex cells i.e; eggs and sperms cells (gametes) is ailed syngamy. The zygote is the first cell of the new living organisms or animal. Each of the two sex cells i.e. egg and sperm or gametes contribute half of the genetic information to the zygote.

The sexually reproducing living organisms (animals) yield gametes by a process called meiosis. The sex cells generally called gametes to have half chromosomes. It keeps the constant number of chromosomes in the upcoming generation. Au of the non-sex cells commonly called somatic cells in the bodies of most living organisms (animals) has the diploid (2N) number of chromosomes.

The process of  Meiosis occurs in specialized cells of the ovaries and testes in living organisms. Firstly, It reduces the chromosomal number to the haploid number (1N). The nuclei of the two gametes combine at the time of fertilization and restore the diploid number(2N).

The process of Meiosis proceeds after the G-2 phase in the cell cycle after DNA (deoxyribonucleic acid) replication. Two nuclear divisions occur during the process of meiosis. These are called meiosis I and meiosis II. These both two nuclear divisions of the process meiosis result in four daughter cells. Each and every cell has half of the number of chromosomes of the parental cell. These daughter cells are not genetically similar or identical.. Meiosis is a continuous operation or procedure or process. It is further divided into two main phases or divisions:

THE FIRST MEIOTIC DIVISION

Prophase I

Following processor operations or activities take place occurs during prophase I:

1. Leptotene

In this stage of meiosis, the folding of chromatin material takes place/occur. In Leptotene stage, after folding of chromatin material the chromosomes become visible under a light Microscope. The cell contains the diploid number (2N) of chromosomes in this earlier phase. It has a copy of each kind or type of chromosome from each original/primary or genuine parent cell. The chromosomes are existed/present in homologous/similar/identical pairs. The homologous chromosomes carry genes for the same/matching/similar/identical traits, have the same length/extent, and have a similar/same staining design/pattern. It makes them identifiable as complement/matching pairs.

2. Zygotene.

The homologous/similar/identical chromosomes line up/arrange side by side. This process/arrangement is called/known as synapsis. It forms/develop/produce a tetrad of chromatids. It is also called/known as/termed as a bivalent. The tetrad contains/carry/accommodate the two homologous/similar/identical chromosomes. Each and every homologous/similar chromosome contains/accommodate its copy in the form/type/kind of chromatid. A network of protein and RNA Ribonucleic Acid is present between the sister chromatids of the two similar/homologous chromosomes. This network holds the homologous/sister chromatids. Thus there is an exact/correct/perfect matching of genes. Therefore, each and every gene is direct across with its sister gene on the opposite homologous/similar/identical chromosome.

3. Pachytene

Synapsis (occurs in the zygotene) also initiates/originate/launch crossing over. The non-sister/non-identical/dissimilar chromatids of the homologous chromosomes interchange/exchange DNA Deoxyribonucleic Acid segments during crossing over. This process/operation/procedure redistributes genetic information/statistics among/between the paired homologous chromosomes. Crossing over produces/develop/give rise to a new combination of genes on the different/various chromatids in homologous pairs. Thus, every/each chromatid ends with new combinations of traits. Crossing over is a type/kind/form of genetic recombination and is a basic/major source of genetic differences/variation in a population of a given species.

4. Diplotene

The same/identical/similar/homologous chromosomes offend/repel each other. Thus the tetrad unfolds/opens longitudinally/lengthwise.

5. Diakinesis,

The chaismata among/between the homologous/identical chromosomes are dissolved. Now the similar/identical/homologous chromosomes spread in the entire cell.

Metaphase I    '

The microtubules generate/develop/form a spindle apparatus. Each and every pair of homologs arranged/lines up in the medium/center of the cell. Their centromeres are on each place/portion/corner/side of the equator of the spindle.

Anaphase I.

The similar/identical/homologous chromosomes detached/separate. They start/begin to locomote/move toward each side/pole;

The orientation of every/each group/pair of identical/homologous chromosomes is random. Therefore, every side/pole gets/receives common/specific chromosomes from each pair of homologs.

Telophase I                                                                                             ;

It is identical/same/similar to mitotic telophase. The chromosome unwinds/uncoils. The nuclear membrane around every/each group/set of chromosomes seems/look/appears.

Interkinesis                                               ;

The transition to the 2nd/IInd/second nuclear break-up/splitting/division is called interkinesis. The DNA deoxyribonucleic acid does not produce copies/replicate during the stage interkinesis. After a varying duration/time period, meiosis II occurs.                    .          .

TH SECOND MEIOTIC DIVISION..,,

The II nd/2nd/second meiotic division similar/same/resembles an ordinary mitotic division. The quantity/number of chromosomes has been lessened/reduced by half. The processes/phases are prophase II, metaphase_II, anaphase II, and telophase-II. The final products of these processes/divisions of meiosis are IV/4/four haploid cells. These cells finally form gametes i.e. eggs in females and sperms cells in male (sex cell).

SPERMATOGENESIS AND 00GENESIS

The process/operation/system/procedure of meiosis generates/produces sperm and egg cells in most living organisms/animals.

Spermatogenesis                        ;

The development/production/formation of sperm cells in the testes is called/termed as/known as spermatogenesis. It carries/follows the pattern/sequence of meiosis and generates/produces mature sperm cells(gametes). All (IV/four products of the process meiosis acquire a flagellum for locomotion/movement. They also get a cap like make-up/structure. This cap like make-up assists/helps in the penetration of the egg (gametes).

Oogenesis                                     ;

The development/production/formation of ova or egg in the ovaries of females is called/known/termed as oogenesis. It /generates/produces a mature ovum or egg(female sex hormone). Only one of the IV/ four meiotic products produces/develops into the working/functional gamete/sex cells. So it alters/differs from the process of spermatogenesis. The other 3/III/three results/products of meiosis are called polar bodies. These polar bodies disintegrate. In some living organisms/ animals, the mature egg is the result/product of the Ist/first meiotic division. It finishes/completes the process/operation meiosis only if it is fertilized.