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The process of meiosis

A model of the entire process of meiosis.

Farabee Kabir

on 4 April 2013

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Transcript of The process of meiosis

Meiosis is the process of cell division where haploid gametes are formed from a diploid cell. It also forms cells with half the number of chromosomes normally found in cells of the species. In this process there are two divisions, meiosis 1 and meiosis 2; so in the end, four haploid cells are formed. The cells formed are genetically different from the original one due to the crossing overs. The steps of meiosis are interphase, prophase 1, prometaphase 1, metaphase 1, anaphase 1, telophase 1 and cytokinesis, then again, prophase 11, prometaphase 11, metaphase 11, anaphase 11, telophase 11, and finally cytokinesis. Most human cells normal contain 46 chromosomes. Forty-six is known as the diploid number. The symbol 2n is used for the diploid number. Sex cells or gametes contain 23 chromosomes. The process of meiosis Interphase Meiosis starts off with the process of interphase which consists of three phases, G1, S, and G2. The first phase, G1, which is also known as the first gap phase, focuses on cell growth merely. As for the S phase, second phase of the process, is where the replication of DNA occurs. Lastly, the second gap phase, G2, and the final step of interphase prepares for the rest of the process of meiosis.

While the S phase is going under DNA duplication, sister chromatids, which are the two identical copies produced from the replication of each chromosome, are formed. The sister chromatids are held together at the centrosome by cohesin proteins, until anaphase 11. Centrosomes, an organelle from which the spindle fibers develops, also goes through replication. Thus helps the cell to undergo the first meotic phase of prophase 1. This is the longest phase of meiosis. Prior to prophase 1, as shown in the previous diagram, the DNA is loosely entangled in chromatin bundles, but during prophase 1, these loose loops then condenses by shortening and thickening to form into highly ordered elongated structures, which is called the chromosomes. It is now visible under microscope.

During prophase 1, the centrosomes separate and move towards opposite poles of the cell, organizing the microtubules or spindle fibers, which guide the movement of chromosomes in later phases of mitosis. Each daughter cell will inherit one centrosome.

As the nucleoli and nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other, this is one of the important process of this phase and it is called the synapsis. The second important step of this phase is called crossing over. Prophase 1 Prometaphase 1 Metaphase 1 is where the homologous pairs randomly assemble at the midway point between the two poles of the cell to form the metaphase plate, with the kinetochores still facing opposite poles. This random and at the metaphase plate, is the physical basis for the creation of the second form of genetic variation in offspring.

The reason being for the randomness is because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome. Also due to the random orientation of the homologous pairs, any maternally inherited chromosome may face either pole of the cell and same goes for any paternally inherited chromosome. The orientation of each tetrad is independent of the orientation of the other 22 tetrads, so it doesn't affect one another. As for each other cells that undergoes the meiotic process, the arrangement of the tetrads is also different. The number of variations is dependent on the number of chromosomes making up a set.

So far to summarise the genetic consequences of meiosis 1, crossing over between each homologous pair in prophase 1, leads maternal and paternal genes to be recombined. Moreover, in this phase, the random and independent assortment of homologous chromosomes on the metaphase plate is the second mechanism that produces a unique variation of maternal and paternal chromosomes into the gametes or spores. Metaphase 1 In the process of synapsis as the two homologous chromosomes come near each other, it forms tetrads. Tetrads are the combination of four chromatids, which resulted from the pair up with the two chromosomes, which consisted two chromatids each.

After synapsis has taken place, the homologous chromosomes exchange genetic material in a process called crossing over. In this process, gentically new chromatids are formed by segments of DNA from one chromatid in the tetrad, passing to another chromatid in the tetrad. Crossing over contributes to the genetic variation of sexual reproduction. After crossing over has taken place, the four chromatids of the tetrad are genetically different from the original four chromatids. The key event in prometaphase 1 is the attachment of the kinetochore proteins at the centromeres to the spindle fiber microtubules. Kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart.

In order to pull sister chromatids apart more easily, the microtubles at opposite poles of the cell moves more towards the middle of the cell, closer to the chromosomes. As this is happening, a kinetochore microtuble spindle fiber that attaches to a kinetochore) attaches itself to the kinetochore of a synapsed homologous chromosome, and it pulls the tetrad toward the originating pole. This then leads the tetrad being pulled into a position favourable for microtubules from the opposing pole to locate and attach to the fused kinetochore of the other homologous chromosome.

By the end of prometaphase I, each homologous chromosomes are facing towards their own poles due to each of their tetrads being attached to microtubules from both poles. Nevertheless, so far the homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely. Meiosis 1:
The role of meiosis 1 is to separate homologous chromosomes into producing two haploid cells (N chromosomes, 23 in humans), and that is why this part of the meiotic process, meiosis 1, is also referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis 1, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes. This is because later, in Anaphase 1, the sister chromatids will remain together as the spindle fibers pull the pair toward the pole of the new cell. Meiosis 1 proceeds through the following phases... Anaphase 1 In anaphase I, the kinetochore microtubules shorten, segregating the homologous chromosomes apart. Since each chromosome has only one fuctional unit of a pair of kinetochores, whole chromosomes are pulled toward opposing poles, forming two haploid sets. In other words, one homologous chromosome (consisting of two chromatids) moves to one side of the cell, while the other homologous chromosome moves to the other side of the cell. This obtains the result of 23 chromosomes (each consisting of two chromatids) move to one pole, and 23 chromosomes (each consisting of two chromatids) move to the other pole. Essentially, the chromosome number of the cell is halved which leads the process being a reduction-division.

Nevertheless, the sister chromatids still remain tightly bound together at the centromere but the chiasmata are broken, in this phase, as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.

During this time disjunction occurs, which is one of the processes leading to genetic diversity as each chromosome can end up in either of the daughter cells. Non-kinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center. Telophase 1 and Cytokinesis Telophase 1 is where the first meiotic division effectively ends when the separated chromosomes arrive at opposite poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. After the microtubules that make up the spindle network disappear, the remainder of the typical telophase 1 events may or may not occur, depending on the type of species. In some organisms, chromosomes decondenses back into chromatin and a new nuclear membrane surrounds the chromatids in this phase. In other organisms, cytokinesis—physical division of the cell, bringing about the separation into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow, or cell membrane, whilst in plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls completing the creation of two daughter cells. Sister chromatids remain attached during telophase I.

Therefore, the end result of this first meiotic division are the two haploid cells. The results can now be called haploid cells because at each pole, there is just one of each pair of the homologous chromosomes. Thus, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. However, although the sister chromatids were once duplicates of the same chromosome, they are not identical at this stage because of crossovers.

Cells may enter a period of rest known as interkinesis or straight onto the second meiotic division to obtain the complete set of haploid gametes, depending on the species. No DNA replication occurs during this stage. Meiosis 2:
Meiosis 2, also known as equational division, is the second part of the meiotic process. The end result is production of four haploid cells (23 chromosomes, N in humans) from the two haploid cells (23 chromosomes, N * each of the chromosomes consisting of two sister chromatids) produced in meiosis 1. Meiosis 2 resembles the mitotic division of a haploid cell. Meiosis 2 proceeds through the following phases... Like prophase 1, the chromosomes condense again by shortening and thickening, if decondensed back in telophase 1. Same for the nuclear membrane and nucleoli, it starts to break down again if it was formed in the end of the first division. This time, the centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. Prophase 2 Prometaphase 2 By now, in prometaphase 2, the nuclear envelopes are completely disappeared, and the spindle is fully formed. The spindle fibers from microtubules at opposite poles, engage the individual kinetochore, formed by each sister chromatid. Metaphase 2 In metaphase 2 of meiosis, the 23 sister chromatid pairs are maximally condensed and gathered at the center of the cell prior to separation. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. Anaphase 2 This phase starts off where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart, this again is random segregation.Which sister chromatid of each pair ends up in a daughter cells to gamete is random and determined completely independently of the separation of any other gene pair. This further leads to genetic variation. As they are moving towards opposing poles, the 46 sister chromatids by convention are now 46 sister chromosomes. Then the 46 chromosomes separate from one another. Spindle fibers move one chromosome from each pair to one pole of the cell and the other member of the pair to the other pole. In all, 23 chromosomes move to each pole. Non-kinetochore microtubules elongate the cell. Telophase 2 and Cytokinesis The whole meiotic process ends with telophase 2, which is similar to telophase 1, where the chromosomes arrive at opposite poles and begin to uncoil, decondense and form a mass of chromatin, again. Nuclear envelopes reform around the chromosomes and the nucleoli reappears. Spindle fiber, disappears for once more.

Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs, the segregation of sister chromatids and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.

Meiosis is now complete and ends up with four new daughter cells. DNA - where the variation of genes originates from.. Crossing over Crossing over introduces genetic variation because the arms of homologous chromosomes exchange genetic information as genes that are in the same chromosomes are linked. Those linked genes on the chromosomes are inherited independently of each other and this is done over synapsis. The exchange in the arms of the homologous chromosomes introduces genetic variation causes the mixing of paternal and maternal genes and the result leads to an increased number of combinations of genes that may be transmitted by gametes to offspring, thereby increasing genetic variation. independent assortment Random segregation Since the chromosomes in each pair of homologous chromosomes segregate in this phase, one entire chromosome of each pair moves into a daughter cell. Thus, it's called random segregation; random because it can be any chromosome moving into the daughter cell. Independent assortment lead to this random orientation of chromosomes. This separation of chromosomes ensures the chromosome number in the resulting gametes will be half that of the original cell.

The paternal and maternal chromosomes sort themselves independently of each other. For example, the maternal chromosomes do not all move into one gamete and the paternal into another. Where the chromosomes of each pair ends up in a gamete is random and determined completely independently of the separation of any other gene pair. Therefore as a result, the mixing of maternal and paternal chromosomes occurs and so independent assortment enhances the genetic variation.

Sexual reproduction is important in increasing genetic variability— the amount by which individuals in a population vary from each other genetically.
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