How does reduction and rearrangement happen in meiosis

Log in. See Answer. Best Answer. Study guides. Q: Explain how the reduction and rearrangement are accomplished in meiosis? Write your answer Related questions. Meiosis is a process of cell reduction Explain this statement? Reduction of the number of sets of chromosomes occurs during? When does the reduction division in meiosis occur? How is reduction accomplished in meiosis? What is the process of reduction division?

How many divisions are in meiosis? Is meiosis known as reduction division? Meiosis II is identical to? What Process was once called reduction division? Meiosis is referred to as this type of division? What meiosis stage is considered the reduction stage?

Why is meiosis 1 called a reduction division? Why meiosis cell division is called reduction cell division? Explain the reduction in chromosome number that occurs during meiosis?

At which stage of meiosis do homologous chromosomes separate? What is another name for Meiosis you? Reduction division is the same as what form of cell division? Does the chromosome number reduce in half in mitosis? What is reduction division? What is the significance of the reduction in the chromosomes number during meiosis? Recombination sites are at the junction between black and grey chromosome segments. In anaphase of meiosis I, arm cohesion is released and half-bivalents separate.

Kinetochore arrangement: one kinetochore faces one pole, while its sister kinetochore faces the opposite pole. Consequently, sister kinetochores separate from one another in anaphase of meiosis II. Chromosome cohesion: in early stages of meiosis II, only the cohesion between centromeres remains; it is removed in anaphase II. Determinants for the pattern of chromosome attachment to the spindle and release of chromosome cohesion are built into the chromosome.

A metaphase I grasshopper spermatocyte was fused to a metaphase II spermatocyte. Spindle poles are indicated by asterisks, manipulated meiosis I chromosomes by straight arrows, unmanipulated meiosis I chromosomes by curved arrows, manipulated meiosis II chromosomes by filled arrowheads, and unmanipulated meiosis II chromosomes by open arrowheads.

The fused cell contains two spindles. A bivalent was detached from the meiosis I spindle and placed near the meiosis II spindle 0 and 8 min, straight arrows. The bivalent attached to the meiosis II spindle with a pair of sister kinetochores facing each pole 48 min, straight arrows. Pairs of sister chromatids segregated to each pole 69 min, straight arrows. Unmanipulated bivalents on the meiosis I spindle had a pair of sister kinetochores facing each pole 48 min, curved arrows.

In anaphase in unmanipulated bivalents, pairs of sister chromatids separated from one another 69 min, curved arrows. A meiosis II chromosome 12 min, filled arrowhead was detached from the meiosis II spindle and placed near the meiosis I spindle 36 min, filled arrowhead. The meiosis II chromosome attached to the meiosis I spindle with a single sister kinetochore facing each pole 48 min, filled arrowhead , and single sister chromatids moved to opposite poles in anaphase 69 min, filled arrowheads.

Unmanipulated meiosis II chromosomes attached with a single sister kinetochore facing each pole 48 min, open arrowheads and moved to opposite poles in anaphase 69 min, open arrowheads. The way a meiosis I chromosome attaches to the spindle and releases cohesion does not depend on its initial spindle attachment. A late-prophase I spermatocyte and a metaphase II spermatocyte were fused 0 min. The prophase nuclear envelope was still present 0 min, arrowheads.

After nuclear envelope breakdown, a bivalent that had not yet attached to the meiosis I spindle 40 min, arrow was placed near the meiosis II spindle 60 min, arrows.

The manipulated bivalent attached to the meiosis II spindle 85 min, arrows. Pairs of sister chromatids segregated to opposite poles in anaphase.

The upper pair is more clearly visible in the min image, while the lower pair is more clearly visible in the min image. The way a meiosis II chromosome attaches to the spindle and releases cohesion does not depend on its initial spindle attachment.

A prophase II spermatocyte and a metaphase I spermatocyte were fused 0 min. After nuclear envelope breakdown, a meiosis II chromosome that had not yet attached to the meiosis II spindle 0 min, arrow moved near the meiosis I spindle 22 min, arrows. It attached with one chromatid facing each pole 22, 35, and 50 min, arrows , and single sister chromatids segregated to opposite poles in anaphase 57 and 59 min, arrows.

Chromosomes acquire meiosis II properties after anaphase I. A spermatocyte in anaphase I was fused to a spermatocyte in metaphase I 0 min. The spindles are outlined: anaphase I above, metaphase I below. Two different anaphase chromosomes were studied in this experiment, one indicated by an arrow and the other by an arrowhead. The chromosomes were detached from the anaphase I spindle 0 min, arrow; 18 min, arrowhead and placed near the metaphase I spindle 20 min, arrows; 60 min, arrowheads.

The chromosomes attached to the metaphase I spindle, with a sister kinetochore facing each pole 20 and 60 min, arrows; 60 min, arrowheads. The manipulated meiosis I chromosomes behaved just like meiosis II chromosomes when the cell entered anaphase, sending a single chromatid to each pole and min, arrowheads and arrows.

Bivalents can be induced to attach to the spindle with a single sister kinetochore facing each pole, but they neither attach nor separate in the normal meiosis II manner. Bivalents in unfused spermatocytes were micromanipulated. A One pair of sister kinetochores of the bivalent was induced to attach to opposite spindle poles 0 min, arrows.

The chromosome remained, with stretched-out kinetochores, at the equator of the spindle after anaphase onset 6 min, arrows. B Another example of a bivalent in which one pair of sister kinetochores was induced to attach to opposite poles 0 min, arrows. In anaphase I, sister kinetochores were greatly stretched towards their spindle poles, but the sister chromatids did not separate from one another 4 and 17 min, arrows. C One pair of sister kinetochores of the bivalent was induced to attach to opposite poles 0 min, arrow , while the other pair of sister kinetochores attached to the same pole 0 min, arrowhead.

In the pair that did attach to opposite poles, the sister chromatids did not separate from one another 27 and 32 min, arrows. The other pair of sister kinetochores attached to the lower spindle pole 0 and 27 min, arrowheads and moved together to that pole in anaphase 27 and 32 min, arrowheads.

Sign In or Create an Account. Advanced Search. User Tools. Sign In. Skip Nav Destination Article Navigation. Article September 18 Paliulis , Leocadia V. This Site. Google Scholar. Bruce Nicklas R. Bruce Nicklas. Author and Article Information. Received: June 09 Revision Requested: August 09 Accepted: August 11 Online Issn: J Cell Biol 6 : — Article history Received:. Revision Requested:. Cite Icon Cite. Genetic interactions between mei-S and ord in the control of sister-chromatid cohesion.

Search ADS. The role of sister chromatid cohesiveness and structure in meiotic behaviour. Kinetochore structure and its role in chromosome orientation during the first meiotic division in male D. The Drosophila mei-s gene promotes sister-chromatid cohesion in meiosis following kinetochore differentiation. Mei-S, a Drosophila protein required for sister-chromatid cohesion, can localize to meiotic centromere regions.

A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis. Meiosis in Drosophila melanogaster. The effect of orientation disruptor ord on gonial mitotic and the meiotic divisions in males.

The centromeric sister chromatid cohesion site directs Mcd1 binding to adjacent sequences. The cohesion protein MEI-S localizes to condensed meiotic and mitotic centromeres until sister chromatids separate. Microtubules, chromosome movement, and reorientation after chromosomes are detached from the spindle by micromanipulation.

Spindle microtubules and their mechanical associations after micromanipulation in anaphase. The ties that bindlocalization of the sister-chromatid cohesin complex on yeast chromosomes. A comparative study of orientation at behavior of univalent in living grasshopper spermatocytes.

Involvement of chromatid cohesiveness at the centromere and chromosome arms in meiotic chromosome segregationa cytological approach. Identification of cohesin association sites at centromeres and along chromosome arms. Cohesin Rec8 is required for reductional chromosome segregation at meiosis. Figure 1. View large Download slide. Figure 2. Figure 3. Figure 4. Figure 6. In each cell that undergoes meiosis, the arrangement of the tetrads is different.

The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate; the possible number of alignments therefore equals 2 n , where n is the number of chromosomes per set.

Humans have 23 chromosome pairs, which results in over eight million 2 23 possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover.

Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition Figure 3. To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

Figure 3. In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2 n , where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes.

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart Figure 4. In telophase, the separated chromosomes arrive at opposite poles.

The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components 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 constriction of the actin ring that leads to cytoplasmic division.

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 that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, 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.

Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes except for changes that occurred during crossing over. In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells. In some species, cells enter a brief interphase, or interkinesis , before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony.

During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.

Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis. If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.

The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. The sister chromatids are maximally condensed and aligned at the equator of the cell. The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.

Non-kinetochore microtubules elongate the cell. Figure 4. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II.

In anaphase II, the sister chromatids are separated. The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. 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 and because of the recombining of maternal and paternal segments of chromosomes with their sets of genes that occurs during crossover.

The entire process of meiosis is outlined in Figure 5. Figure 5. Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells.

They share some similarities, but also exhibit distinct differences that lead to very different outcomes Figure 6. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells.

In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only.

This is half the number of chromosome sets in the original cell, which is diploid. The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad.

All of these events occur only in meiosis I. When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis. Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes only one set of them line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles.

During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossover, the two products of each individual meiosis II division would be identical like in mitosis.

Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I. Figure 6. Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions.

The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.