The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. 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 interphase I 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 pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell. Meiosis I vs. 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. Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I.
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. The purpose of mitosis is cell regeneration, growth, and asexual reproduction,while the purpose of meiosis is the production of gametes for sexual reproduction.
Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new daughter 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 haploid daughter 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.
Comparing Meiosis and Mitosis : 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. The main differences between mitosis and meiosis occur in meiosis I. In meiosis I, the homologous chromosome pairs become associated with each other and are bound together with the synaptonemal complex.
Chiasmata develop and crossover occurs between homologous chromosomes, which then 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 tetrad is broken up and the homologous chromosomes move to opposite poles, the ploidy level is 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 similar to a mitotic division. In this case, the duplicated chromosomes only one set, as the homologous pairs have now been separated into two different cells line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles.
During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles. The two daughter cells of mitosis, however, are identical, unlike the daughter cells produced by meiosis. They are different because there has 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.
Meiosis II is, therefore, referred to as equatorial division. Privacy Policy. Skip to main content. Meiosis and Sexual Reproduction. Search for:. The Process of Meiosis. Introduction to Meiosis Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction. Learning Objectives Describe the importance of meiosis in sexual reproduction. Had there been no reduction, the number of chromosome would have multiplied generation after generation.
Meiosis 1 , besides causing reduction in number of chromosomes, is of significance as described below:. Meiosis is responsible for independent segregation of genes present on two homologous chromosomes, which are inherited from male and female parent. Independent segregation occurs during: 1.
Crossing over taking place during prophase 1 stage. Random orientation of bivalents at metaphase 1 stage. Mendel's laws of segregation and law of independent assortment can be explained on the basis of chromosome behaviour during meiosis 1.
Why does reduction of chromosomes takes place in meiosis 1? What is its significance? Krishan T. If membrane tearing in one cell was repaired by membrane flow in the adjacent cell, fusion occurred. After fusion, images of cells were recorded on an optical disk recorder ; Panasonic. After fusion, chromosomes were manipulated with a microneedle. Fusion did not cause cell death, showing that the physiological differences between meiosis I and meiosis II cells do not make the two different cell types incompatible.
The cells retained both a meiosis I and a meiosis II spindle in the same cell Fig. We detached a bivalent from the meiosis I spindle. Such detachment is genuine—the old kinetochore microtubules are lost, so the chromosome must start fresh in forming microtubule attachments Nicklas and Kubai The detached chromosome was then moved to the side of the meiosis II spindle farthest from the meiosis I spindle so that it would have no option but to attach to the meiosis II spindle.
The bivalent promptly attached to the meiosis II spindle Fig. In anaphase, sister chromatids behaved in the normal meiosis I fashion and moved together to their associated pole Fig.
Cohesion behavior was also chromosome intrinsic. Meiosis I chromosomes on meiosis II spindles lost cohesion only between chromatid arms Fig. 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. We also did the reciprocal experiment in these six cells. A meiosis II chromosome was placed near the meiosis I spindle Fig. The meiosis II chromosome attached to the meiosis I spindle and congressed to the spindle equator with sister kinetochores facing opposite poles Fig. In anaphase, single chromatids moved apart to opposite poles Fig.
The results were the same in all 12 experiments in six fused cells. The chromosomes invariably attached and segregated in their normal manner according to chromosome type.
However, it was possible that these characteristics of chromosome behavior were impressed on the chromosome as a result of its first spindle attachment. Therefore, we also did experiments in which chromosomes that had never attached to a spindle in one meiotic division were exposed to the unexpected condition of attaching to a spindle of the other meiotic division.
A cell in late prophase of meiosis I was fused with a cell in metaphase II Fig. After nuclear envelope breakdown of the prophase I nucleus, one of the just-condensed meiosis I chromosomes was placed near the meiosis II spindle Fig. The meiosis I chromosome attached to the meiosis II spindle just as it normally would to a meiosis I spindle, with each pair of sister kinetochores associating with one spindle pole Fig. Experiments of this type performed in three fused cells yielded identical results.
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.
Immediately after nuclear envelope breakdown in the prophase II nucleus, a meiosis II chromosome was placed near the meiosis I spindle Fig. It attached to the meiosis I spindle with a single sister kinetochore facing each pole Fig. Sister chromatids separated from one another in anaphase Fig.
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.
We wanted to determine when chromosomes become capable of attaching to the spindle in the meiosis II manner with a single sister kinetochore facing each spindle pole and releasing cohesion in the meiosis II manner releasing centromere cohesion.
Bivalents in late metaphase I cells were induced to orient with sister kinetochores facing opposite poles a meiosis II type of attachment. This was accomplished by detaching a chromosome from the spindle and repeatedly placing it so its centromeres faced the center of the spindle Fig. The chromosome would eventually attach to the spindle with one kinetochore facing one pole and the sister kinetochore facing the other Fig.
These chromosomes invariably remained at the spindle equator for a long time after anaphase onset with both kinetochores greatly stretched toward the poles Fig. Two of the five chromosomes observed eventually separated sister chromatids to opposite poles Fig.
Three of the five chromosomes observed did not separate sister chromatids to opposite poles Fig. This shows that chromosomes can exhibit meiosis II attachment characteristics by late metaphase I. However, sister chromatids cannot properly separate from one another in anaphase. Separation is either delayed Fig. Such rigorous experiments, in which kinetochores are repeatedly detached from the spindle, might be thought to damage the kinetochores, so that movement in anaphase is abnormal.
That is not the case, however. The whole bivalent was manipulated in these experiments so that both pairs of sister kinetochores one pair on each half-bivalent were treated equally.
In four of the five experiments, only one of the two half-bivalents attached with sister kinetochores to opposite poles Fig. The kinetochores of the other half-bivalent attached to the same spindle pole; they moved normally in anaphase and without delay to that spindle pole Fig. Thus it is the form of the attachment, not the manipulation, that determines how the chromosomes behave in anaphase. 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.
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. We determined when meiosis I chromosomes acquire the ability to behave like normal chromosomes in meiosis II; i.
Prometaphase I or metaphase I cells were fused to anaphase I cells. After fusion, anaphase I chromosomes were detached from the anaphase spindle and placed near the metaphase I spindle Fig. These anaphase I chromosomes attached to the metaphase spindle with a single sister kinetochore facing each pole Fig. These data differ from what we saw in metaphase I cells Fig.
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