Like mitosis, meiosis is a formof eukaryotic cell division. However, these two processes distribute geneticmaterial among the resulting daughter cells in very different ways. Mitosiscreates two identical daughter cells that each contain the same number ofchromosomes as their parent cell. In contrast, meiosis gives rise to fourunique daughter cells, each of which has half the number of chromosomes as the parentcell. Because meiosis creates cells that are destined to become gametes (or reproductive cells), thisreduction in chromosome number is critical — without it, the union of twogametes during fertilization would result in offspring with twice the normalnumber of chromosomes!


Apart from this reduction in chromosome number, meiosis differs from mitosisin yet another way. Specifically, meiosis creates new combinations of geneticmaterial in each of the four daughter cells. These new combinations result fromthe exchange of DNA between paired chromosomes. Such exchange means that thegametes produced through meiosis exhibit an amazing range of genetic variation.

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Finally, unlike mitosis, meiosis involves two rounds of nuclear division, notjust one. Despite this fact, many of the other events of meiosis are similar tothose that occur in mitosis. For example, prior to undergoing meiosis, a cellgoes through an interphase period in which it grows, replicates its chromosomes,and checks all of its systems to ensure that it is ready to divide. Likemitosis, meiosis also has distinct stages called prophase, metaphase, anaphase,and telophase. A key difference, however, is that during meiosis, each of thesephases occurs twice — once during the first round of division, called meiosisI, and again during the second round of division, called meiosis II.


As previously mentioned, the first round of nuclear division that occurs duringthe formation of gametes is called meiosisI. It is also known as the reductiondivision because it results in cells that have half the number ofchromosomes as the parent cell. Meiosis I consists of four phases: prophase I,metaphase I, anaphase I, and telophase I.


During prophase I, the chromosomescondense and become visible inside the nucleus. Because each chromosome wasduplicated during the S phase that occurred just before prophase I, each nowconsists of two sister chromatids joined at the centromere. This arrangement meansthat each chromosome has the shape of an X.


Once this chromosomal condensation has occurred, the members of eachchromosome pair (called homologouschromosomes, because they are similar in size and contain similar genes),align next to each other. At this point, the two chromosomes in each pairbecome tightly associated with each other along their lengths in a processcalled synapsis. Then, while thehomologous chromosomes are tightly paired, the members of each pair tradeadjacent bits of DNA in a process called crossingover, also known as recombination(Figure 1). This trading of genetic material creates unique chromosomes thatcontain new combinations of alleles.


At the end of prophase I, the nuclear membrane finallybegins to break down. Outside the nucleus, the spindle grows out fromcentrosomes on each side of the cell. As in mitosis, the microtubules of thespindle are responsible for moving and arranging the chromosomes duringdivision.
", "182", "http://www.ubraintv-jp.com/ubraintv-jp.com_education", "At the end of metaphase one, homologous chromosomes line up in the center of the cell. Each chromosome looks like an elongated X-shaped structure. In the pair of chromosomes at top, the chromosome at left is mostly green, but the lower region of the right chromatid is orange. The chromosome at right is mostly orange, but the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair. Mitotic spindles are located at each side of the cell. Each spindle apparatus is composed of several white lines, representing fibers, emanating from two oval-shaped structures, representing centrosomes. The fibers attach the centrosomes to the centromeres of each chromosome. Shorter fibers also emanate from the mitotic spindle but are not attached to chromosomes.")" class="inlineLinks">Figure Detail
At the start of metaphase I, microtubules emerge from the spindle and attach to the kinetochore near the centromere of each chromosome. In particular, microtubules from one side of the spindle attach to one of the chromosomes in each homologous pair, while microtubules from the other side of the spindle attach to the other member of each pair. With the aid of these microtubules, the chromosome pairs then line up along the equator of the cell, termed the metaphase plate (Figure 2).
Figure 3:During anaphase I, the homologous chromosomes are pulled toward opposite poles of the cell.
", "182", "http://www.ubraintv-jp.com/ubraintv-jp.com_education", "In the pair of chromosomes at top, the chromosome at left is moving toward the left-hand mitotic spindle; the chromosome is mostly green, but the lower region of the right chromatid is orange. The chromosome at right is moving toward the right-hand mitotic spindle. The chromosome is mostly orange, but the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair, mirroring the movements of the chromosomes above.")" class="inlineLinks">Figure Detail
During anaphase I, themicrotubules disassemble and contract; this, in turn, separates the homologouschromosomes such that the two chromosomes in each pair are pulled toward oppositeends of the cell (Figure 3). This separation means that each of the daughtercells that results from meiosis I will have half the number of chromosomes ofthe original parent cell after interphase. Also, the sister chromatids in each chromosomestill remain connected. As a result, each chromosome maintains its X-shapedstructure.
Figure 4:Telophase I results in the production of two nonidentical daughter cells, each of which has half the number of chromosomes of the original parent cell.
As the new chromosomes reach the spindle during telophase I, the cytoplasm organizes itself and divides in two. There are now two cells, and each cell contains half the number of chromosomes as the parent cell. In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I (Figure 4).
At this point, the first division of meiosis is complete. The cell now restsfor a bit before beginning the second meiotic division. During this period,called interkinesis, thenuclear membrane in each of the two cells reforms around the chromosomes. Insome cells, the spindle also disintegrates and the chromosomes relax (althoughmost often, the spindle remains intact).It is important to note, however, that no chromosomal duplication occurs during this stage.

During meiosis II, the two cellsonce again cycle through four phases of division. Meiosis II is sometimesreferred to as an equational divisionbecause it does not reduce chromosome number in the daughter cells — rather, thedaughter cells that result from meiosis II have the same number of chromosomesas the "parent" cells that enter meiosis II. (Remember, these "parent" cellsalready have half the number of chromosomes of the original parent cell thanksto meiosis I.)


As prophaseII begins, the chromosomes once again condense into tight structures, andthe nuclear membrane disintegrates. In addition, if the spindle was disassembledduring interkinesis, it reforms at this point in time.
The events of metaphase II are similar to those of mitotic metaphase — in bothprocesses, the chromosomes line up along the cell"s equatorial plate, alsocalled the metaphase plate, in preparation for their eventual separation(Figure 5).
During anaphaseII, microtubules from each spindle attach to each sister chromatid at thekinetochore. The sister chromatids then separate, and the microtubules pullthem to opposite poles of the cell. As in mitosis, each chromatid is nowconsidered a separate chromosome (Figure 6). This means that the cells thatresult from meiosis II will have the same number of chromosomes as the "parent"cells that entered meiosis II.
Finally, in telophase II, nuclearmembranes reform around the newly separated chromosomes, which relax and fadefrom view. As soon as the cytoplasm divides, meiosis is complete. There are nowfour daughter cells — two from each of the two cells that entered meiosis II —and each daughter cell has half the normal number of chromosomes (Figure 7).Each also contains new mixtures of genes within its chromosomes, thanks torecombination during meiosis I.

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Meiosis is important because it ensures that allorganisms produced via sexual reproduction contain the correct number ofchromosomes. Meiosis also produces genetic variation by way of the process ofrecombination. Later, this variation is increased even further when two gametesunite during fertilization, thereby creating offspring with unique combinationsof DNA. This constant mixing of parental DNA in sexual reproduction helps fuelthe incredible diversity of life on Earth.