Like mitosis, meiosis is a formof eukaryotic bio cell division. However, these two processes distribute geneticmaterial among the result daughter cell in really different ways. Mitosiscreates two identical daughter cells that each contain the exact same number ofchromosomes as their parental cell. In contrast, meiosis provides rise to fourunique daughter cells, each of which has half the variety of chromosomes as the parentcell. Because meiosis creates cell that space destined to end up being gametes (or reproductive cells), thisreduction in chromosome number is vital — without it, the union the twogametes throughout fertilization would result in offspring v twice the normalnumber the chromosomes!
Apart native this palliation in chromosome number, meiosis differs from mitosisin yet another way. Specifically, meiosis creates brand-new combinations that geneticmaterial in each of the four daughter cells. These brand-new combinations result fromthe exchange the DNA between paired chromosomes. Such exchange method that thegametes developed through meiosis exhibit an amazing range of genetic variation.
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Finally, unlike mitosis, meiosis entails two ring of nuclear division, notjust one. Regardless of this fact, countless of the other occasions of meiosis are similar tothose that happen in mitosis. For example, before undergoing meiosis, a cellgoes through an interphase period in which that grows, replicates that is chromosomes,and checks every one of its solution to ensure the it is all set to divide. Likemitosis, meiosis likewise has unique stages referred to as prophase, metaphase, anaphase,and telophase. A vital difference, however, is that throughout meiosis, every of thesephases occurs double — once during the an initial round the division, called meiosisI, and also again during the second round that division, called meiosis II.
As previously mentioned, the an initial round the nuclear division that occurs duringthe development of gametes is dubbed meiosisI. The is also known as the reductiondivision due to the fact that it outcomes in cells that have half the number ofchromosomes together the parental cell. Meiosis I is composed of 4 phases: prophase I,metaphase I, anaphase I, and also telophase I.
During prophase I, the chromosomescondense and become visible inside the nucleus. Due to the fact that each chromosome wasduplicated throughout the S phase that occurred just before prophase I, every nowconsists of 2 sister chromatids join at the centromere. This plan meansthat every chromosome has the form of one X.
Once this chromosomal condensation has occurred, the members that eachchromosome pair (called homologouschromosomes, because they are similar in size and also contain similar genes),align next to each other. At this point, the 2 chromosomes in each pairbecome tightly associated with each various other along your lengths in a processcalled synapsis. Then, if thehomologous chromosomes space tightly paired, the members of every pair tradeadjacent bits the DNA in a procedure called crossingover, likewise known as recombination(Figure 1). This trade of hereditary 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 every side of the cell. Together in mitosis, the microtubules that thespindle are responsible because that moving and arranging the chromosomes duringdivision.
", "182", "http://www.cg-tower.com/cg-tower.com_education", "At the finish of metaphase one, homologous chromosomes heat up in the center of the cell. Every chromosome looks prefer an elongated X-shaped structure. In the pair the chromosomes in ~ top, the chromosome at left is greatly green, but the lower region of the best chromatid is orange. The chromosome at best is greatly orange, however the lower an ar of the left chromatid is green. A second pair the chromosomes exhibiting the same pattern the coloration on their arms is shown below the topmost pair. Mitotic spindles are situated at every side the the cell. Each spindle apparatus is composed of number of white lines, representing fibers, emanating from two oval-shaped structures, representing centrosomes. The fibers attach the centrosomes come the centromeres of each chromosome. Shorter fibers also emanate from the mitotic spindle yet are no attached to chromosomes.")" class="inlineLinks">Figure Detail
At the start of metaphase I, microtubules emerge from the spindle and also attach come the kinetochore close to the centromere of each chromosome. In particular, microtubules indigenous one next of the spindle affix to among the chromosomes in each homologous pair, when microtubules indigenous the various other side that the spindle attach to the various other member of every pair. With the help of these microtubules, the chromosome pairs then line up along the equator that the cell, termed the metaphase key (Figure 2).
Figure 3:During anaphase I, the homologous chromosomes are pulled toward opposite poles the the cell.
", "182", "http://www.cg-tower.com/cg-tower.com_education", "In the pair of chromosomes at top, the chromosome at left is relocating toward the left-hand mitotic spindle; the chromosome is mainly green, however the lower region of the appropriate chromatid is orange. The chromosome at ideal is moving toward the right-hand mitotic spindle. The chromosome is mainly orange, however the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the very same pattern of coloration on your arms is shown below the topmost pair, mirroring the movements of the chromosomes above.")" class="inlineLinks">Figure Detail
During anaphase I, themicrotubules disassemble and also contract; this, in turn, separates the homologouschromosomes such the the 2 chromosomes in each pair are pulled toward oppositeends of the cell (Figure 3). This separation means that every of the daughtercells that results from meiosis I will certainly have half the number of chromosomes ofthe original parent cabinet after interphase. Also, the sister chromatids in each chromosomestill remain connected. As a result, every chromosome maintains its X-shapedstructure.
Figure 4:Telophase I results in the manufacturing of 2 nonidentical daughter cells, every of which has half the variety of chromosomes of the initial parent cell.
As the new chromosomes with the spindle during telophase I, the cytoplasm organizes itself and divides in two. Over there are currently two cells, and also each cell contains half the number of chromosomes as the parental cell. In addition, the two daughter cells room not genetically identical to each other since of the recombination that emerged during prophase i (Figure 4).
At this point, the first division of meiosis is complete. The cell now restsfor a bit prior to beginning the 2nd meiotic division. During this period,called interkinesis, thenuclear membrane in each of the two cells reforms around the chromosomes. Insome cells, the spindle additionally disintegrates and also the chromosomes be sure (althoughmost often, the spindle remains intact).It is necessary to note, however, the no chromosomal duplication occurs during this stage.
During meiosis II, the 2 cellsonce again bicycle through four phases that division. Meiosis II is sometimesreferred to together an equational divisionbecause that does not alleviate chromosome number in the daughter cells — rather, thedaughter cell that an outcome from meiosis II have actually the same number of chromosomesas the "parent" cell that enter meiosis II. (Remember, these "parent" cellsalready have fifty percent the variety of chromosomes the the original parent cell thanksto meiosis I.)
As prophaseII begins, the chromosomes when again condense into tight structures, andthe atom membrane disintegrates. In addition, if the spindle was disassembledduring interkinesis, it reforms at this suggest in time.
The events of metaphase II are comparable to those that mitotic metaphase — in bothprocesses, the chromosomes line up follow me the cell"s equatorial plate, alsocalled the metaphase plate, in preparation for their ultimate separation(Figure 5).
During anaphaseII, microtubules from every spindle attach to each sister chromatid in ~ thekinetochore. The sister chromatids then separate, and the microtubules pullthem come opposite poles the the cell. As in mitosis, every chromatid is nowconsidered a separate chromosome (Figure 6). This means that the cell thatresult from meiosis II will have actually the same variety of chromosomes as the "parent"cells that gone into meiosis II.
Finally, in telophase II, nuclearmembranes reform around the freshly separated chromosomes, which relax and also fadefrom view. As soon as the cytoplasm divides, meiosis is complete. There room nowfour daughter cells — two from each of the 2 cells that gotten in meiosis II —and each daughter cabinet has half the normal variety of chromosomes (Figure 7).Each also contains brand-new mixtures of gene within that is chromosomes, thanks torecombination during meiosis I.
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Meiosis is important because it ensures that allorganisms created via sex-related reproduction save the correct number ofchromosomes. Meiosis also produces genetic variation by method of the process ofrecombination. Later, this variation is increased also further once two gametesunite throughout fertilization, thereby creating offspring with distinct combinationsof DNA. This consistent mixing of parental DNA in sex-related reproduction help fuelthe incredible diversity the life ~ above Earth.