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You are watching: Which cells in a multicellular organism undergo meiosis

Cooper GM. The Cell: A molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.


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The somatic cabinet cycles discussed so much in this chapter result in diploid daughter cells with identical hereditary complements. Meiosis, in contrast, is a devoted kind of cabinet cycle the reduces the chromosome number by half, causing the production of haploid daughter cells. Uncellular cells eukaryotes, such together yeasts, can undergo meiosis as well as reproducing by mitosis. Diploid Saccharomyces cerevisiae, because that example, experience meiosis and produce spores when confronted with unfavorable eco-friendly conditions. In multicellular plants and animals, however, meiosis is limited to the germ cells, wherein it is crucial to sex-related reproduction. Vice versa, somatic cell undergo mitosis to proliferate, the germ cell undergo meiosis to develop haploid gametes (the sperm and the egg). The advance of a brand-new progeny organism is climate initiated by the blend of these gametes in ~ fertilization.


The procedure of Meiosis

In contrast to mitosis, meiosis outcomes in the division of a diploid parental cell into haploid progeny, each containing only one member of the pair the homologous chromosomes that were existing in the diploid parent (Figure 14.32). This palliation in chromosome number is accomplished by two sequential rounds of nuclear and also cell department (called meiosis I and also meiosis II), which follow a single round that DNA replication. Like mitosis, meiosis i initiates after ~ S phase has actually been completed and the parental chromosomes have replicated to create identical sister chromatids. The sample of chromosome distinction in meiosis I, however, is dramatically different from the of mitosis. Throughout meiosis I, homologous chromosomes very first pair through one another and then segregate to various daughter cells. Sister chromatids stay together, so completion of meiosis I outcomes in the development of daughter cell containing a single member of each chromosome pair (consisting of two sister chromatids). Meiosis i is adhered to by meiosis II, which resembles mitosis in the the sister chromatids separate and also segregate to different daughter cells. Perfect of meiosis II therefore results in the production of four haploid daughter cells, each of which contains only one copy of every chromosome.


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Figure 14.32

Comparison the meiosis and mitosis. Both meiosis and mitosis initiate after DNA replication, so every chromosome consists of 2 sister chromatids. In meiosis I, homologous chromosomes pair and then segregate to different cells. Sister chromatids then separate (more...)


The pairing of homologous chromosomes ~ DNA replication is not just a crucial event underlying meiotic chromosome segregation, but also permits recombination in between chromosomes the paternal and also maternal origin. This vital pairing the homologous chromosomes takes place during an extensive prophase of meiosis I, i beg your pardon is separated into 5 stages (leptotene, zygotene, pachytene, diplotene, and also diakinesis) on the communication of chromosome morphology (Figure 14.33). The initial association of homologous chromosomes is assumed to be mediated by base pairing in between complementary DNA strands throughout the leptotene stage, before the dyed becomes highly condensed. The close combination of homologous chromosomes (synapsis) begins during the zygotene stage. During this stage, a zipperlike protein structure, dubbed the synaptonemal complex, creates along the length of the paired chromosomes (Figure 14.34). This facility keeps the homologous chromosomes very closely associated and also aligned with one one more through the pachytene stage, which have the right to persist for numerous days. Recombination in between homologous chromosomes is completed during their combination at the pachytene stage, leave the chromosomes linked at the website of crossing over (chiasmata; singular, chiasma). The synaptonemal complicated disappears in ~ the diplotene stage and also the homologous chromosomes separate along your length. Importantly, however, lock remain associated at the chiasmata, i m sorry is critical for their correct alignment at metaphase. At this stage, every chromosome pair (called a bivalent) consists of 4 chromatids with clearly evident chiasmata (Figure 14.35). Diakinesis, the last stage the prophase I, to represent the shift to metaphase, during which the chromosomes become totally condensed.


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Figure 14.33

Stages the the prophase the meiosis I. Micrographs illustrating the morphology that chromosomes of the lily. (C. Hasenkampf/Biological photo Service.)


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Figure 14.34

The synaptonemal complex. Dyed loops room attached come the lateral elements, which room joined come each various other by a zipperlike central element.


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Figure 14.35

A bivalent chromosome in ~ the diplotene stage. The bivalent chromosome consists of combine homologous chromosomes. Sister chromatids of each chromosome room joined in ~ the centromere. Chromatids that homologous chromosomes are joined at chiasmata, which are (more...)


At metaphase I, the bivalent chromosomes align on the spindle. In contrast to mitosis (see figure 14.27), the kinetochores that sister chromatids are surrounding to each other and also oriented in the very same direction, when the kinetochores of homologous chromosomes room pointed towards opposite spindle poles (Figure 14.36). Consequently, microtubules native the very same pole of the spindle connect to sisters chromatids, if microtubules from opposite poles affix to homologous chromosomes. Anaphase ns is initiated through disruption of the chiasmata in ~ which homologous chromosomes space joined. The homologous chromosomes then separate, when sister chromatids remain connected at your centromeres. At perfect of meiosis I, each daughter cell has because of this acquired one member of every homologous pair, consist of of two sister chromatids.


Figure 14.36

Chromosome distinction in meiosis I. In ~ metaphase I, the kinetochores of sister chromatids space either unify or adjacent to one another. Microtubules native the exact same pole of the spindle thus attach come the kinetochores the sister chromatids, if microtubules (more...)


Meiosis II initiates instantly after cytokinesis, usually prior to the chromosomes have totally decondensed. In contrast to meiosis I, meiosis II each other a regular mitosis. In ~ metaphase II, the chromosomes align on the spindle with microtubules from opposite poles of the spindle attached to the kinetochores the sister chromatids. The link between the centromeres of sister chromatids is broken at anaphase II, and sister chromatids segregate come opposite poles. Cytokinesis then follows, offering rise come haploid daughter cells.


Regulation that Oocyte Meiosis

Vertebrate oocytes (developing eggs) have actually been particularly useful models for research on the cell cycle, in component because of their big size and also ease that manipulation in the laboratory. A noteworthy example, discussed earlier in this chapter, is listed by the discovery and subsequent purification the MPF from frog oocytes. Meiosis of this oocytes, favor those of various other species, is regulated at two distinct points in the cell cycle, and also studies the oocyte meiosis have actually illuminated novel instrument of cabinet cycle control.

The very first regulatory suggest in oocyte meiosis is in the diplotene phase of the first meiotic division (Figure 14.37). Oocytes deserve to remain arrested at this stage for lengthy periods of time—up come 40 to 50 years in humans. During this diplotene arrest, the oocyte chromosomes decondense and also are proactively transcribed. This transcriptional task is reflected in the tremendous expansion of oocytes throughout this period. Human being oocytes, for example, are around 100 μm in diameter (more than a hundred times the volume the a common somatic cell). Frog oocytes are also larger, with diameters of roughly 1 mm. Throughout this period of cabinet growth, the oocytes accumulate stockpiles the materials, including RNAs and also proteins, that are required to assistance early advance of the embryo. As listed earlier in this chapter, early embryonic cell cycles then happen in the absence of cabinet growth, rapidly splitting the fertilized egg right into smaller cell (see number 14.2).


Figure 14.37

Meiosis that vertebrate oocytes. Meiosis is arrested at the diplotene stage, throughout which oocytes grow to a large size. Oocytes then resume meiosis in solution to hormonal stimulation and also complete the very first meiotic division, through asymmetric cytokinesis (more...)


Oocytes the different types vary as to when meiosis resumes and also fertilization takes place. In some animals, oocytes stay arrested in ~ the diplotene stage until they space fertilized, just then proceeding to finish meiosis. However, the oocytes of most vertebrates (including frogs, mice, and humans) resume meiosis in an answer to hormonal stimulation and proceed v meiosis I before fertilization. Cell division following meiosis ns is asymmetric, causing the manufacturing of a small polar body and also an oocyte the retains its big size. The oocyte climate proceeds to go into meiosis II without having re-formed a cell nucleus or decondensed that is chromosomes. Many vertebrate oocytes are then arrested again in ~ metaphase II, whereby they continue to be until fertilization.

Like the M phase of somatic cells, the meiosis of oocytes is regulated by MPF. The regulation that MPF throughout oocyte meiosis, however, display screens unique features that space responsible for metaphase II arrest (Figure 14.38). Hormone stimulation the diplotene-arrested oocytes at first triggers the resumption that meiosis through activating MPF, as at the G2 come M transition of somatic cells. As in mitosis, MPF then induces chromosome condensation, nuclear envelope breakdown, and also formation that the spindle. Activation that the anaphase-promoting complicated B then leads come the metaphase to anaphase transition of meiosis I, accompanied by a diminish in the activity of MPF. Adhering to cytokinesis, however, MPF activity again rises and also remains high if the egg is arrested at metaphase II. A regulatory mechanism distinctive to oocytes hence acts to preserve MPF activity during metaphase II arrest, staying clear of the metaphase come anaphase shift of meiosis II and the inactivation that MPF that would result from cyclin B proteolysis during a regular M phase.


Figure 14.38

Activity that MPF throughout oocyte meiosis. Hormonal stimulation of diplotene oocytes activates MPF, resulting in progression come metaphase I. MPF activity then drops at the change from metaphase i to anaphase I. Following completion the meiosis I, MPF activity (more...)


The variable responsible for metaphase II arrest was very first identified through Yoshio Masui and Clement Markert in 1971, in the same collection of experiments that led to the discovery of MPF. In this case, however, cytoplasm native an egg arrested in ~ metaphase II was injected into very early embryo cell that was undergoing mitotic cell cycles (Figure 14.39). This injection of egg cytoplasm led to the embryonic cell to arrest at metaphase, indicating that metaphase arrest to be induced through a cytoplasmic factor current in the egg. Because this factor acted to arrest mitosis, the was called cytostatic factor (CSF).


Figure 14.39

Identification of cytostatic factor. Cytoplasm from a metaphase II egg is microinjected right into one cabinet of a two-cell embryo. The injected embryo cell arrests at metaphase, if the uninjected cell proceeds to divide. A variable in metaphase II egg cytoplasm (more...)


More recent experiments have figured out a protein-serine/threonine kinase well-known as Mos as vital component the CSF. Mos is especially synthesized in oocytes approximately the time of perfect of meiosis I and is then forced both for the boost in MPF task during meiosis II and also for the maintain of MPF activity during metaphase II arrest. The action of Mos results from activation that the ERK MAP kinase, which plays a main role in the cabinet signaling pathways discussed in the vault chapter. In oocytes, however, ERK plays a different role; that activates another protein kinase dubbed Rsk, i beg your pardon inhibits activity of the anaphase-promoting facility and arrests meiosis in ~ metaphase II (Figure 14.40). Oocytes have the right to remain arrested in ~ this suggest in the meiotic cell cycle for numerous days, awaiting fertilization.


Figure 14.40

Maintenance the metaphase II arrest by the Mos protein kinase. The Mos protein kinase maintains metaphase II arrest by inhibiting the anaphase-promoting complex. The action of Mos is mediated by MEK, ERK, and also Rsk protein kinases.


Fertilization

At fertilization, the sperm binding to a receptor ~ above the surface ar of the egg and fuses through the egg plasma membrane, initiating the breakthrough of a brand-new diploid biology containing genetic information derived from both parental (Figure 14.41). Not only does fertilization lead to the mix of paternal and maternal chromosomes, yet it additionally induces a number of changes in the egg cytoplasm that are crucial for more development. This alterations activate the egg, bring about the completion of oocyte meiosis and also initiation that the mitotic cell cycles that the early embryo.


Figure 14.41

Fertilization. Scanning electron micrograph of a human being sperm fertilizing one egg. (David M. Philips/Visuals Unlimited.)


A vital signal resulting from the binding of a sperm come its receptor top top the plasma membrane that the egg is rise in the level that Ca2+ in the egg cytoplasm, more than likely as a repercussion of stimulation that the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) (see figure 13.27). One impact of this key in intracellular Ca2+ is the induction of surface ar alterations that prevent extr sperm native entering the egg. Because eggs space usually exposed to big numbers that sperm at one time, this is a crucial event in ensuring the formation of a regular diploid embryo. These surface alterations are thought come result, at the very least in part, native the Ca2+-induced exocytosis of secretory vesicles the are present in big numbers in ~ the egg plasma membrane. Relax of the components of this vesicles transforms the extracellular coat of the egg so regarding block the entrance of added sperm.

The boost in cytosolic Ca2+ adhering to fertilization also signals the completion of meiosis (Figure 14.42). In egg arrested at metaphase II, the metaphase come anaphase shift is triggered by a Ca2+-dependent activation that the anaphase-promoting complex. The resultant inactivation of MPF leads to perfect of the second meiotic division, with asymmetric cytokinesis (as in meiosis I) providing rise to a second small polar body.


Figure 14.42

Fertilization and also completion that meiosis. (A) Fertilization cause the shift from metaphase II come anaphase II, bring about completion that oocyte meiosis and also emission that a 2nd polar body (which commonly degenerates). The sperm nucleus decondenses, (more...)


Following completion of oocyte meiosis, the fertilized egg (now referred to as a zygote) consists of two haploid nuclei (called pronuclei), one acquired from every parent. In mammals, the two pronuclei then get in S phase and also replicate their DNA as they move toward every other. As they meet, the zygote start M step of its very first mitotic division. The two nuclear envelopes break down, and also the condensed chromosomes that both paternal and maternal beginning align on a common spindle. Perfect of mitosis then gives rise to 2 embryonic cells, every containing a new diploid genome. These cells climate commence the collection of embryonic cell divisions that at some point lead come the breakthrough of a brand-new organism.

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