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You are watching: At what point is meiosis ii completed for the female gamete?

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


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The somatic cabinet cycles disputed so far in this chapter result in diploid daughter cells with identical genetic complements. Meiosis, in contrast, is a committed kind of cabinet cycle the reduces the chromosome number by half, causing the production of haploid daughter cells. Unicellular eukaryotes, such together yeasts, have the right to undergo meiosis and also reproducing by mitosis. Diploid Saccharomyces cerevisiae, because that example, undergo meiosis and produce spores when confronted with unfavorable ecological conditions. In multicell plants and also animals, however, meiosis is limited to the germ cells, wherein it is key to sexual reproduction. Vice versa, somatic cells undergo mitosis to proliferate, the germ cell undergo meiosis to produce haploid gametes (the sperm and the egg). The advance of a brand-new progeny organism is then initiated by the fusion of these gametes at fertilization.


The procedure of Meiosis

In contrast to mitosis, meiosis outcomes in the department of a diploid parental cell right into haploid progeny, every containing just one member that the pair of homologous chromosomes that were present in the diploid parent (Figure 14.32). This reduction in chromosome number is accomplished by two sequential rounds of nuclear and cell department (called meiosis I and meiosis II), which follow a solitary round of DNA replication. Choose mitosis, meiosis i initiates after ~ S phase has actually been completed and the parental chromosomes have replicated to produce identical sisters chromatids. The sample of chromosome segregation in meiosis I, however, is dramatically various from the of mitosis. During meiosis I, homologous chromosomes very first pair v one another and also then segregate to various daughter cells. Sisters chromatids remain together, so perfect of meiosis I results in the formation of daughter cell containing a solitary member of each chromosome pair (consisting of 2 sister chromatids). Meiosis i is followed by meiosis II, which each other mitosis in that the sister chromatids separate and segregate to different daughter cells. Completion of meiosis II thus results in the manufacturing of 4 haploid daughter cells, each of which has only one copy of each chromosome.


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Figure 14.32

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


The pairing of homologous chromosomes after ~ DNA replication is not just a key event underlying meiotic chromosome segregation, yet also allows recombination between chromosomes the paternal and also maternal origin. This an essential pairing that homologous chromosomes takes location during an extended prophase the meiosis I, i beg your pardon is separated into five stages (leptotene, zygotene, pachytene, diplotene, and diakinesis) ~ above the communication of chromosome morphology (Figure 14.33). The initial association of homologous chromosomes is thought to it is in mediated by base pairing between complementary DNA strands throughout the leptotene stage, prior to the dyed becomes very condensed. The close combination of homologous chromosomes (synapsis) starts during the zygotene stage. During this stage, a zipperlike protein structure, referred to as the synaptonemal complex, develops along the size of the paired chromosomes (Figure 14.34). This facility keeps the homologous chromosomes carefully associated and aligned with one one more through the pachytene stage, which have the right to persist for number of days. Recombination between homologous chromosomes is completed throughout their combination at the pachytene stage, leaving the chromosomes linked at the sites of crossing over (chiasmata; singular, chiasma). The synaptonemal complex disappears in ~ the diplotene stage and the homologous chromosomes different along their length. Importantly, however, lock remain linked at the chiasmata, i beg your pardon is critical for their correct alignment at metaphase. At this stage, each chromosome pair (called a bivalent) is composed of four chromatids with plainly evident chiasmata (Figure 14.35). Diakinesis, the final stage the prophase I, represents the transition to metaphase, throughout which the chromosomes become fully condensed.


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Figure 14.33

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


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Figure 14.34

The synaptonemal complex. Chromatin loops room attached to the lateral elements, which room joined come each other by a zipperlike main element.


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Figure 14.35

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


At metaphase I, the bivalent chromosomes align on the spindle. In contrast to mitosis (see number 14.27), the kinetochores of sister chromatids are nearby to each other and oriented in the exact same direction, when the kinetochores of homologous chromosomes room pointed towards opposite spindle poles (Figure 14.36). Consequently, microtubules indigenous the very same pole of the spindle affix to sister chromatids, while microtubules from opposite poles connect to homologous chromosomes. Anaphase i is initiated by disruption that the chiasmata at which homologous chromosomes room joined. The homologous chromosomes then separate, while sister chromatids remain associated at your centromeres. At perfect of meiosis I, every daughter cabinet has as such acquired one member of every homologous pair, consisting of two sister chromatids.


Figure 14.36

Chromosome segregation in meiosis I. In ~ metaphase I, the kinetochores that sister chromatids space either unify or adjacent to one another. Microtubules indigenous the exact same pole the the spindle because of this attach to the kinetochores that sister chromatids, while microtubules (more...)


Meiosis II initiates instantly after cytokinesis, usually before the chromosomes have totally decondensed. In contrast to meiosis I, meiosis II each other a typical mitosis. At metaphase II, the chromosomes align top top the spindle through microtubules native opposite poles that the spindle attached to the kinetochores that sister chromatids. The link in 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 of Oocyte Meiosis

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

The an initial regulatory allude in oocyte meiosis is in the diplotene stage of the an initial meiotic division (Figure 14.37). Oocytes can remain arrested in ~ this stage for long periods the time—up come 40 come 50 years in humans. Throughout this diplotene arrest, the oocyte chromosomes decondense and are actively transcribed. This transcriptional activity is reflect in the tremendous expansion of oocytes throughout this period. Human being oocytes, for example, are around 100 μm in diameter (more 보다 a hundred times the volume of a common somatic cell). Frog oocytes are also larger, v diameters of about 1 mm. During this period of cell growth, the oocytes accumulate stockpiles of materials, including RNAs and also proteins, that are necessary to support early advance of the embryo. As noted earlier in this chapter, early on embryonic cell cycles then occur in the absence of cell growth, rapidly separating the fertilized egg right into smaller cells (see number 14.2).


Figure 14.37

Meiosis of vertebrate oocytes. Meiosis is arrested at the diplotene stage, throughout which oocytes thrive to a large size. Oocytes climate resume meiosis in solution to hormone stimulation and complete the an initial meiotic division, through asymmetric cytokinesis (more...)


Oocytes of different types vary regarding when meiosis resumes and also fertilization bring away place. In some animals, oocytes remain arrested at the diplotene phase until they are fertilized, only then proceeding to complete meiosis. However, the oocytes of most vertebrates (including frogs, mice, and also humans) resume meiosis in an answer to hormone stimulation and proceed with meiosis I prior to fertilization. Cell division following meiosis i is asymmetric, causing the manufacturing of a little polar body and also an oocyte that retains its large size. The oocyte then proceeds to go into meiosis II without having re-formed a cell core or decondensed that chromosomes. Most vertebrate oocytes room then arrested again in ~ metaphase II, where they continue to be until fertilization.

Like the M phase of somatic cells, the meiosis that oocytes is controlled by MPF. The regulation the MPF during oocyte meiosis, however, displays unique features that room responsible because that metaphase II arrest (Figure 14.38). Hormonal stimulation the diplotene-arrested oocytes at first triggers the resumption that meiosis through activating MPF, as at the G2 to M transition of somatic cells. Together in mitosis, MPF then induces chromosome condensation, atom envelope breakdown, and also formation the the spindle. Activation of the anaphase-promoting complex B then leads to the metaphase come anaphase shift of meiosis I, attach by a diminish in the task of MPF. Complying with cytokinesis, however, MPF activity again rises and also remains high when the egg is arrested at metaphase II. A regulatory mechanism unique to oocytes for this reason acts to preserve MPF activity during metaphase II arrest, preventing the metaphase come anaphase change of meiosis II and the inactivation the MPF the would result from cyclin B proteolysis throughout a normal M phase.


Figure 14.38

Activity the MPF during oocyte meiosis. Hormonal stimulation that diplotene oocytes activates MPF, bring about progression come metaphase I. MPF task then drops at the change from metaphase i to anaphase I. Following completion the meiosis I, MPF activity (more...)


The aspect responsible because that metaphase II arrest was very first identified by 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 indigenous an egg arrested at metaphase II to be injected into very early embryo cell that was experience mitotic cell cycles (Figure 14.39). This injection that egg cytoplasm brought about the embryonic cabinet to arrest in ~ metaphase, indicating that metaphase arrest was induced by a cytoplasmic factor present in the egg. Because this element acted come arrest mitosis, that was called cytostatic factor (CSF).


Figure 14.39

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


More current experiments have identified a protein-serine/threonine kinase known as Mos as an essential component the CSF. Mos is specifically synthesized in oocytes about the time of completion of meiosis I and is then forced both for the rise in MPF activity during meiosis II and for the maintain of MPF task during metaphase II arrest. The activity of Mos outcomes from activation that the ERK MAP kinase, which plays a main role in the cell signaling pathways disputed in the ahead chapter. In oocytes, however, ERK plays a different role; it activates one more protein kinase dubbed Rsk, which inhibits activity of the anaphase-promoting complicated and arrests meiosis in ~ metaphase II (Figure 14.40). Oocytes have the right to remain arrested in ~ this point in the meiotic cell cycle for numerous days, awaiting fertilization.


Figure 14.40

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


Fertilization

At fertilization, the sperm binding to a receptor on the surface of the egg and fuses with the egg plasma membrane, initiating the advancement of a brand-new diploid biology containing hereditary information derived from both parental (Figure 14.41). Not only does fertilization result in the mixing of paternal and also maternal chromosomes, but it likewise induces a variety of changes in the egg cytoplasm the are an important for more development. This alterations activate the egg, leading to the completion of oocyte meiosis and also initiation that the mitotic cabinet 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 crucial signal resulting from the binding of a sperm to its receptor ~ above the plasma membrane of the egg is rise in the level the Ca2+ in the egg cytoplasm, most likely as a consequence of stimulation of the hydrolysis the phosphatidylinositol 4,5-bisphosphate (PIP2) (see figure 13.27). One result of this key in intracellular Ca2+ is the induction of surface ar alterations that prevent extr sperm from entering the egg. Due to the fact that eggs room usually exposed to large numbers the sperm in ~ one time, this is a vital event in ensuring the development of a common diploid embryo. These surface ar alterations space thought come result, at the very least in part, native the Ca2+-induced exocytosis the secretory vesicles that are current in large numbers in ~ the egg plasma membrane. Relax of the materials of this vesicles alters the extracellular cloak of the egg so as to block the entrance of additional sperm.

The boost in cytosolic Ca2+ following fertilization likewise signals the perfect of meiosis (Figure 14.42). In egg arrested at metaphase II, the metaphase to anaphase shift is motivated by a Ca2+-dependent activation of the anaphase-promoting complex. The result inactivation the MPF leader to perfect of the 2nd meiotic division, with asymmetric cytokinesis (as in meiosis I) offering rise come a 2nd small polar body.


Figure 14.42

Fertilization and completion the meiosis. (A) Fertilization induces the change from metaphase II come anaphase II, resulting in completion the oocyte meiosis and also emission that a 2nd polar body (which typically degenerates). The sperm cell nucleus decondenses, (more...)


Following perfect of oocyte meiosis, the fertilized egg (now referred to as a zygote) contains two haploid nuclei (called pronuclei), one obtained from every parent. In mammals, the two pronuclei then get in S phase and also replicate your DNA as they migrate toward each other. Together they meet, the zygote start M step of its an initial mitotic division. The two nuclear envelopes rest down, and also the condensed chromosomes of both paternal and maternal origin align ~ above a common spindle. Completion of mitosis then gives rise to two embryonic cells, every containing a new diploid genome. These cells climate commence the collection of embryonic cell departments that at some point lead come the development of a brand-new organism.

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