what happens to the chromosome of the egg and sperm during fertilization

The somatic cell cycles discussed then far in this chapter outcome in diploid daughter cells with identical genetic complements. Meiosis, in contrast, is a specialized kind of cell cycle that reduces the chromosome number by half, resulting in the product of haploid daughter cells. Unicellular eukaryotes, such as yeasts, can undergo meiosis besides equally reproducing past mitosis. Diploid Saccharomyces cerevisiae, for example, undergo meiosis and produce spores when faced with unfavorable environmental atmospheric condition. In multicellular plants and animals, however, meiosis is restricted to the germ cells, where it is fundamental to sexual reproduction. Whereas somatic cells undergo mitosis to proliferate, the germ cells undergo meiosis to produce haploid gametes (the sperm and the egg). The evolution of a new progeny organism is and then initiated past the fusion of these gametes at fertilization.

The Process of Meiosis

In contrast to mitosis, meiosis results in the sectionalisation of a diploid parental cell into haploid progeny, each containing simply one member of the pair of homologous chromosomes that were present in the diploid parent (Figure 14.32). This reduction in chromosome number is accomplished past two sequential rounds of nuclear and cell division (called meiosis I and meiosis Ii), which follow a unmarried circular of Dna replication. Like mitosis, meiosis I initiates later on S phase has been completed and the parental chromosomes take replicated to produce identical sister chromatids. The pattern of chromosome segregation in meiosis I, nonetheless, is dramatically different from that of mitosis. During meiosis I, homologous chromosomes first pair with one another and then segregate to different daughter cells. Sister chromatids remain together, and so completion of meiosis I results in the formation of girl cells containing a single member of each chromosome pair (consisting of two sister chromatids). Meiosis I is followed past meiosis 2, which resembles mitosis in that the sister chromatids divide and segregate to different daughter cells. Completion of meiosis II thus results in the product of four haploid daughter cells, each of which contains simply ane copy of each chromosome.

Figure fourteen.32

Comparing of meiosis and mitosis. Both meiosis and mitosis initiate afterwards DNA replication, so each chromosome consists of two sister chromatids. In meiosis I, homologous chromosomes pair and and then segregate to different cells. Sister chromatids so split up (more than...)

The pairing of homologous chromosomes after Dna replication is not only a key event underlying meiotic chromosome segregation, but also allows recombination between chromosomes of paternal and maternal origin. This critical pairing of homologous chromosomes takes place during an extended prophase of meiosis I, which is divided into 5 stages (leptotene, zygotene, pachytene, diplotene, and diakinesis) on the basis of chromosome morphology (Figure fourteen.33). The initial association of homologous chromosomes is idea to be mediated by base pairing betwixt complementary Dna strands during the leptotene stage, before the chromatin becomes highly condensed. The close association of homologous chromosomes (synapsis) begins during the zygotene stage. During this stage, a zipperlike protein construction, called the synaptonemal complex, forms along the length of the paired chromosomes (Figure fourteen.34). This complex keeps the homologous chromosomes closely associated and aligned with i another through the pachytene stage, which can persist for several days. Recombination between homologous chromosomes is completed during their association at the pachytene stage, leaving the chromosomes linked at the sites of crossing over (chiasmata; atypical, chiasma). The synaptonemal complex disappears at the diplotene stage and the homologous chromosomes separate along their length. Chiefly, however, they remain associated at the chiasmata, which is critical for their right alignment at metaphase. At this stage, each chromosome pair (called a bivalent) consists of four chromatids with clearly axiomatic chiasmata (Figure 14.35). Diakinesis, the final stage of prophase I, represents the transition to metaphase, during which the chromosomes become fully condensed.

Figure fourteen.33

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

Figure xiv.34

The synaptonemal complex. Chromatin loops are fastened to the lateral elements, which are joined to each other by a zipperlike primal element.

Figure 14.35

A bivalent chromosome at the diplotene stage. The bivalent chromosome consists of paired homologous chromosomes. Sister chromatids of each chromosome are joined at the centromere. Chromatids of 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 of sis chromatids are adjacent to each other and oriented in the same direction, while the kinetochores of homologous chromosomes are pointed toward opposite spindle poles (Figure 14.36). Consequently, microtubules from the aforementioned pole of the spindle attach to sister chromatids, while microtubules from contrary poles adhere to homologous chromosomes. Anaphase I is initiated by disruption of the chiasmata at which homologous chromosomes are joined. The homologous chromosomes so separate, while sis chromatids remain associated at their centromeres. At completion of meiosis I, each daughter cell has therefore acquired one member of each homologous pair, consisting of 2 sis chromatids.

Figure 14.36

Chromosome segregation in meiosis I. At metaphase I, the kinetochores of sister chromatids are either fused or adjacent to one some other. Microtubules from the same pole of the spindle therefore adhere to the kinetochores of sister chromatids, while microtubules (more...)

Meiosis II initiates immediately after cytokinesis, ordinarily before the chromosomes take fully decondensed. In contrast to meiosis I, meiosis 2 resembles a normal mitosis. At metaphase 2, the chromosomes align on the spindle with microtubules from reverse poles of the spindle fastened to the kinetochores of sister chromatids. The link between the centromeres of sister chromatids is broken at anaphase II, and sister chromatids segregate to opposite poles. Cytokinesis and so follows, giving rise to haploid daughter cells.

Regulation of Oocyte Meiosis

Vertebrate oocytes (developing eggs) have been especially useful models for enquiry on the cell cycle, in part because of their big size and ease of manipulation in the laboratory. A notable instance, discussed earlier in this affiliate, is provided past the discovery and subsequent purification of MPF from frog oocytes. Meiosis of these oocytes, like those of other species, is regulated at two unique points in the cell cycle, and studies of oocyte meiosis have illuminated novel mechanisms of cell cycle control.

The first regulatory indicate in oocyte meiosis is in the diplotene stage of the kickoff meiotic sectionalisation (Figure 14.37). Oocytes tin remain arrested at this stage for long periods of time—up to 40 to l years in humans. During this diplotene arrest, the oocyte chromosomes decondense and are actively transcribed. This transcriptional activity is reflected in the tremendous growth of oocytes during this period. Human oocytes, for example, are near 100 μm in diameter (more than a hundred times the book of a typical somatic cell). Frog oocytes are even larger, with diameters of approximately 1 mm. During this menstruum of cell growth, the oocytes accumulate stockpiles of materials, including RNAs and proteins, that are needed to support early on development of the embryo. As noted earlier in this chapter, early on embryonic cell cycles and then occur in the absence of prison cell growth, rapidly dividing the fertilized egg into smaller cells (encounter Figure fourteen.ii).

Figure 14.37

Meiosis of vertebrate oocytes. Meiosis is arrested at the diplotene phase, during which oocytes grow to a big size. Oocytes and so resume meiosis in response to hormonal stimulation and complete the first meiotic division, with disproportionate cytokinesis (more...)

Oocytes of unlike species vary as to when meiosis resumes and fertilization takes identify. In some animals, oocytes remain arrested at the diplotene stage until they are fertilized, but and so proceeding to consummate meiosis. Still, the oocytes of most vertebrates (including frogs, mice, and humans) resume meiosis in response to hormonal stimulation and go on through meiosis I prior to fertilization. Cell division following meiosis I is asymmetric, resulting in the production of a small polar body and an oocyte that retains its large size. The oocyte and then proceeds to enter meiosis Two without having re-formed a nucleus or decondensed its chromosomes. Most vertebrate oocytes are then arrested over again at metaphase 2, where they remain until fertilization.

Like the Thou phase of somatic cells, the meiosis of oocytes is controlled by MPF. The regulation of MPF during oocyte meiosis, however, displays unique features that are responsible for metaphase II arrest (Figure fourteen.38). Hormonal stimulation of diplotene-arrested oocytes initially triggers the resumption of meiosis by activating MPF, every bit at the Thousand_two to M transition of somatic cells. Equally in mitosis, MPF then induces chromosome condensation, nuclear envelope breakdown, and formation of the spindle. Activation of the anaphase-promoting complex B then leads to the metaphase to anaphase transition of meiosis I, accompanied by a decrease in the activeness of MPF. Following cytokinesis, however, MPF activity once again rises and remains high while the egg is arrested at metaphase II. A regulatory mechanism unique to oocytes thus acts to maintain MPF activity during metaphase 2 arrest, preventing the metaphase to anaphase transition of meiosis Two and the inactivation of MPF that would event from cyclin B proteolysis during a normal Grand phase.

Effigy 14.38

Activity of MPF during oocyte meiosis. Hormonal stimulation of diplotene oocytes activates MPF, resulting in progression to metaphase I. MPF activity then falls at the transition from metaphase I to anaphase I. Following completion of meiosis I, MPF activity (more than...)

The cistron responsible for metaphase Ii arrest was beginning identified by Yoshio Masui and Clement Markert in 1971, in the same series of experiments that led to the discovery of MPF. In this example, however, cytoplasm from an egg arrested at metaphase II was injected into an early embryo cell that was undergoing mitotic jail cell cycles (Effigy 14.39). This injection of egg cytoplasm acquired the embryonic cell to arrest at metaphase, indicating that metaphase arrest was induced by a cytoplasmic factor present in the egg. Because this factor acted to arrest mitosis, it was chosen cytostatic factor (CSF).

Figure 14.39

Identification of cytostatic gene. Cytoplasm from a metaphase II egg is microinjected into one cell of a two-cell embryo. The injected embryo cell arrests at metaphase, while the uninjected cell continues to divide. A factor in metaphase Ii egg cytoplasm (more...)

More recent experiments have identified a protein-serine/threonine kinase known as Mos as an essential component of CSF. Mos is specifically synthesized in oocytes around the time of completion of meiosis I and is then required both for the increment in MPF activity during meiosis II and for the maintenance of MPF action during metaphase 2 arrest. The action of Mos results from activation of the ERK MAP kinase, which plays a primal role in the cell signaling pathways discussed in the previous chapter. In oocytes, however, ERK plays a dissimilar part; it activates another poly peptide kinase called Rsk, which inhibits activeness of the anaphase-promoting complex and arrests meiosis at metaphase II (Figure xiv.xl). Oocytes can remain arrested at this point in the meiotic jail cell cycle for several days, awaiting fertilization.

Figure xiv.forty

Maintenance of metaphase II arrest past the Mos protein kinase. The Mos protein kinase maintains metaphase 2 arrest by inhibiting the anaphase-promoting complex. The action of Mos is mediated by MEK, ERK, and Rsk protein kinases.

Fertilization

At fertilization, the sperm binds to a receptor on the surface of the egg and fuses with the egg plasma membrane, initiating the development of a new diploid organism containing genetic data derived from both parents (Figure fourteen.41). Not only does fertilization lead to the mixing of paternal and maternal chromosomes, but it also induces a number of changes in the egg cytoplasm that are critical for further evolution. These alterations actuate the egg, leading to the completion of oocyte meiosis and initiation of the mitotic jail cell cycles of the early embryo.

Effigy 14.41

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

A fundamental signal resulting from the bounden of a sperm to its receptor on the plasma membrane of the egg is an increase in the level of Caⁱⁱ⁺ in the egg cytoplasm, probably as a consequence of stimulation of the hydrolysis of phosphatidylinositol four,5-bisphosphate (PIP₂) (see Figure 13.27). 1 effect of this pinnacle in intracellular Caⁱⁱ⁺ is the induction of surface alterations that forestall boosted sperm from entering the egg. Because eggs are commonly exposed to big numbers of sperm at one time, this is a critical result in ensuring the germination of a normal diploid embryo. These surface alterations are idea to result, at least in part, from the Ca²⁺-induced exocytosis of secretory vesicles that are present in large numbers below the egg plasma membrane. Release of the contents of these vesicles alters the extracellular glaze of the egg so as to cake the entry of additional sperm.

The increment in cytosolic Ca²⁺ following fertilization besides signals the completion of meiosis (Effigy 14.42). In eggs arrested at metaphase II, the metaphase to anaphase transition is triggered past a Ca²⁺-dependent activation of the anaphase-promoting circuitous. The resultant inactivation of MPF leads to completion of the second meiotic partitioning, with asymmetric cytokinesis (as in meiosis I) giving ascent to a second small polar body.

Figure 14.42

Fertilization and completion of meiosis. (A) Fertilization induces the transition from metaphase II to anaphase 2, leading to completion of oocyte meiosis and emission of a second polar body (which ordinarily degenerates). The sperm nucleus decondenses, (more...)

Following completion of oocyte meiosis, the fertilized egg (now chosen a zygote) contains two haploid nuclei (called pronuclei), one derived from each parent. In mammals, the 2 pronuclei then enter Southward phase and replicate their DNA as they drift toward each other. As they run across, the zygote enters One thousand phase of its get-go mitotic division. The two nuclear envelopes break down, and the condensed chromosomes of both paternal and maternal origin align on a common spindle. Completion of mitosis then gives rise to two embryonic cells, each containing a new diploid genome. These cells and then commence the series of embryonic prison cell divisions that somewhen lead to the evolution of a new organism.

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Source: https://www.ncbi.nlm.nih.gov/books/NBK9901/