What Happens to a Zygote Following Fertilization? | Sciencing
Whereas somatic cells undergo mitosis to proliferate, the germ cells undergo meiosis to produce haploid gametes (the sperm and the egg). Recombination between homologous chromosomes is completed during their association at the . of oocyte meiosis, the fertilized egg (now called a zygote) contains two haploid . Meiosis reduces chromosome number from diploid (2n) to haploid (n) Fertilization of the egg by the sperm restores the diploid number of . 23 chromosomes) restores the diploid number and creates a zygote (2n, 46) (awww . the female egg and the male sperm fuse to create a zygote cell which then turns They also have other adaptations to increase the chances of fertilisation and.
The egg retains a set, while the other is dispatched to a polar body that separates from the egg and eventually degrades. Sciencing Video Vault Pronuclei Stage The tightly packed chromosomes in the sperm now begin to decondense and are surrounded by a temporary membrane that forms the paternal pronucleus.
Enzymes from the egg cell assist in the formation of the paternal pronucleus. The egg cell develops its own pronucleus as well.
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During the next 12 to 18 hours, the DNA in each pronucleus replicates, forming chromosomes with twin attached chromatids. The two pronuclei attach to an array of microtubules anchored by a structure called an aster. The microtubules pull the two pronuclei together. Mitosis Once the pronuclei are drawn together, their membranes dissolve.
The fertilized egg now prepares for mitosis, which is the distribution of duplicated chromosomes to two daughter cells. During mitosis, the chromosomes align on a central spindle where the duplicates separate and are pulled to either end of the cell.
Frog oocytes are even larger, with diameters of approximately 1 mm. During this period of cell growth, the oocytes accumulate stockpiles of materials, including RNAs and proteinsthat are needed to support early development of the embryo.
As noted earlier in this chapter, early embryonic cell cycles then occur in the absence of cell growth, rapidly dividing the fertilized egg into smaller cells see Figure Meiosis is arrested at the diplotene stage, during which oocytes grow to a large size.
Oocytes then resume meiosis in response to hormonal stimulation and complete the first meiotic division, with asymmetric cytokinesis more Oocytes of different species vary as to when meiosis resumes and fertilization takes place.The difficult journey of the sperm - Signs
In some animals, oocytes remain arrested at the diplotene stage until they are fertilized, only then proceeding to complete meiosis. However, the oocytes of most vertebrates including frogs, mice, and humans resume meiosis in response to hormonal stimulation and proceed 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 then proceeds to enter meiosis II without having re-formed a nucleus or decondensed its chromosomes.
Most vertebrate oocytes are then arrested again at metaphase II, where they remain until fertilization. Like the M 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 Hormonal stimulation of diplotene -arrested oocytes initially triggers the resumption of meiosis by activating MPF, as at the G2 to M transition of somatic cells.
As in mitosisMPF 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 activity of MPF. Following cytokinesishowever, MPF activity 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 II arrest, preventing the metaphase to anaphase transition of meiosis II and the inactivation of MPF that would result from cyclin B proteolysis during a normal M phase.
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 The factor responsible for metaphase II arrest was first identified by Yoshio Masui and Clement Markert inin the same series of experiments that led to the discovery of MPF.
In this case, however, cytoplasm from an egg arrested at metaphase II was injected into an early embryo cell that was undergoing mitotic cell cycles Figure This injection of egg cytoplasm caused 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 mitosisit was called cytostatic factor CSF.
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.
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A factor in metaphase II egg cytoplasm more Mos is specifically synthesized in oocytes around the time of completion of meiosis I and is then required both for the increase in MPF activity during meiosis II and for the maintenance of MPF activity during metaphase II arrest. The action of Mos results from activation of the ERK MAP kinase, which plays a central role in the cell signaling pathways discussed in the previous chapter.
In oocytes, however, ERK plays a different role; it activates another protein kinase called Rsk, which inhibits action of the anaphase-promoting complex and arrests meiosis at metaphase II Figure Oocytes can remain arrested at this point in the meiotic cell cycle for several days, awaiting fertilization.
Maintenance of metaphase II arrest by the Mos protein kinase. The Mos protein kinase maintains metaphase II arrest by inhibiting the anaphase-promoting complex. Fertilization At fertilization, the sperm binds to a receptor on the surface of the egg and fuses with the egg plasma membraneinitiating the development of a new diploid organism containing genetic information derived from both parents Figure Not only does fertilization lead to the mixing of paternal and maternal chromosomesbut it also induces a number of changes in the egg cytoplasm that are critical for further development.