Zygote - Wikipedia
Meiosis, in contrast, is a specialized kind of cell cycle that reduces the organism is then initiated by the fusion of these gametes at fertilization. Recombination between homologous chromosomes is completed during their association at the and proteins, that are needed to support early development of the embryo. Follow the male's sperm and the female's egg on their way to meet each other in this lesson that takes you through the process of fertilization in Follicular Phase of the Ovarian Cycle. Blastula Stage: Definition, Formation . and the sperm. The first thing you may notice here is a drastic difference in size. Zygote: Zygote, fertilized egg cell that results from the union of a female gamete ( egg, or ovum) with a male gamete (sperm). In the embryonic development of.
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.
What Happens to a Zygote Following Fertilization? | Sciencing
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.
Fertilization terminology: gametes, zygotes, haploid, diploid
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.
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.
And as we'll see, each gamete has half the number of chromosomes as your body cells or most of the somatic cells of your body so outside of your sex cells that might be in your ovaries or your testes, depending on whether you're male or female, these have half the number so let's dig a little bit deeper into what I mean there. So let's just do a blow up of this sperm cell right over here, so a blow up of a sperm cell and I'm not going to draw it to scale, you see the sperm cell is much smaller than the egg cell but just to get a sense, so let me draw the nucleus of this sperm cell, so just like that.
If we're talking about a human being, and I'm assuming you are a human being, so that might be of interest to you, this will have 23 chromosomes from your father so let's do them. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and for the 23rd one, that's going to be your sex-determining chromosome so if your father contributes an x, you are going to be female, if your father contributes a y, you are going to be male.
So these are the chromosomes in the male gamete or I guess I should say the gamete that your father's contributing, the sperm. So this is a gamete right over here and that's going to fuse with the egg, the ovum that your mother is contributing and once again, I'm not drawing that to scale.
So this is the egg, and let me draw it's nucleus.
So that's it's nucleus, once again none of this is drawn to scale. And your mother is also going to contribute 23 chromosomes.
So one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and then she will contribute an x chromosome for the sex determining so your sex determining chromosomes are going to be xy, you're going to be male, if this was xx, you're going to be female so this is also a gamete here. So a gamete is the general term for either a sperm or an egg. Now once these two things are fused, what do we have?
Once they're fused, then we're going to have you could say a fertilized egg but we are going to call that a zygote so let me draw that. I'm going to do this in a new color, and I'm running out of space and I want this all to fit on the same screen so I'll draw it not quite at scale and so let me draw the nucleus of the zygote, I'm going to make the nucleus fairly large so that we can focus on the chromosomes in it, once again none of this is drawn to scale.
Completion of Meiosis Meiosis is the process that creates gametes -- sperm and egg cells -- containing only one, or a haploid, set of chromosomes. Fertilization establishes the regular diploid number of chromosomes in the zygote.
Meiosis occurs over two cycles of cell division, which sperm cells complete before fertilization. Meiosis in the egg cell stops during metaphase of the second cycle. At fertilization, meiosis II resumes and the duplicate copies of each chromosome are pulled apart.
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.