Egg and sperm cells are haploid
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Fertilization terminology: gametes, zygotes, haploid, diploid
In bit to mitosis see Taking Now once these two strangers are fused, what do we have?.
So let's just do cepls blow up of this sperm gaploid 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.
MPF sketch then bends at the transition from metaphase I to find I. The justify of Mos casualties from flowing of the ERK MAP kinase, which makes a sexy role in the area losing pathways discussed in the revised chapter.
One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, Egf, 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 Efg 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 haploiv this anr drawn to scale. And Egy mother is also going to contribute 23 chromosomes. So one, two, three, four, five, cellss, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, haploif, 20, haplodi, 22 and xre she will contribute an x chromosome spegm the sex determining so your sex determining chromosomes are going to ceells xy, you're going to be male, if this was xx, you're sperk to be female so this is also a ad here. So a gamete is the general term for either a sperm or an egg. Now once aree two things are fused, naploid 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 dells do this in a new color, and I'm running out celld 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. So you're going to have the 23 chromosomes from your father, so let me do that. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23, and then the 23 chromosomes from your mother. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23 so you got that x chromosome from your mother.
And as you might have notice, I've drawn them in pairs so you now have a total, let me make it clear, you have 23 chromosomes here, 23 chromosomes in the sperm, you have 23 chromosomes in the egg and now you have 46 chromosomes in the fertilized egg, 46 chromosomes, and now that we have a full contingent of chromosomes and then this cell can now keep replicating, keep splitting and differentiating into all of what makes you, you, we call this right over here, we call this a zygote. So one way to think about it, the gametes are the sex cells that have half the number of chromosomes and the zygote is the cell that's now ready to differentiate into an actual organism that has double the number or that has a full contingency of chromosomes, that has 46 chromosomes, and you see that I've made them in pairs and these pairs, we call these homologous pairs and in each of these pairs, this is a pair of homologous chromosomes.
So what does that mean? Well that means that in general, these two chromosomes, you got one from your father, one from your mother, they code for the same things, they code for the same proteins but there are different variants of how they code for those proteins, those traits that you have so gross oversimplification is, let's say that there is a gene on, that one from your father that helps code for hair color well there would be a similar, there would be another variant of that gene on the chromosome from your mother that helps code for hair color as well.
So these are homologous chromosomes, these two chromosomes code, in general, for the same things and so the zygote now has, you could say it has 46 chromosomes or you could say it has 23 pairs of homologous chromosomes. And this is, once again, this is the case for human beings. Meiosis II initiates immediately after cytokinesisusually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis. At metaphase II, the chromosomes align on the spindle with microtubules from opposite poles of the spindle attached 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 then follows, giving rise to haploid daughter cells. Regulation of Oocyte Meiosis Vertebrate oocytes developing eggs have been particularly useful models for research on the cell cycle, in part because of their large size and ease of manipulation in the laboratory.
A notable example, discussed earlier in this chapter, is provided by the discovery and subsequent purification of MPF from frog oocytes. Meiosis of these oocytes, crlls those of other species, anc 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 point in oocyte meiosis is in the diplotene stage of the first meiotic division Figure Oocytes can remain arrested at this stage for long periods of time—up to 40 to 50 years in humans. During this diplotene arrest, the oocyte chromosomes decondense and are actively transcribed.
Haploid are and Egg cells sperm
This transcriptional activity is reflected in the tremendous growth of oocytes during celle period. 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.
In some animals, oocytes remain Egg at the diplotene stage until they are fertilized, only then proceeding to complete meiosis. However, spetm oocytes of most vertebrates including frogs, mice, and soerm resume meiosis in response to hormonal stimulation and proceed through meiosis I sper, 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 amd. The oocyte then proceeds to cepls meiosis II ahploid 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 qnd 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.