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8

The frequency rises with maternal age due to a peculiarity of meoisis in female mammals. Meiosis is originated in the fetal ovary, arresting at metaphase I with the homologous chromosomes aligned for segregation. Cells remain in this state until the time of ovulation, often decades later in humans. The longer cells remain in the arrested ...


8

Primary oocytes are formed prenatally and reain suspended in prophase of meiosis I for years until the onset of puberty. An oocyte completes meiosis I as its follicle matures (during ovulation) resulting in a secondary oocyte and the FIRST polar body. After ovulation, each oocyte continues to metaphase of meiosis II. Meiosis II is completed only if ...


7

This is a venerable fact. Exceptions in D. melanogaster strain Y-007 have been observed and D. ananassae has consistent male cross-overs, but this work dates back to the 1970s. This Current Biology 2002 paper is not exactly new, but sheds some light on the issue in question. In male Drosophila melanogaster, meiosis occurs in the absence of ...


6

If the question is about the one and only most important difference between mitosis and meiosis, then the answer "meiosis reduces ploidy" is probably correct. But if the list of important differences is open, it would be critical to add that mitosis generates identical cells (identical to each other and any ancestral cells, barring rare new mutations), while ...


6

Meiosis consists of two divisions. Both are somehow similar to "ordinary" type of cell division - mitosis, but there is no DNA replication between them. As mitosis, each of two meiosis divisions might be divided into 5 stages: Profaze (condensation of chromosomes, formation of microtubular spindle apparatus between two centrosomes) Prometaphase (the ...


6

Oocytes, or immature female eggs, develop in the fetus's ovaries during pregnancy. This graph (U. New South Wales) shows the oocyte population over time in a human female: Although the x-scale is kind of confusing (months when negative, years when positive), you can see that the fetus has all the oocytes it will ever have at the peak 18-22 weeks after ...


5

When females are in their mother's womb all their Oogenium (plural Oogenia) are being made, they they undergo mitotic divisions to become a primary oocyte. Then, the primary oocytes start to undergo meiosis I - but meiosis I is arrested. This is the first meiotic arrest. When does meiosis I continues? when a female hits puberty, every month one or more ...


5

I'm actually not sure myself. If I were to use something, I would go with "Mitos'd" and "Meios'd". However, you may not win over many fans, depending on the audience. If it's with students or maybe a professor, you could get away with shortening the processes. If it's in any formal setting, be as precise and descriptive as possible. It's not a lot of ...


5

I'm not sure about the ubiquity of this but, in many animals, each primary oocyte that undergoes oogenesis only produces one mature egg. The other products of meiosis are polar bodies, which are not fertilised. These cells often degenerate but can sometimes play supportive roles in embryogenesis. To answer your question, each mature egg is necessarily ...


5

In humans and mice anyway ,a lot of it boils down to the recognition of a specific sequence that marks recombination hotspots by PRDM9. http://www.sciencemag.org/content/327/5967/836 Edit - I'm expanding in response to the comment below... Meiotic recombination occurs at vastly greater frequencies in some locations in the genome than others and these are ...


4

To the best of my knowledge there is no strong evidence as to the reason why. The most reasonable explanation seems to be that it evolved as a crude mechanism for preventing recombination of the male sex-chromosome. You might then ask why a mechanism targeted to the sex-chromosome specifically (as in humans) did not evolve to which I'd suggest that ...


4

Meiosis starts with a diploid cell and produces four haploid cells. In animals, the starting diploid cell is usually called a germ cell and the surviving haploid cells become gametes (sperm and ova). (In animals, the female mitotic sequence produces only one ovum; the other three haploid cells become "polar bodies".) In other organisms such as plants, the ...


4

In automixy the meiotic cells give rise to diploid offsprings. This can happen by diploidization of the haploid cell (1n->2n), which will produce homozygotes or endomitosis prior to meiosis (4n->2n) which produces heterozygotes. Examples: Cnemidophorus uniparens : 4n->2n Sphyrna tiburo: 1n->2n I don't know of any case where there is fusion ...


4

The question is very broad and complicated, since the situation may differ in prokaryotes and eukaryotes. Nevertheless, I'm citing a good paper that is closely related to your question: Studies in yeast show that initiation of recombination, which occurs by the formation of DNA double-strand breaks, determines the distribution of gene conversion and ...


3

While your question asking about birds, reptiles and fish (oh my!) may be too broad, hopefully looking at frog oogenesis can show some differences in large offspring number v. small offspring number. Some frogs even give birth to live offspring (Iskandar et al. 2014). Much of this explanation can be found in Developmental Biology, 6th edition by Gilbert ...


3

Trisomy is due to non-disjunction in meiosis (the process in which eggs and sperm are created). This happens before fertilization. Trisomies are more frequently seen in children of older women. It's not fully understood why this happens, but it is likely related to the fact that the oocytes do not complete meiosis (and become eggs) until ovulation. Here's ...


3

No, meiosis is not a cycle like mitosis. In mitosis, haploid or diploid cells divide to create two genetically identical cells, and this process can go on and on. On the other hand, meiosis results in 4 genetically unique daughter cells which are also haploid. If a haploid cell was to try to undergo meiosis, there would not be enough genetic material and you ...


3

The paper you cite says that the break points are single stranded DNA which have specific proteins bound to them. I'm not an expert here, but if thats the cause of meitotic break points there are some interesting possibilities for detecting them: you could detect them with a tiling array. - that's an micro array which has an oligomer every 40 bp or so ...


2

According to all I've learned and heard, the only thing consistent about meiosis is that it reduces ploidy because homologous chromosomes are separated. (Not necessarily diploid to haploid - it can be polyploid as well, although odd ploidies usually seem to mess it up.) Mitosis on the other hand mainly serves to separate two copies of a genome into ...


2

Meiosis, as you know, have two stages, Meiosis I and II. The oocyte is arrested during metaphase II of MEOISIS II. This arrest is facilitated by a complex called "Cytostatic Factor" (CSF). After fertilization, the sperm induces a rise in intracellular calcium ion which activates and enzyme, Calmodulin Kinase II. This complex, through a series of ...


2

Untill puberty the follicles grown from primordial to antral stage (secondary folliccle) and oocytes are arrested in diplotene of profase I of meiosis I, without polar body, with a nucleous called germinative vesicle. After puberty, with gonadotrophis, they can grow more and ovulate. With LH surge they are stimulated to continuing meiosis and they lose the ...


2

As far as I can tell there is a distinction. A tetrad refers to the entire group of four chromatids after they have come together for crossing over in Prophase I (synapses). A synaptonemal complex as you would expect is formed in synapses. This is a protein-RNA complex that connects the intervening regions of matched chromosomes in some circumstances - ...


2

There are many questions in your question. I'll try to answer each question pointwise. Which flags are used by the enzymes in the process of making the centromere to tell them that it is the right spot There are some centromere associated repeats in the DNA which mark the site for centromere assembly. There is no particular consensus sequence of this ...


2

I think that Fair meiosis (I assume that you are referring to that chromosomes have an equal chance of transmission) can be seen as a byproduct of recombination at meiosis, which makes every chromosome a mosaic of maternal and paternal chromosomes. Therefore, selection does not act on chromosomes as a single unit, and "unfair" meiosis becomes meaningless. ...


2

In an evolution mutations are often random and lead to differences in phenotype that can be adaptive under certain pressures. A lot of times mutation is a random process, but here are three cases I can think of off of the top of my head where I would say the organism is 'trying' to do it: HIV is a retrovirus, which means in its viral form its genome is ...


2

The chance of having a child with Down's Syndrome does not only have to do with cell division, but the mechanism that allows spontaneous abortion to occur within the uterus of the mother. There is strong evidence for uterine selection against genetically disadvantaged embryos. However, as women approach the menopause and the risk of future infertility ...


2

The number of spindle fibres is actually more than total number of kinetochore pairs. The fibres attached to kinetochores are called K-fibres and the others are called polar fibres. I cant surely say that there is exactly one K-fibre per kinetochore but as per its definition and from the microscopic images you can conclude that there is one per kinetochore. ...


2

According to this book, during disassembly of the nuclear envelope, the nuclear membranes are broken down into vesicles. The nuclear membranes reform at the end of mitosis as the vesicles bind to the surface of chromosomes and fuse with each other to form a double membrane around the chromosomes (how this happens is not clear, except that integral membrane ...


1

A recombinase only active during meiosis - Dmc1 and a general recombinase Rad51 coat the single-stranded DNA to form nucleoprotein filaments. These filaments hold the homologs together at the chiasma, the point of attachment during crossing over.


1

Couldn't fit in a comment... To me, your question sounds like "what are the possible advantages of sexual reproduction over asexual reproduction?" but in the meantime you're saying that you're not interested neither in the advantage of recombination nor in the advantage of "independent assortment". I don't quite see what you mean by "independent assortment" ...



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