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12

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 ...


10

When females are in their mother's womb all their Oogonium (plural Oogonia) 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. Meiosis I continues when a female hits puberty, every month one or more primary oocytes ...


10

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 state, ...


10

After meiosis I, those n=23 chromosomes have two chromatides. Meiosis II just separates them into single chromatides.


7

Assuming that you have studied megasporogenesis and microsporogenesis. To produce a seed, you require the production of pollen(n) and egg(n) and their fusion. Let's start with pollen grain(n): 4 pollen grains are produced after 1 meiotic division in the pollen sacs. $$4~\text{pollen grains} = 1~\text{meiotic division}$$ To produce 1 pollen grain. $$ ...


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 ...


6

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 from ...


6

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 ...


5

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 ...


5

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 ...


5

At the start, all the cells are 2n, diploid cells. By far the largest difference between meiosis I and mitosis is that mitosis results in genetically identical, diploid somatic cells. Meiosis, in it's entirety, results in gametes of haploid genetic information, but the genetic information is not identical due to crossing-over events that happened during ...


5

Well, in my opinion, the entirety of the meiosis is a process (reproduction of sex cell) in which two levels of division occur, it's all kind of one process. Though meiosis II may seem to have many similarities with mitosis, meiosis II can only occur with sex cells, to my understanding that is the main factor which differentiates meiosis I and II from ...


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 of ...


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, 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 ...


4

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 SF. ...


4

Meiosis does not determine sexual form. Eukaryotes use meiosis and fertilization to recombine genes to form new combinations. Meiosis does produce haploid cells from diploid cells, but that has nothing much to do with the sexual forms involved. In the case of the algal genus Chara, the organism's life cycle is entirely haploid except for the single-...


4

ID characteristics that can help you recognize diplotene better: diplotene : the only difference between this phase and Diakinesis is that The centrosomes reach the poles. you can see the photos of diplotene and diakinesis here : http://www1.biologie.uni-hamburg.de/b-online/e09/meiosea.htm (a) polyploid nutri-tive cells with many heteropicnotic ...


4

In case of gametogenesis (let us talk about spermatogenesis) gametes are formed from meiotic division of Primary spermatocytes. In Primates Primary spermatocytes are cells that that are formed from mitotic division of B spermatogonia (which is another class of germ cells) which inturn are formed from mitosis of Ap spermatogonia which arise from mitotic ...


4

For producing $x$ number of seeds (or say zygote) $x$ number of egg cells must fuse with $x$ number of male gametes. In angiosperms, 1 meiotic division of Megaspore mother cell leads to formation of one egg cell. So $x$ egg cells are formed from $x$ number of meiotic division. But 1 meiotic division of Sperm mother cell (2n) leads to formation off 4 ...


4

It is the other way around. Meiosis I (or reductional division) splits chromosome pairs so each cell gets half of the chromosomes of the parent. Meiosis II (or equational division) splits double-chromatid chromosomes (making two single-chromatid chromosomes), so cells retain the number of chromosomes, like in mitosis. If more clarification needed: before ...


3

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 ...


3

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. ...


3

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 ...


3

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. ...


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

This phenomena is known as achiasmy, where recombination is absent in one sex in a species. The Haldane Huxley rule states that in achiasmic species, the sex without recombination will always be the heterogametic sex (XY or ZW). This is basically the only consistent rule relating to recombination. There seem to be exceptions to every other theory and pattern ...


3

I think you really need to go back to the very basics and try to understand the DNA molecule, what a chromosome is, DNA replication and mitosis, what homologous chromosomes are, what non homologous chromosomes are, what sister chromatids are, what non sister chromatids are before you attempt to understand meioses. And I would definitely recommend you learn ...


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