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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

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

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


8

It depends on the regions of sequence homology between the two chromosomes. Crossing over occurs through pairing of homologous regions. If there's a substantial stretch of chromosome without a matching homologous region on its pair, that non-homologous region should loop out and not be involved in crossing over. Crossovers will occur only in paired ...


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

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 ...


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

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

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

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

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

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

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

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, 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

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

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 will talk about humans only and I will not talk about the special case of sexual chromosomes (and anomalies such as trisomy 21) from the answer just to keep things easy. There is a lot to say, so I am just making a very short overview of different subjects. Get some coffee first, read slowly and don't hesitate to click on the wikipedia links to further ...


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 ...


3

"How are germ cells not reduced in number?" It does happen. Germ cells do eventually run out. It is called menopause in women. And age related infertility in men. As for your question of where do germ cell come from? Much like were do muscles cells come from?, you have to be specific. Germ cells comes from the gonads (ovaries and testis) would be a simple ...


3

I assume that you mean phase of cell division. First off all i will write down a definition of word polyploid. Polyploid is cell which has two or more pairs of homologus chromozoms. There are two basic types of cell division are meiosis and mitosis Technicly you can say that in reduction meiosis when chromozomes go from diploity to haploid state (which is ...


3

That graphic is just confusing you. The cell doesn't keep track of all the chromosomes from Mother versus all the chromosomes from Father so it can sort them out later. That I know of! [I don't think imprinting counts because its effect varies between genes] But the paternal and maternal chromosomes are still separate before meiosis - there hasn't been ...


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 ...


2

For a male, one primary spermatocyte produces two secondary spermatocytes through meiosis I, which in turn produce two spermatids each through meiosis II. So one primary spermatocyte produces four spermatids. That means that $x$ spermatids are produced after $x/4$ meiotic divisions consisting of once meiosis I and twice meiosis II. However, we don't count ...


2

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" ...


2

We need to make a distinction between the genetic map of a chromosome, which is usually built up from meiotic recombination frequencies between linked genetic markers, the physical map of a chromosome, which used to be made up of clones and contigs, but is now usually derived from the reference genome sequence, and the cytological map of a chromosome, which ...


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