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Question on Chromatids

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Question on Chromatids

Postby Chromatidious » Sun Dec 19, 2010 3:15 pm

In fertilization when sperm and ovum come together they form a single original chromatid. The chromatid makes a copy of itself and the two chromatids are connected at the centromere and all of the chromosomes are formed in this way. The zygote stem cell is created and forms around the chromosomes. When the zygote stem cell divides the chromatids separate by the mitotic spindles and go into two nascent daughter stemcells. Technically there is one original chromatid and three unoriginal chromatids at this point. When both of these cells divide again there is now 4 cells, but there is still only one original chromatid and 7 unoriginal chromatids. Is it technically possible for this original chromatid to still be in a human if the zygote stemcell or its first daughter stem cell became a braincell?

If I am right or wrong about specific things could you explain as detailed as possible and examples in layman terms would be helpful. Thank you so much.
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Postby kolean » Tue Dec 21, 2010 3:04 pm

I like to use the words paternal genome and maternal genome. Even though the genomes are inside the oocyte, they are still seperated and called pronucleuses (Mmm....spelling? perhaps I should say -male pronucleus and female pronucleus). They are still being 'worked on' inside their own pronucleus, till the cell cycle begins and they merge to become one nucleus.
Doubling of the DNA then occurs, and centromeres are made to attach to the asters of the centrioles (mitotic spindle), which will guide the chromatids/sister chromatids during the cell cycle. Since the DNA is replicated semi-conservative, the original paternal and maternal strand will be left completely whole, and if they are split in anaphase to the original centriole and stay together in one of the cells at the 2-cell stage, is not known to me. (Anybody else know about this on the forum?)
It would be interesting though to see if there is a cell within the blastocyst (ICM - InnerCellMass - for humans) that contains all the original paternal and maternal genome.
Why would you pick the original cell to become a brain cell? I would guess that it would become a sex germ cell. That way the original paternal/maternal genome could be passed on to the next generation, and sex germ cells are established during the blastocyst stage and sequestered from environmental influences, so its epigenetic state remains undisturbed.
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