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Chapter 15 Blog: The Eukaryotic Cell, Mitosis, and Meiosis (Semon)

Page history last edited by Semon Rezchikov 13 years, 2 months ago

A.  Blog

This chapter discussed cell division, and, naturally, since this is a biology class, how everything sometimes goes horribly wrong.


Cells have several phases in their cell cycle. The G0 (G for growth) phase is a phase in which the cell is not growing at all; many fully differentiated cells are frozen at this stage. In G1, the cell grows, making more protein and cytoplasm. If it passes the G1 checkpoint, a crucial control on cell division (regulated by cyclin activity), the cell enters S (S for synthesis) phase and copies its genetic material. (Note: copying all the DNA in a cell has lots of potential for wonderful and terrible mutations because there's so much of it. Mostly terrible ones, sadly.) In the G2 phase it grows some more; there is another checkpoint at the end of G2 to make sure dividing should really occur and nothing went horribly wrong so far. The cell then enters M (M for mitosis) phase and divides. There's a checkpoint in the middle of M (during metaphase) to make sure chromosome alignment occurred correctly. Clearly, if the checkpoints don't function correctly, things go horribly wrong, possibly resulting in nonfunctioning, misfunctioning, or cancerous cells. 


The normal division of a cell into two identical cells is called mitosis. In prophase the nucleus dissapears and the chromatin condenses into chromosomes. They line up in the middle of the cell in metaphase, and then are pulled apart by proteins attatched to microtubules radiating from centromeres at opposite poles of the cell during anaphase. Around this time, the cell cleaves its cytoplasm in half (cytokinesis) and the nuclear envelope reforms during telophase. If the sister chromatids (the identical copuies of chromosomes) aren't pulled apart properly during anaphase, you get a nondisjunction and thus daughter cells with irregular numbers of chromosomes. This is bad. This causes Down Syndrome, which sucks.


In meiosis, first the chromosomes pair with their homologous partners, cross over, and separate, producing 2 haploid cells; the haploid cells then undergo mitosis to get to a haploid non-duplicated state. Because each pair of homologous chromosomes assorts independently on the metaphase plate, there can be 2^n possible genotypes of the daughter cells of the first mitotic division, even assuming no crossing over (where n is haploid number). Yay, genetic diversity!



B.  Useful Materials

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Such an awesome video. Microscopy techniques are great.



Very, very, very in-depth description of various mitosis studies.

The images are stunning. Note the use of sophisticated microscopic techniques.


Nondisjunction for Dummies. :-)



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