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The Cell Cycle and Apoptosis.docx

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BIOL 3096
Joel Sheffield

Lecture 38 – The Cell Cycle andApoptosis November 27, 2013 The Cell Cycle • Most of the cells in the body are not dividing, e.g. muscles, nerves, they are in G 0 • Two exceptions are the skin, the gut • People have found that cells not normally dividing can be induced to divide. Studying Cell division • DNAsynthesis, once it starts, is not a smooth rate, there are faster periods and slower periods. • Rao and Johnson used the Sendai Virus to fuse cells in different phases, S, G etc. 1hey used techniques like radioactive thymidine to identify the phase of cells ◦ G ad1ed to mitotic cells: G mate1ial started to condense too. ◦ S added to mitotic cells: S material also condensed, but because replicating DNAis especially sensitive to damage, the result was condensed “pulverized” chromosomes, i.e. little fragments of condensed chromosome material. ◦ G added to mitotic cells: G material condensed 2 2 • Another experiment involved injecting the cytoplasm of cells in different phases into unfertilized Xenopus embryos to see if it caused the embryos to mature and divide, ◦ Isolated from the cytoplasm was a protein known as Mitosis Promoting Factor (MPK, originally maturation promoting factor), every time the cell was undergoing mitosis, this factor increased in concentration. ◦ MPF is composed of two subunits, one has kinase activity, the other is a protein called cyclin, which varied its concentration in the cytoplasm on a “cyclic” basis. ◦ Cyclin activates the kinase subunit. ◦ Kinases that phosphorylate threonine and serine were also found in the cytoplasm, that remain at constant levels ◦ As cyclin levels would increase in the cytoplasm, these kinase subunits binds to the cyclin creating active MPF, thus, Cyclin helps trigger mitosis. ◦ Along with cyclin that stimulates mitosis, a cyclin was found that stimulates entry into the S phase. Thus there are Mitotic Cyclins and G Cycli1s. ◦ Each of these cyclins has its own Kinases, generically called Cell cycle dependent kinases (cdc2) 1. cdc2 in its in inactive form binds to a Tyrosine-P, a Threonine-P, and Cyclin. 2. Another gene, cdc25, removes the tyrosine-P, activating the cdc2/cyclin complex 3. This stimulates mitosis 4. Afterwards, threonine-P and cyclin dissociate from the cdc2, i.e. complex is degraded. Degradation • An example is ubiquination
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