14 - Cell Cycle To produce two genetically identical...

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Cell Cycle
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To produce two genetically identical daughter cells, the DNA in each chromosome must be replicated and the replicated chromosomes must be segregated into the two daughter cells. Finally, most cells must also duplicate their organelles and macromolecules and divide these into the daughter cells. So to explain how cells reproduce, we must consider three questions: How do cells duplicate their contents? How do the cells partition the duplicated contents and split into two? How do the cells coordinate these activities? We’ve looked at the first question in some detail earlier in the semester. The physical process is discussed in chapter 19. So this lecture will focus on how cells coordinate these activities.
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The division of a eukaryotic cell involves duplicating their contents and dividing, a process called the cell cycle .
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The cell cycle is divided into four phases: 1. M Phase 2. G 1 Phase 3. S Phase 4. G 2 Phase
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A central controller triggers each process in a set sequence. It monitors, through feedback mechanisms, where events are in the cell by keeping track of several checkpoints.
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Two important cellular checkpoints occur. One occurs in G1 that allows the cell to confirm that the environment is favorable for cell proliferation and that DNA is intact. The second occurs in G2 and ensures that the cells do not enter mitosis until damaged DNA is repaired and DNA replication is complete.
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The cell-cycle control system ensures correct progression through the cell cycle. These activities are governed by the cyclic activation and deactivation of protein complexes via phosphorylation. The protein kinases that are responsible for “throwing the switch’ are dependent on cyclins. Cyclines are a set of proteins that have no enzymatic activity themselves, but they bind to cell-cycle kinases and influence their activation. The concentration of the cyclins varies in a cyclic fashion throughout the cell cycle enabling the protein kinases to demonstrate a cyclic pattern of activation as well.
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So the question becomes, how is this cyclic degradation of the cyclins accomplished?
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Proteosome Review: Most proteins are degraded in cytosol by proteosomes. The proteosome structurally contains a central cylinder stoppered on either end by protein complexes thought to bind labeled proteins for degradation. Ubiquitin is a small protein that appears to act as the degradation tag. Once labeled with ubiquitin, the stoppered proteins appear to funnel the protein into its core for lysis.
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In the case of cyclins: The activities of the cyclins is likewise regulated by degradation. Ubiquitination marks the proteins for destruction and the proteosomes breakdown the tagged molecules. This leaves the Cdk (cyclin dependent protein kinases) free of the cyclins that cause its activation and inactivation.
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But what controls when the cyclin is labeled with ubiquitin? In the case of M-cyclin, a protein complex called
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This note was uploaded on 04/13/2008 for the course BIOL 308 taught by Professor Ragsdale during the Fall '07 term at Winona.

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14 - Cell Cycle To produce two genetically identical...

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