BIS104 Note13

BIS104 Note13 - Lecture 13 Bio Sci 104 Winter 2010 Cell...

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Lecture 13 1 Bio Sci 104 Winter 2010 Cell Cycle I. Overview A. Single cell organisms must coordinate their replication to the environmental conditions. Multicellular organisms require cell proliferation during embryogenesis and subsequent growth. In addition, organisms require continued cell reproduction in adult to replace wear and tear. Cell division must only be in response to proper signals and under controls. In addition, the complex processes involved in replication must be carefully coordinated and timed. Thus, control of cell division is tightly regulated in eukaryotic cells. Regulation is really at two levels: cell has to decide whether to replicate or not, and once committed to replication, cell must coordinate the necessary molecular events. B. Decision to replicate or not: At any given time, most cells in an adult multicellular organism are not actively replicating (denoted as in G o ); in fact, "terminally differentiated" cells normally cannot replicate at all. In response to proper internal and external signals, however, certain cell types can replicate (leave G o , enter "the cell cycle"). We will talk in subsequent lectures about the signals involved in deciding to enter into the cell cycle or not. In this lecture, we will focus on the events once the cell is actively cycling C. Once decision to replicate is made, eukaryotic cells go through an orderly and necessary progression - the cell cycle, consisting of G 1 (growth phase 1) , S (DNA synthesis), G 2 (growth phase 2) , M (mitosis) phases. This may take from hrs to many days, depending on cell type. It is crucial for cell survival that these events take place in correct sequence, and that early events be completed before later events occur. 1. The cell cycle is regulated by a central control system which achieves these goals by timing the processes involved, and by monitoring each step (checkpoints). e.g.: G 1 checkpoint- Is last mitosis complete? Is cel l large enough? Is environment favorable? G 2/ M checkpoint- Is all DNA replicated? Metaphase-anaphase transition ( late M checkpo int)- Are a l l chromosomes on mitotic spindle? Central controller will, if necessary, delay later steps to ensure completion of earlier ones. 2. Central controller will also monitor for accidents and errors. For example, DNA damage (generated by radiation or chemicals ) must be repaired before the DNA is
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Lecture 13 2 1. Kinases themselves form an interrelated gene family denoted cyclin-dependent protein kinases (Cdk). When Cdk are active, they phosphorylate other proteins on serine and threonine residues, thus activating or repressing the functions of the multiple downstream proteins involved in specific events in the cell cycle. In yeast, there is
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BIS104 Note13 - Lecture 13 Bio Sci 104 Winter 2010 Cell...

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