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Objective 7: TSWBAT identify factors which stimulate and inhibit cell division.
The timing and rates of cell division in different parts of an animal or plant are crucial for normal growth, development, and maintenance. The frequency of cell division varies with cell type. Some human cells divide frequently throughout life (skin cells), others have the ability to divide, but keep it in reserve (liver cells), and mature nerve and muscle cells do not appear to divide at all after maturity. Investigation of the molecular mechanisms regulating these differences provide important insights into how normal cells operate, but also how cancer cells escape controls. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The cell cycle appears to be driven by specific chemical signals in the cytoplasm. Fusion of an S phase and a G 1 phase cell, induces the G 1 nucleus to start S phase. Fusion of a cell in mitosis with one in interphase induces the second cell to enter mitosis. A molecular control system drives the cell cycle Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.12
The distinct events of the cell cycle are directed by a distinct cell cycle control system. These molecules trigger and coordinate key events in the cell cycle. The control cycle has a built-in clock, but it is also regulated by external adjustments and internal controls. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.13
A checkpoint in the cell cycle is a critical control point where stop and go signals regulate the cycle. Many signals registered at checkpoints come from cellular surveillance mechanisms. These indicate whether key cellular processes have been completed correctly. Checkpoint also register signals from outside the cell. Three major checkpoints are found in the G 1, G 2, and M phases. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
For many cells, the G 1 checkpoint, the restriction point in mammalian cells, is the most important. If the cells receives a go-ahead signal, it usually completes the cell cycle and divides. If it does not receive a go-ahead signal, the cell exits the cycle and switches to a nondividing state, the G 0 phase. Most human cells are in this phase. Liver cells can be “called back” to the cell cycle by external cues (growth factors), but highly specialized nerve and muscle cells never divide. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Rhythmic fluctuations in the abundance and activity of control molecules pace the cell cycle. Some molecules are protein kinases that activate or deactivate other proteins by phosphorylating them. The levels of these kinases are present in constant amounts, but these kinases require a second protein, a cyclin, to become activated. Level of cyclin proteins fluctuate cyclically. The complex of kinases and cyclin forms cyclin- dependent kinases (Cdks). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cyclin levels rise sharply throughout interphase, then fall abruptly during mitosis. Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin concentration. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.14a
MPF (“maturation-promoting factor” or “M-phase- promoting-factor”) triggers the cell’s passage past the G 2 checkpoint to the M phase. MPF promotes mitosis by phosphorylating a variety of other protein kinases. MPF stimulates fragmentation of the nuclear envelope. It also triggers the breakdown of cyclin, dropping cyclin and MPF levels during mitosis and inactivating MPF. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.14b
The key G 1 checkpoint is regulated by at least three Cdk proteins and several cyclins. Similar mechanisms are also involved in driving the cell cycle past the M phase checkpoint. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
While research scientists know that active Cdks function by phosphorylating proteins, the identity of all these proteins is still under investigation. Scientists do not yet know what Cdks actually do in most cases. Some steps in the signaling pathways that regulate the cell cycle are clear. Some signals originate inside the cell, others outside. Internal and external cues help regulate the cell cycle Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The M phase checkpoint ensures that all the chromosomes are properly attached to the spindle at the metaphase plate before anaphase. This ensures that daughter cells do not end up with missing or extra chromosomes. A signal to delay anaphase originates at kinetochores that have not yet attached to spindle microtubules. This keeps the anaphase-promoting complex (APC) in an inactive state. When all kinetochores are attached, the APC activates, triggering breakdown of cyclin and inactivation of proteins uniting sister chromatids together. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
A variety of external chemical and physical factors can influence cell division. Particularly important for mammalian cells are growth factors, proteins released by one group of cells that stimulate other cells to divide. For example, platelet-derived growth factors (PDGF), produced by platelet blood cells, bind to tyrosine-kinase receptors of fibroblasts, a type of connective tissue cell. This triggers a signal-transduction pathway that leads to cell division. Each cell type probably responds specifically to a certain growth factor or combination of factors. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The role of PDGF is easily seen in cell culture. Fibroblasts in culture will only divide in the presence of medium that also contains PDGF. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.15
In a living organism, platelets release PDGF in the vicinity of an injury. The resulting proliferation of fibroblasts help heal the wound.
Growth factors appear to be a key in density- dependent inhibition of cell division. Cultured cells normally divide until they form a single layer on the inner surface of the culture container. If a gap is created, the cells will grow to fill the gap. At high densities, the amount of growth factors and nutrients is insuffi- cient to allow continued cell growth. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.16a
Most animal cells also exhibit anchorage dependence for cell division. To divide they must be anchored to a substratum, typically the extracellular matrix of a tissue. Control appears to be mediated by connections between the extracellular matrix and plasma membrane proteins and cytoskeletal elements. Cancer cells are free of both density-dependent inhibition and anchorage dependence. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.16b
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