The Eukaryotic Cell Cycle – Controls
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The Eukaryotic Cell Cycle – Controls Slide 2 It is essential for all organisms to carefully control the process of cell division. In prokaryotic organisms, we saw that the rate of cell division is largely influenced by environmental cues, such as nutrient availability. In eukaryotic organisms, cell division is affected by factors both internal and external to the cell or organism. Since many eukaryotic organisms are multicellular and have highly differentiated, or specialized, tissues and organs, controlled cell division is an important aspect of the development of these organisms. Slide 3 As a eukaryotic cell moves through the cell cycle, the transition between the different phases of the cell cycle, such as G1 to S, or G2 to M, is controlled by cyclin- dependent kinases. Kinases are enzymes that add phosphate groups to other molecules, in a process called phosphorylation. The addition of a phosphate group to a molecule often results in a change in the three dimensional shape of the molecule. In some instances, phosphorylation of an enzyme dramatically changes its activity. In one such case, cyclin dependent kinases phosphorylate enzymes that are essential for different parts of the cell cycle such as DNA replication or chromosome condensation. In order to be active cyclin dependent kinases must be bound to a second type of protein, called cyclin; hence the name cyclin dependent kinase. Slide 4 In addition to internal factors, the cell cycle may also be influenced by factors that are external to the cell, such as growth factors and hormones. These external signals are typically proteins, lipids, or other small molecules that bind to the cell membrane of targeted cells, and initiate a response within the cell. To illustrate the effects of these types of molecules on the cell cycle, imagine that you have a small cut on your finger. As your finger bleeds, platelets in your blood help clot the blood. The platelets serve a second purpose as well – they release a growth factor called platelet-derived growth factor, which diffuses through the blood to nearby skin cells, stimulating them to divide. The growth and division of cells due to this stimulus eventually results in the healing of the wound. Slide 5 Finally, you should also note that signals affecting rates of cell division may come completely from outside an organism. For example, consider the new growth of plants in the spring, or flowers that are produced at specific times of the year. In these instances, factors external to the organism, such as the amount of sunlight, air and soil temperatures, or the amount of water available may all affect the cell division that results in the growth and development of new leaves or flowers. Slide 6 In most instances, mitosis and the rest of the cell cycle occur at rates appropriate for the tissue or organism. Effective control of the cell cycle can then result in the proper growth and development of the organism. However, in some cases, the cell cycle runs out of control. This can lead to the unregulated growth of tissues, examples of which are the tumors associated with different types of cancer. Generally, problems with the cell cycle are caused by some malfunction of the control mechanisms we have just discussed. For example, some cancerous cells produce an excess of cyclin, which over-stimulates cyclin dependent kinases and the division of cells. In other cases, cancerous cells may produce their own growth factor, or proceed through the cell cycle without requiring any growth factor stimulation. In these cases, control of the cell cycle is lost, resulting in rapidly dividing cells and the growth of tumors.