The Eukaryotic Cell Cycle – Controls

							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.