Lecture 24 Meiosis
ed. Chap. 12 pp. 230-232 and Chap. 13 pp. 238-249
LEARN FIGURE 13.8!!!
: Finish up Regulation of the Cell Cycle
Most cells in our bodies stay quiescent (meaning that they do not divide) most of the time.
However, entry into the cell cycle can be triggered by external factors when needed. An example is
in wound healing. The cells in your finger (for instance) do not normally divide much, and your
finger stays about the same size. However, you may get a cut. Cells at the edge of the cut enter the
cell cycle and the start to divide, filling in the gap. Over time, the wound is completely healed.
This happens because when you get a cut, small cells called platelets that circulate in the blood are
stimulated to release a small molecule called a growth factor. The name of this particular growth
factor is PDGF (platelet-derived growth factor). PDGF binds to the PDGF receptor, a tyrosine
kinase receptor on the surface of nearby cells (those next to the wound) and triggers a signal
transduction pathway. The result of this pathway is to increase the amount of the G1 cyclin. The
G1 cyclin binds to the appropriate cdk (cyclin-dependent kinase) to activate the cdk. This causes
the cell to enter the cell cycle again and to divide, generating new cells that fill in the wound. Once
the wound has healed, the cells leave the cell cycle again, and stop dividing.
Scientists take advantage of the ability of growth factors to stimulate cell division in the
laboratory. Many experiments are done using mammalian cells grown in the laboratory. We start
out with cells take from tissue. Often, these are cells taken from an animal (such as a mouse or a
rat). In some cases, they can even be human cells. [We didn’t discuss this in class, and it won’t be
on the exam. It’s just for interest. There are several ways of obtaining human cells for study
without harming people. For instance, when foreskins are removed from baby boys at circumcision,
the cells from the foreskin can be used for experiments. Similarly, after a baby is born, the cells
from the umbilical cord can be grown for awhile in the lab and used for study.] In any case, this is a
very useful method for scientists to obtain large numbers of cells for studying a wide variety of
Cancer Cells Grow Even Without Growth Factors
As we’ve seen, normal cells need growth factors in order to enter the cell cycle. Occasionally,
however, abnormal cells are formed that enter the cell cycle and divide even without growth factors.
These cells divide and grow uncontrollably. They eventually form a tumor, or mass of abnormal
cells. This can lead to cancer. Because cells that enter the cell cycle when they shouldn’t can cause
cancer, understanding how the cell cycle normally works, and how it’s regulated, are very
important. We may be able to use this information in treating or preventing cancer.