NEL Cell Division 563
Not all cells of the body have the same ability to undergo mitosis. Age is one reason
cells stop dividing. However, division is usually stopped by cell specialization. Relatively
unspecialized cells, such as skin cells and the cells that line the digestive tract, reproduce
more often than do the more specialized muscle cells, nerve cells, and secretory cells.
Only two cell types in the human body divide endlessly: the sperm-producing cells,
called spermatogonia, and the cells of a cancerous tumour. Males are capable of pro-
ducing as many as one billion sperm cells a day from the onset of puberty well into old
age. However, once the sperm cells are formed, they lose the ability to divide further.
Cancer cells divide at such an accelerated rate that the genes cannot regulate the prolif-
eration and cannot direct the cells toward specialization.
It would appear that the more specialized a cell is, the less able it is to undergo mitosis.
The fertilized egg cell is not a specialized cell; differentiation begins to occur only after
its third division, which results in eight cells. Interestingly, it is at the point where dif-
ferentiation begins that the biological clock within the cell is turned on.
Section17.- Cell division produces new cells for cell growth and for the replacement of
worn-out cells in the body. - Cell division involves a series of steps that produce two genetically identical
daughter cells. Two divisions occur during cell division: nuclear division
(mitosis) and cytoplasmic division (cytokinesis). - During interphase, genetic material is replicated.
- Cells seem able to divide only a finite number of times.
- Cells lose the ability to divide as they specialize.
SUMMARY The Cell Cycle
Purpose Design Analysis
Problem Materials Evaluation
Hypothesis Procedure Synthesis
Prediction EvidenceTo perform this investigation, turn to page 587.
Frequency of Cell Division
In this activity, you will view and compare cells from onion cells
and from a whitefish blastula in various stages of mitosis.
Because slides are used, the cell divisions you will be viewing are
frozen in time. Therefore, it will not be possible for you to watch a
single cell progress through the stages of mitosis. Based on your
observations, you will determine the frequency of cell division
and construct a clock representing the division cycle, given the
time taken to complete one cycle of mitosis. In a table, you will
record the number of cells in each stage of mitosis.INVESTIGATION 17.1Introduction Report Checklist
Cancer and Metastasis
Cells that divide uncontrollably
can become cancer. This
animation shows how cancer cells
can spread from one part of the
body to another.http://www.science.nelson.com GO