Download Chapter 5 Cells

Science 21B
Presentation for Section 5.1
The Cell Cycle
Slide 1
What do you think of when you hear the word immortal? You would probably say it means “to
live forever.” How do you think scientists define immortality for cells? Division without limits?
What would you infer about cell division in a normal cell and how it compares to cell division in
a cancer cell? You would probably think that division in normal cells is limited or regulated in
some way. The underlying idea is that uncontrolled cell growth can be unhealthy for an
organism. The cell divisions of normal cells are controlled by the body. They can take place in
response to conditions in the body that may relate to injury or infection, developmental changes,
or the need for maintenance. You should understand that it is really the cell line, not the single
cell, that is immortal.
This photograph shows a lung cancer cell undergoing cell division. Unlike healthy cells cancer
cells can divide without limit; they are what scientists call immortal. This property is useful to
scientists who culture cancer cells for research purposes. However, cancer cells are very
dangerous in the body where they can form tumors and invade tissues.
Think about doubling and division in reference to baking cookies. If you wanted two batches of
cookies, not just one, what would you need to do to the recipe? Double it, of course. Doubling a
recipe means doubling all the ingredients in the original recipe. If you apply this to cell division,
for cells to be functional after division, what needs to be present? A complete set of DNA,
organelles, cytoplasm, cell membrane, and a cell wall, if it’s a plant cell.
Slide 2
You will be seeing the terms cycles, phases, and stages. A cycle is a repeating series of events
that has an obvious relationship to the Latin word cyclus meaning “circle.” The word phase
comes from the Greek phasis meaning “appearance.” One common example of phases is the
phases of the moon. Each phase of mitosis has a distinctive appearance. The word stage comes
from the Latin word status for “stand.” You might think in terms of stagecoaches and their
stopping points, or stands, along a journey.
The cell must move through the stages in the cell cycle to reach a point where it can successfully
divide. Here are some familiar words used together: divide is to separate; duplicate is an identical
copy; double is to increase twofold; and copy is a reproduction of the original. Put all together
you might say, “Cells double their DNA in order to divide and create two nearly identical
duplicates, or copies.”
Why do you always have to cut your hair, fingernails, and the lawn? Growth is caused by new
cells being made. The cell cycle is arranged in clockwise arrangement. There are four main
stages in the cell cycle: gap 1, synthesis, gap 2, and mitosis.
Slide 3
What can you infer about the relative amount of time a cell spends in each stage? An actively
dividing cell spends the least amount of time in mitosis. The length of gap 1 varies the most and
is usually the longest.
The word gap suggests a break or an interruption. Scientists named the two stages of the cell
cycle gaps because they did not know the cell was active during these stages.
Three important checkpoints in the cell cycle ensure that specific events occur correctly and in
the proper order, thus signaling that a cell is ready to divide. The first checkpoint begins at the
end of gap 1, during which a cell checks the DNA for any damage before copying begins and
also makes sure that conditions are right for division. By the second checkpoint, at the end of gap
2, both DNA and the centrosome need to have been successfully copied, and those parts of the
cytoskeleton that help to pull the two halves apart must be in place. The third and final
checkpoint occurs halfway through mitosis, to check that the two copies of DNA have been
properly attached to the cytoskeleton that will separate them.
One misconception is that cytokinesis is a part of the process of mitosis, especially given its
inclusion as part of the M stage of the cell cycle. Mitosis and cytokinesis are two distinct
processes. Mitosis is the division of the cell nucleus and its contents, resulting in two identical
nuclei. Cytokinesis is the division of the cell cytoplasm, resulting in two cells, each of which
contains one of the nuclei from mitosis. The two processes overlap: together, mitosis and
cytokinesis make-up of the mitosis stage of the cell cycle.
Slide 4
You might think in terms of cell population when relating the rate of cell division to the length of
a cell’s life span. You, for example, would expect a muscle cell to divide at much slower rate
than a population of skin cells.
The specialization in cells sometimes includes cells whose job it is to divide to supply new cells.
These are called “stem cells.” Many of the cells in the human body that need constant
replacement, such as blood cells and surface skin cells, are unable to divide. Skin cells undergo a
lot of wear and tear because they are exposed on the outside of the body. Stem cells that are part
of the cell population provide new cells that, once produced, differentiate into the type of cell
needed.
Slide 5
Do you remember the formulas from math class for surface area and volume? Surface area can
be calculated for a cell by the following formula: it is its length times its width times its number
of sides (l × w × s). The volume of a cell can be calculated by the formula length times width
times height (l × w × h). The surface area of a cube increases by the length squared and its
volume increases by the length cubed.
Slide 6
To maintain a suitable cell size, growth and division must be coordinated. Surface area to
volume ratio, if it’s too small, the cell cannot move materials into and out of the cell at a
sufficient rate or in sufficient quantities.