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P A R T 2
Chemotherapeutic agents
CELL CYCLE
Most cells have the ability to reproduce themselves
through the process of mitosis. The genetic makeup of a
particular cell determines the rate at which that cell can
multiply. Some cells reproduce very quickly (e.g. the cells
lining the gastrointestinal tract have a generation time
of 72 hours), and some reproduce very slowly (e.g. the
cells found in breast tissue have a generation time of a
few months). In some cases, certain factors influence
cell reproduction. Erythropoietin, a hormone produced
by the kidney, can stimulate the production of new red
blood cells. Active leucocytes release chemicals that
stimulate the production of white blood cells when the
body needs new ones. Regardless of the rate of repro-
duction, each cell has approximately the same life cycle.
The life cycle of a cell, called the
cell cycle
, consists of
four active phases and a resting phase (see Figure 7.6).
G
0
phase
During the G
0
phase, or resting phase, the cell is stable. It
is not making any proteins associated with cell division
and is basically dormant as far as reproduction goes.
These cells are just functioning to do whatever they are
supposed to do. Cells in the G
0
phase cause a problem
in the treatment of some cancers. Cancer chemotherapy
usually works on active, dividing cells, leaving resting
cells fairly untouched. When the resting cells are stimu
lated to become active and regenerate, the cancer can
return, which is why cancer chemotherapeutic regimens
are complicated and extended over time, and why a
5-year cancer-free period is usually the basic guide for
considering a cancer to be cured.
G
1
phase
When a cell is stimulated to emerge from its resting
phase, it enters what is called the G
1
phase, which lasts
from the time of stimulation from the resting phase until
the formation of DNA. During this period, the cell syn-
thesises substances needed for DNA formation. The cell
is actively collecting materials to make these substances
and producing the building blocks for DNA.
S phase
The next phase, called the S phase, involves the actual
synthesis of DNA, which is an energy-consuming
activity. The cell remains in this phase until the amount
of cellular DNA has doubled.
G
2
phase
After the cellular DNA has doubled in preparation for
replication, the G
2
phase begins. During this phase,
the cell produces all of the substances required for the
manufacture of the mitotic spindles.
M phase
After the cell has produced all of the substances nec-
essary for formation of a new cell, or daughter cell, it
undergoes cell division. This occurs during the M phase
of the cell cycle. During this phase, the cell splits to form
two identical daughter cells, a process called mitosis.
KEY POINTS
■■
All cells progress through a cell cycle, which allows
them to reproduce.
■■
Each cell goes through a resting phase (G
0
); a
gathering phase (G
1
), when the components needed
for cell division are collected by the cell; a synthesising
phase (S), when DNA and other components are
produced; a final gathering phase (G
2
), when the
last substances needed for division are collected and
produced; and an M phase, when actual cell division
occurs, producing two identical daughter cells.
CHAPTER SUMMARY
■■
The cell is composed of a nucleus, which contains
genetic material and controls the production of
proteins by the cell; a cell membrane, which separates
the inside of the cell from the outside environment;
and a cytoplasm, which contains various organelles
important to cell function.
■■
The cell membrane functions as a fluid barrier
made of lipids and proteins. The arrangement of the
lipoprotein membrane controls what enters and leaves
the cell.
■■
Proteins on the cell membrane surface can act
either as receptor sites for specific substances or as
histocompatibility markers that identify the cell as a
self-cell (i.e. a cell belonging to that individual).
KEY POINTS
Phases of
cell cycle
G
1
G
0
G
2
S
M
5-FU
FIGURE 7.6
Diagram of the cell cycle, showing G
0
, G
1
, S, G
2
and
M phases.
