CELL DIVISION
Cell division is the process by which a cell reproduces
itself. There are two types of cell division, mitosis and
meiosis. Although both types involve cell reproduc-
tion, their purposes are very different.
MITOSIS
Each of us began life as one cell, a fertilized egg. Each
of us now consists of billions of cells produced by the
process of mitosis. In mitosis, one cell with the
diploid numberof chromosomes (the usual number,
46 for people) divides into two identical cells, each
with the diploid number of chromosomes. This pro-
duction of identical cells is necessary for the growth of
the organism and for repair of tissues (see also Box
3–3: Abnormal Cellular Functioning—Cancer).
Before mitosis can take place, a cell must have two
complete sets of chromosomes, because each new cell
must have the diploid number. The process of DNA
replicationenables each chromosome (in the form of
chromatin) to make a copy of itself. The time during
which this takes place is called interphase, the time
between mitotic divisions. Although interphase is
sometimes referred to as the resting stage, resting
means “not dividing” rather than “inactive.” The cell
is quite actively producing a second set of chromo-
somes and storing energy in ATP.
The long, thin, and invisible chromatin molecules
then begin to coil very precisely and extensively, and if
we were looking at the nucleus of a living cell under a
microscope, we would see the duplicated chromo-
somes appear. Each would look somewhat like the let-
ter X, because the original DNA molecule and its copy
(now called chromatids) are still attached.
The stages of mitosis are prophase, metaphase,
anaphase, and telophase. What happens in each of
these stages is described in Table 3–4. As you read the
events of each stage, refer to Fig. 3–5, which depicts
mitosis in a cell with a diploid number of four.
As mentioned previously, mitosis is essential for
repair of tissues, to replace damaged or dead cells.
Some examples may help illustrate this. In several
areas of the body, mitosis takes place constantly. These
sites include the epidermis of the skin, the stomach
lining, and the red bone marrow. For each of these
sites, there is a specific reason why this constant mito-
sis is necessary.
What happens to the surface of the skin? The dead,
outer cells are worn off by contact with the environ-
ment. Mitosis of the epidermal cells in the lower liv-
ing layer replaces these cells, and the epidermis
maintains its normal thickness.
The stomach lining, although internal, is also con-
stantly worn away. Gastric juice, especially hydrochlo-
ric acid, is very damaging to cells. Rapid mitosis of the
several kinds of lining cells replaces damaged cells and
keeps the stomach lining intact.
One of the functions of red bone marrow is the
production of red blood cells. Because red blood cells
have a life span of only about 120 days, new ones are
needed to replace the older ones that die. Very rapid
mitosis in the red bone marrow produces approxi-
mately 2 million new red blood cells every second.
These dividing cells in the red bone marrow are
among the stem cells present in the body. A stem cell
is an unspecialized cell that may develop into several
different kinds of cells. Stem cells in the red bone mar-
row may become red blood cells, white blood cells, or
platelets. These marrow stem cells are often called
adult stem cells, and many, if not all, of the body’s
organs have such cells. Embryonic stem cells will be
described in a later chapter; these are cells in which all
of the DNA still has the potential to be active. They
may become any of the more than 200 different kinds
of human cells. The stem cells found in the umbilical
cords of newborns are between the adult and embry-
onic cells in terms of their potential.
It is also important to be aware of the areas of the
body where mitosis does not take place. In an adult,
most muscle cells and neurons (nerve cells) do not
reproduce themselves. If they die, their functions are
also lost. Someone whose spinal cord has been severed
will have paralysis and loss of sensation below the level
of the injury. The spinal cord neurons do not undergo
mitosis to replace the ones that were lost, and such an
injury is permanent.
Skeletal muscle cells are capable of limited mitosis
for repair. The heart is made of cardiac muscle cells,
which, like neurons, seem to be incapable of mitosis. A
heart attack (myocardial infarction) means that a por-
tion of cardiac muscle dies because of lack of oxygen.
These cells are not replaced, and the heart will be a
less effective pump. If a large enough area of the heart
muscle dies, the heart attack may be fatal.
Some research has found evidence for the potential
for mitosis after damage in both the central nervous
system and the heart. Such cell division may be that of
neurons or muscle cells that were stimulated to divide60 Cells