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Cell Division and Cancer
Perhaps you already know that you have one chance in three of getting cancer; one chance in five of dying of it.
Perhaps you have, or have had, friends or family members with cancer.
Each year, cancer kills almost a half-million Americans; it is second only to heart disease as a cause of death in the
United States.
In its most fundamental sense, cancer is a family of diseases, primarily of the old and the very young, in which
cells divide, move around the body, and secrete things as if the rest of the organism had no control over them.
Let's look at how cells reproduce. The process is known as cell division where the cellular contents are divided
between two new daughter cells.
An individual grows by taking in raw materials from the environment and uses these raw materials to synthesize
(produce) new structural and functional molecules. When the cell reaches a certain critical size and metabolic
state, it divides.
The new cells are structurally and functionally similar to each other. I say similar because the two daughter cells
receive about half rather than exactly half of their parent's cytosol and organelles.
Much more important, however, is that each daughter cells inherits an exact replica (copy) of the heredity
information or genome of the parent cell.
In prokaryotes, distributing exact replicas of hereditary information is relatively simple. Prokaryotes have their
hereditary material present as a single, long, circular strand of DNA with their associated proteins. This molecule,
the prokaryote's chromosome, is replicated just prior to division.
Each of the two daughter chromosomes attaches to the parent plasma membrane and as the membrane elongates,
the chromosomes move apart. After the chromosomes are well separated, the cell pinches apart to become two new
cells.
In eukaryotes, the problem of distributing the hereditary material equally is much more complex. A typical
eukaryote cell contains perhaps 1,000 times as much DNA as a prokaryote cell and this DNA is linear, forming a
number of distinct chromosomes. Humans, for instance, have 46 chromosomes.
When human cells divide, each daughter cell has to receive one copy-- and only one copy--of each chromosome.
The solution is elaborate. In a series of steps called mitosis, each daughter cell is provided with complete set of
chromosomes.
Mitosis is usually followed by cytokinesis or cytoplasmic division, a process that divides the parent cell into two
daughter cells.
Dividing eukaryote cells pass through a regular, repeated sequence of cell growth and division known as the cell
cycle. The cycle consists of five major phases: G1, S, G2(interphase), mitosis, cytokinesis
Completion of the cycle takes from a few hours to several days depending on environmental conditions. Some few
cells are permanently arrested and never divide.
Before a cell can begin mitosis and actually divide, it must replicate its DNA, synthesize some associated proteins,
produce a supply of organelles sufficient for two daughter cells and assemble the machinery necessary for mitosis
and cytokinesis. These things occur during the G1, S, and G2 phases of the cell cycle and are known collectively
as interphase.
The extremely important process of DNA replication occurs during the S phase (synthesis phase) of the cell cycle.
Here also, many of the DNA-associated proteins are synthesized. G phases (gap phases) precede and follow the S
phase.
Some cell types pass through successive cell cycles throughout the life of the organism. Other cell types
occasionally divide; still others are permanently arrested and never divide.
Division Rate
Cells that do not divide after tissue is differentiated
Cell Type
Nerve cells
Muscle cells
Cells that do not normally divide but can be
stimulated to do so
Liver cells
Cells that divide constantly and rapidly
Skin cells
Epithelial cells
Sperm cells
Bone marrow cells
The stem cells in human blood marrow are a good example of cells that divide constantly and rapidly. The average
red blood cell lives only about 120 days. There are about 2.5 trillion of them in an adult body. To maintain this
number, about 2.5 million new red blood cells must be produced each second by the divison of stem cells. Mature
red blood cells have no capacity to divide.
Cells in the human liver do not normally divide in the adult. If, however, a portion of the liver is removed
surgically, the remaining cells divide until the liver returns to its old size.
Consider this-- all told, about 2 trillion cell divisions occur in an adult human every 24 hours; about 25 million a
second!
It is obviously of critical importance that various cell types divide at only a sufficient rate to produce the needed
cells for growth and replacement. If any particular cell type divides more rapidly than is necessary, the normal
organization and functions of the organism will be disrupted as specialized tissues are invaded and interfered with
by the rapidly dividing cells. This is the course of events in a cancer.
In simple terms, cancer is a group of more than 100 diseases that develop across time and involve the uncontrolled
division of the body's cells. Although cancer can develop in virtually any of the body's tissues, and each type of
cancer has its unique features, the basic processes that produce cancer are quite similar in all forms of the disease.
Cancer begins when a cell breaks free from the normal restraints on cell division and begins to follow its own
agenda for proliferation (Figure 3). All of the cells produced by division of this first, ancestral cell and its progeny
also display inappropriate proliferation. A tumor, or mass of cells, formed of these abnormal cells may remain
within the tissue in which it originated (a condition called in situ cancer), or it may begin to invade nearby tissues
(a condition called invasive cancer). An invasive tumor is said to be malignant, and cells shed into the blood or
lymph from a malignant tumor are likely to establish new tumors (metastases) throughout the body. Tumors
threaten an individual's life when their growth disrupts the tissues and organs needed for survival.
Cell Division and Cancer
Directions: Read and this article; highlight key points, and be ready to discuss it in class.
Perhaps you already know that you have one chance in three of getting cancer; one chance
in five of dying of it. Perhaps you have, or have had, friends or family members with
cancer.
Each year, cancer kills almost a half-million Americans; it is second only to heart disease
as a cause of death in the United States.
In its most fundamental sense, cancer is a family of diseases, primarily of the old and the
very young, in which cells divide, move around the body, and secrete things as if the rest
of the organism had no control over them.
Let's look at how cells reproduce. The process is known as cell division where the cellular
contents are divided between two new daughter cells.
An individual grows by taking in raw materials from the environment and uses these raw
materials to synthesize (produce) new structural and functional molecules. When the cell
reaches a certain critical size and metabolic state, it divides.
The new cells are structurally and functionally similar to each other. I say similar because
the two daughter cells receive about half rather than exactly half of their parent's
cytosol and organelles.
Much more important, however, is that each daughter cells inherits an exact replica (copy)
of the heredity information or genome of the parent cell.
In eukaryotes, the problem of distributing the hereditary material equally is much more
complex. A typical eukaryote cell contains perhaps 1,000 times as much DNA as a
prokaryote cell and this DNA is linear, forming a number of distinct chromosomes.
Humans, for instance, have 46 chromosomes.
When human cells divide, each daughter cell has to receive one copy-- and only one copy-of each chromosome.
The solution is elaborate. In a series of steps called mitosis, each daughter cell is
provided with complete set of chromosomes.
Mitosis is usually followed by cytokinesis or cytoplasmic division, a process that divides
the parent cell into two daughter cells.
Dividing eukaryote cells pass through a regular, repeated sequence of cell growth and
division known as the cell cycle.
Completion of the cycle takes from a few hours to several days depending on
environmental conditions. Some few cells are permanently arrested and never divide.
Before a cell can begin mitosis and actually divide, it must replicate its DNA, synthesize
some associated proteins, produce a supply of organelles sufficient for two daughter cells
and assemble the machinery necessary for mitosis and cytokinesis. These things occur
during the G1, S, and G2 phases of the cell cycle and are known collectively as interphase.
The extremely important process of DNA replication occurs during the S phase (synthesis
phase) of the cell cycle. Here also, many of the DNA-associated proteins are synthesized.
G phases (gap phases) precede and follow the S phase.
Some cell types pass through multiple cell cycles throughout the life of the organism.
Other cell types occasionally divide; still others are permanently arrested and never
divide.
The stem cells in human blood marrow are a good example of cells that divide constantly
and rapidly. The average red blood cell lives only about 120 days. There are about 2.5
trillion of them in an adult body. To maintain this number, about 2.5 million new red blood
cells must be produced each second by the divison of stem cells. Mature red blood cells
have no capacity to divide.
Cells in the human liver do not normally divide in the adult. If, however, a portion of the
liver is removed surgically, the remaining cells divide until the liver returns to its old size.
Consider this-- all told, about 2 trillion cell divisions occur in an adult human every 24
hours; about 25 million a second!
It is obviously of critical importance that various cell types divide at only a sufficient rate
to produce the needed cells for growth and replacement. If any particular cell type
divides more rapidly than is necessary, the normal organization and functions of the
organism will be disrupted as specialized tissues are invaded and interfered with by the
rapidly dividing cells. This is the course of events in a cancer.