Download Part I: The Cell Cycle

Cellular Differentiation
Cell Structure and Function
Part I: The Cell Cycle
During your lifetime, trillions of your cells will undergo the cell cycle. This process allows you to
grow, heal, and maintain your vital tissues and organs. In order for organisms to grow and thrive,
certain cells must constantly undergo mitosis, such as the skin and stomach cells in animals. The
process of creating new cells is part of the cell cycle, or a series of events that a eukaryotic cells
undergoes in order to divide and replicate. The cell cycle may renew other somatic cells, or nonreproductive cells, in the body. The entire cell cycle can be divided up into four different phases:
Gap 1 (G1), Synthesis (S), Gap 2 (G2), and the Mitotic Phase (M). The M Phase includes both
mitosis and cytokinesis.
Interphase
Interphase is the first stage of the cell cycle, and
the longest phase. The cell undergoes normal
functioning and growth during this phase. DNA is
replicated in preparation for the cell to divide.
Interphase is broken into three phases: G1 Phase,
Synthesis (S) Phase, and G2 Phase.
G1 Phase of Interphase
Here the cell is in a phase where it grows in size,
but is not duplicating any genetic material.
S Phase of Interphase
During this phase the genetic material, or DNA, of
the cell is duplicated. In this phase, the DNA is in
the form of chromosomes.
G2 Phase of Interphase
This is the phase where the cell prepares for
division. Centrosomes are formed during this
phase. These structures aid in later phases of
cellular division.
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Cellular Differentiation
Cell Structure and Function
Part I: The Cell Cycle, continued
Mitosis
After a cell has moved through all three phases of interphase, it is ready to begin the process of
mitosis, or the process of nuclear division to form two distinct nuclei. This process occurs in the
following four steps:
Prophase
During the first phase of mitosis, the loosely
bundled DNA begins to condense by coiling
tightly into more organized sister chromatids,
which are joined by a protein bundle called a
centromere. During prophase, the chromosomes
are visible under a light microscope. In late
prophase, the nuclear membrane breaks into
fragments and disappears, freeing the genetic
contents of the nucleus. In the cytoplasm,
microtubules grow from the centrosomes. The
cell’s two centrosomes begin moving toward the
opposite ends of the cell.
Metaphase
During metaphase, the centromeres of each
chromosome line up in the center of the cell,
known as the metaphase plate. The
centrosomes are at opposite poles and are
connected to the centromeres by spindle fibers
that are made up of microtubules and proteins.
Some microtubules connect opposite
centrosomes and are used to create a pushing
force during cell division.
Chromatids
Centromeres
Centrosomes
Metaphase Plate
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Cellular Differentiation
Cell Structure and Function
Part I: The Cell Cycle, continued
Anaphase
This third phase of mitosis begins when the
two centromeres of each chromosome part
ways and separate the sister chromatids.
Each chromatid is now considered a complete
chromosome. Microtubules connected to the
centromeres shorten, pulling the
chromosomes toward the opposite poles of
the cell. Microtubules not connected to
centromeres elongate, pushing the poles
further apart. When anaphase ends, there is a
complete set of chromosomes at each pole of
the cell.
Telophase/Cytokinesis
During telophase, the cell begins to elongate,
and a membrane forms around the two sets of
chromosomes. The chromosomes begin to
uncoil and nucleoli reappear in each nucleus.
Cytokinesis, or the division of cytoplasm,
begins during telophase. In animal cells, a
cleavage furrow forms where the metaphase
plate used to be. This cleavage furrow
pinches together until two new daughter cells
are formed with identical copies of DNA. In
plant cells, a new cell wall grows at the site of
the metaphase plate until the two daughter
cells are separated. When cytokinesis is
complete, the two new daughter cells begin
their own cell cycles.
Complete Part I of your Student Journal.
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Cellular Differentiation
Cell Structure and Function
Part II: Cell Cycle Regulation
There are several factors responsible for regulating the cell cycle. DNA contains many different
genes that produce specific proteins. Some proteins called cyclins control different phases of the
cell cycle. Cyclins, in turn, are regulated by another class of proteins known as growth factors.
Some growth factors trigger cyclins that cause cells to divide, while others stop cell growth by
blocking the action of cyclins. Cells also have specialized proteins that check the DNA for
sequence errors during replication. Some errors, or mutations, can be repaired. If too many
mutations are identified, cells don’t progress through the cell cycle.
Cell growth and division can also be affected by contact inhibition. When a cell is in close contact
with other cells, proteins are produced that cause cells to remain in the G1 phase. If you sustain a
cut or scrape on the skin, proteins are produced promoting cell growth. Cells in the damaged area
will begin to divide until the wound is healed, when contact inhibition again triggers cells to remain
in the G1 phase. In fact, there is something called the G1 checkpoint, or the restriction point. If the
cell is not properly signaled to divide at the G1 checkpoint, the cell will move into a G0 phase where
it does not divide at all. Most somatic cells in animals exist in the G0 phase, as once developed
they won’t (usually) divide again.
The cell cycle also plays a role in what is known as cellular differentiation. As you already know,
your body is a collection of many different cell types. In animals, stem cells are unspecialized cells
that are constantly reproducing, especially during the growth and development phases of the
animal. These cell types have the ability to differentiate in many different types of cells based on
both internal and external signals.
Complete Part II of your Student Journal.
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Cellular Differentiation
Cell Structure and Function
Part III: Specialized Cells
As animals grow and develop, the stem cells differentiate into more and more specific and
specialized cell types. These specialized cells may be found only in certain areas of the body, such
as in epithelial or bone cells, or may be found throughout the body, such as blood cells in the
circulatory system. Some of the very specific types of cells found in animals include blood cells,
muscle cells, epithelium cells, bone cells, and nerve cells.
Blood cells are specialized cells that are suspended within
plasma in the blood of animals. These types of cells may
carry oxygen throughout the body or fight infections. There
are two types of blood cells. White blood cells are called
leukocytes and they fight infection. Red blood cells are
called erythrocytes and carry oxygen through the body.
Epithelial cells make up the epithelial tissues of the body,
and there are many different types. Epithelial cells include
skin cells, as well as the lining of many organs and organ
systems, such as the lining of the lungs or the digestive
tracts.
Muscle cells may fall into three categories. Smooth muscle
cells are spindle shaped and are found in organs. Skeletal
muscle is made of long fibers that are bundled into
structures called myofibrils. Cardiac muscles are found in the
heart and are connected to synchronize a heartbeat.
There are a variety of connective tissues that each have
their own cell type. These tissues include bone cells,
cartilage, connective tissues, and fat cells.
Nerve cells, or neurons, are a major component of the
nervous tissues. These types of cells transmit electrical
signals between any part of the body and the brain. Many
neurons are connected to either muscle or connective
tissues.
Complete Part III of your Student Journal.
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Cellular Differentiation
Cell Structure and Function
Part IV: DNA, RNA, and Differentiation
So what controls how cells know to differentiate into a wide variety of specialized cells? It has to do
with the genetic material contained within each cell. Remember that during interphase in somatic
cells, the cells are growing and the DNA is being duplicated. Then, at some point there is a signal
to start the replication process, usually at the G1 checkpoint. It is at this point that the genetic
material, or DNA, of each cell is triggered to do work.
Every single cell in your body contains the exact same copy of DNA as every other cell. That DNA
contains individual sections called genes. There are genes that will code for specific proteins. It is
these genes found on the DNA strands that will determine whether a cell becomes a skin cell or a
nerve cell. So how exactly does this occur?
All of the cells within an organism are constantly sending signals to each other. It is these specific
signals that turn on certain genes along the DNA strand. As the DNA is replicated, or transcribed, it
is read by another form of genetic material known as RNA. The RNA will, in turn, synthesize the
specific proteins coded for on the DNA strand. It is these proteins that determine what type of cell it
will become.
In other words, each gene found on the DNA codes for its own protein. As the RNA reads each
gene, the information contained in the DNA segment is then translated by the RNA. The RNA then
uses this translation to build a protein. For example, cells in the interior of the body will be signaled
by genes to become either muscle or connective tissues, while other cells on the exterior of the
body will be signaled to become epithelial cells. This process is known as gene expression. As you
can imagine, any errors, or mutations, to the gene may cause errors in the translation process.
These errors may lead to a variety of disorders and diseases.
GENE EXPRESSION
DNA (gene is signaled) à RNA à Protein Synthesis à Muscle Cell
Complete Part IV of your Student Journal.
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Cellular Differentiation
Cell Structure and Function
Part V: Disruptions in the Cell Cycle
As you can imagine, if regulatory checks or signals fail, cells can grow and divide without restraint.
For example, if the “go” signal is not given at the G1 checkpoint, but the cell begins to divide
anyway, this can lead to the rapid an unregulated growth of somatic cells that might otherwise
remain in the G0 phase.
The result can be a mass of cells that known as a tumor. Some tumors may show unregulated
growth, but they do not spread to other parts of the body. These types of tumor are known as
benign tumors, and once removed do not return. Other tumors may continue to grow and invade
other tissues, or may break away and spread to other parts of the body. These are known as
malignant tumors, and are associated with cancer.
Cancer refers to a specific class of disease caused by unregulated cell growth. Cancer cells do not
respond to normal signals to stop growth and reproduction. Sometimes a mutation in a gene can
be responsible for this type of disruption to the cell cycle. Mutated genes that cause cancer are
known as oncogenes. The process by which normal cells are transformed into cancer cells is
known as carcinogenesis, or oncogenesis.
This transformation affects gene expression in that the genes responsible for normal cell division
are mutated. However, it is important to note that more than one gene must mutate in order for
carcinogensis to occur. Several related genes must undergo progressive mutations in order for a
normal cell to transform into a cancer cell. The gene mutations that lead to cancer may be caused
by many sources. Overexposure to UV rays from the sun may disrupt the cell cycle. Other
environmental factors that may cause cancers by disrupting the cell cycle include carcinogens
taken in by smoking, chemical exposures, and overexposure to X-rays. There are also certain
viruses that have been linked to cancer causing tumors.
Complete Part V of your Student Journal and then complete the Reflections and Conclusions.
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