Download Chapter 12: The Cell Cycle

Chapter 12: The Cell Cycle
Cell division functions in reproduction, growth, and repair.
Cell Division - the reproduction of cells
Cell Cycle - the life of a cell from its origin in the division of a parent
cell until its own division into two
Cell division enables unicellular organisms to divide themselves,
forming two separate organisms, and also enables sexually
reproducing organisms to develop from a single cell- the fertilized
egg, or zygote. After an organism is fully grown, cell division
continues to function in renewal and repair.
Cell division distributes identical sets of chromosomes to
daughter cells
• A cell’s endowment of DNA is called its genome. Before a cell can
divide, all of this DNA must be copied and then separated so that
each daughter cell ends up with a complete genome.
• During the replication and distribution of DNA, the DNA molecules
are packed into chromosomes. Each eukaryotic species has a
characteristic number of chromosomes in each nucleus.
• Somatic cells – all body cells except the reproductive cells
• Gametes – sperm and egg cells
Chromosomes are duplicated and distributed in
Mitosis, which is the division of the cell’s nucleus
• In each eukaryotic chromosome there is a long, linear DNA
molecule representing hundreds of thousands of genes. The DNA is
associated with various proteins that maintain the structure of the
chromosome and help control the activity of genes. This DNAprotein complex, called chromatin, condenses after a cell
duplicates its DNA in preparation for division. This allows us to see
the chromosomes with a light microscope.
The Mitotic Cell Cycle
Mitosis is just one part of the cell cycle. The mitotic (M) phase is
usually the shortest part of the cell cycle. Mitotic cell division
alternates with the much longer interphase, where the cell grows
and copies its chromosomes.
Interphase can be divided into
subphases:
-The G1 phase
-The S phase (DNA synthesis)
-The G2 phase
During all three subphases,
the cell grows by producing
proteins and cytoplasmic
organelles.
Mitosis is broken down into five subphases.
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase and
Cytokinesis
Prophase: The chromatin fibers
become more tightly coiled,
condensing into discrete
chromosomes. The nucleoli
disappear, and each duplicated
chromosome appears as two
identical sister chromatids joined
together. In the cytoplasm the mitotic
spindle begins to form, which is
made of microtubules extending from
the two centrosomes.
Prometaphase: The nuclear
envelope fragments. The
chromosomes have become more
condensed, and the microtubules
extend from each pole toward the
middle of the cell. Each of the two
chromatids of a chromosome has
a specialized structure called a
kinetochore, located at the
centromere region. Microtubules
attach to the kinetochores.
Metaphase: The centrosomes
are now at opposite poles of the
cell. The chromosomes line up
on the metaphase plate with
their centromeres on the plate.
The entire apparatus of
microtubules is called the
spindle.
Anaphase: The paired
centromeres of each chromosome
separate, separating the sister
chromatids from each other. Each
chromosome begins moving to
the opposite pole of the cell as the
kinetochore microtubules shorten.
The poles of the cell move farther
apart as the nonkinetochore
microtubules lengthen.
Telophase and Cytokinesis:
At telophase, the daughter
nuclei form at the two poles of
the cell. Nuclear envelopes
arise from the fragments of the
parent cell’s nuclear envelope
and other portions of the
endomembrane system. The
chromatin fiber of each
chromosome becomes less
tightly coiled. Cytokinesis, the
division of the cytoplasm, is
usually well underway by this
time. In animal cells,
cytokinesis involves the
formation of a cleavage furrow,
which pinches the cell in two.
Cell
Plate
Mitosis in eukaryotes may have evolved from
binary fission in bacteria
• Prokaryotes reproduce by binary fission, literally
meaning “division in half.”
Regulation of the Cell Cycle
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The sequential events of the cell cycle are directed by the cell cycle
control system, a cyclically operating set of molecules in the cell that both
triggers and coordinates key events in the cell cycle.
A checkpoint in the cell cycle is a critical control point where stop and goahead signals can regulate the cycle. Animal cells have built in stop signals
that halt the cell cycle until overridden by go-ahead signals. The signals
report whether crucial cellular processes up to that point have been
completed correctly and whether or not the cell should proceed. Three
major checkpoints are found in the G1, G2, and M phases.
If a cell does not receive a go ahead-signal, it will exit the cycle, switching
into a nondividing state called the G0 phase.
Cyclins and Cyclin-Dependent Kinases
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Flunctuations in the abundance and activity of cell cycle control molecules
pace the events of the cell cycle. Some of these molecules are protein
kinases, enzymes that activate or inactivate other proteins by
phosphorylating them. Protein kinases give the go-ahead signals at the G1
and G2 checkpoints.
The kinases that drive the cell cycle are always present, but are in the
inactive form much of the time. To be active, a kinase has to be attached to
a cyclin, which is a protein. These kinases are called cyclin-dependent
kinases, or Cdks.
The activity of a Cdk rises and falls with changes in the concentration of its
cyclin partner.
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The first cyclin-Cdk complex discovered is called MPF, or “maturationpromoting factor”
The peaks of MPF activity correspond to the peaks of cyclin concentration.
The cyclin level rises during interphase, then drops during mitosis.
MPF triggers the cell’s passage past the G2 checkpoint into M phase. It
causes the nuclear envelope to fragment by phosphorylating proteins of the
nuclear lamina.
Later in the M phase, MPF switches itself off by starting a process that
leads to the destruction of its cyclin.
The non-cyclin part, Cdk, still exists in the cell, but is inactive until it
associates with new cyclin molecules synthesized during interphase of the
next round of the cycle.
Internal Signals: Messages from the Kinetochores
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The M phase checkpoint will not allow anaphase to start unless all of the
chromosomes are properly attached to the spindle at the metaphase plate.
This ensures that the daughter cells do not end up with a missing or extra
chromosome.
A signal that delays anaphase originates at kinetochores that are not yet
attached to spindle microtubules. These proteins trigger a signaling pathway
that keeps an anaphase-promoting complex (APC) in an inactive state.
When all the kinetochores are attached to the spindle, the “wait” signal
stops. The APC then becomes active and triggers the breakdown of cyclin
and the inactivation of proteins holding the sister chromatids together,
allowing then to separate.
External Signals: Growth Factors
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There are many external factors,
both chemical and physical, that
can influence cell division.
A growth factor is a protein
released by certain body cells that
stimulates other cells to divide.
Ex: platelet-derived growth factor
(PDGF), which is made by blood
cells called platelets. Fibroblasts,
a type of connective cell tissue,
have PGDF receptors on their
plasma membranes. PGDF
molecules bind to these receptors,
which leads to stimulation of cell
division.
When an injury occurs, platelets
release PGDF, which causes
fibroblasts to grow, healing the
wound.
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Density-dependent inhibition
describes when crowded cells
stop dividing. When a cell
population reaches a certain
density, the amount of required
growth factors and nutrients
available to each cell becomes
insufficient to allow continued
cell growth.
Most animal cells also exhibit
anchorage dependence. In
order to divide, they must be
attached to something, such as
the inside of a culture jar or the
extracellular matrix of a tissue.
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Cancer cells do not exhibit density-dependent
inhibition or anchorage dependence.
They divide excessively and invade other
tissues
Cancer cells can go on dividing indefinitely if
they are given a continual supply of nutrients
Nearly all normal mammalian cells divide only
about 20 to 50 times before they stop dividing.
Cancer starts in the body when a single cell
undergoes transformation, the process that
converts a normal cell to a cancer cell. The
body’s immune system normally recognizes the
cell and destroys it, but sometimes it evades
destruction.
The cell can then divide to form a tumor, a
mass of abnormal cells. It the cells remain at
the original site, it is called a benign tumor,
which mostly does not cause serious problems.
If the tumor becomes invasive enough to impair
the functions of one or more organs, it is called
a malignant tumor.
The spread of cancer cells to locations distant
from the original site is called metastasis.