Chapter 12: The Cell Cycle

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Chapter 12 The Cell Cycle
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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.

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Cell division distributes identical sets of
chromosomes to daughter cells

• A cells 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

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Chromosomes are duplicated and distributed in
Mitosis, which is the division of the cells
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 DNA-protein 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.

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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.
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Mitosis is broken down into five subphases.

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase and Cytokinesis

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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.
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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.
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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 cells 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
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Mitosis in eukaryotes may have evolved from
binary fission in bacteria

• Prokaryotes reproduce by binary fission,
  literally meaning division in half.

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Regulation of the Cell Cycle

• 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 go-ahead 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.

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Cyclins and Cyclin-Dependent Kinases

• 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 maturation-promoting 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 cells 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.

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Internal Signals Messages from the Kinetochores

• 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.

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External Signals Growth Factors

• 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
  bodys 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.