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Chapter 12
The Cell Cycle
The Genome
• All of the genetic material contained
within the DNA of an organism.
• Most prokaryotes contain a single
strand of circular DNA
• Most Eukaryotes have multiple linear
strands of DNA.
Chromosomes
• Are the structures that make DNA
replication and distribution manageable.
• The name chromosome comes from
their ability to take up dye when being
prepared for microscopy.
• All eukaryotes have a characteristic
number of c-somes contained within the
nucleus of the cell.
Gametes
• The reproductive cells of organisms are
called gametes and they contain 1/2 the
number of c-somes normally found in
somatic cells.
Chromatin and Eukaryotic
Cells
• The chromosomes
of eukaryotic cells
are made up of
chromatin.
• Chromatin is a
composed of DNA
and associated
proteins.
Chromatin and Eukaryotic
Cells
• The DNA found on each c-some
contains a few hundred to a few
thousand genes specifying an
organisms traits.
• The associated proteins help to
maintain the structure of the c-some
and control the activity of the genes.
Non-Dividing Cells
• During this time, the chromatin of each
c-some is in a long thin configuration
distributed throughout the cell.
• The DNA is duplicated in preparation for
cell division and when finished, the
chromatin begins to condense.
Dividing Cells
• Once the c-some has been duplicated,
there are now 2 sister chromatids which
contain identical DNA molecules and are
attached by proteins along their
lengths.
• The region where the sister chromatids
appear to be pinched together is called
the centromere.
Mitosis
• During mitosis, the sister chromatids
separate, the nucleus divides, and
cytokinesis separates the cytoplasm of
the cell.
• Each new cell now contains one copy of
DNA from the parent cell and the cycle
repeats.
Meiosis
• In a completely different form of cell division,
meiosis yields cells that are not identical and
contain only one set of c-somes.
• Occurs in the testes and ovaries and results
in a sperm or an egg.
• During fertilization, the egg and the sperm
fuse and restore the normal number of csomes of the organism.
Meiosis in Humans
• Meiosis reduces the number of c-somes
from 46 to 23. 22 autosomes and 1 sex
c-some.
• Fertilization then restores the c-some
number back to 46. 44 autosomes and
2 sex c-somes.
2 Main Phases of the Cell
Cycle
• Interphase which is
broken into:
• G1
• S-Phase
• G2
• Mitosis (M-Phase)
which is broken into:
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis
Interphase
• Interphase is the longest part of the cell
cycle and is broken into:
• G1 phase is which is also called the 1st
gap phase or 1st growth phase
• S phase is where DNA is synthesized
and the cell continues to grow.
• G2 phase is more growth and
preparation for cell division.
Mitosis-Prophase
• Prophase is when
the chromatin
becomes tightly
coiled.
• Prometaphase is
when each c-some
has a distinct
centromere.
Prophase
• Prophase
Prometaphase
• Prometaphase
Mitosis-Metaphase
• Metaphase is when
the c-somes align at
the metaphase
plate.
Metaphase
• Metaphase
Mitosis-Anaphase
• Anaphase occurs
when sister
chromatids begin
to move apart.
Anaphase
• As the proteins which bind the sister
chromatids together become
inactivated, anaphase begins as the
sister chromatids are separated and
begin to move to opposite ends of the
cell.
• Motor proteins walk the chromatids to
the poles of the cell.
Anaphase
• Anaphase
Mitosis-Telophase
• Telophase occurs
when the 2 daugher
nuclei being to form.
Telophase
• Telophase
Mitosis-Cytokinesis
• Cytokinesis is when
a cleavage furrow
forms which pinches
the cell into two new
daughter cells.
Cytokinesis
• Cytokinesis
Interphase and Centrosomes
• The centrosomes are nonmembranous
organelles which organize the
microtubules.
• Sometimes the centrosomes are called
the microtubule organizing center.
• During interphase they duplicate and
move to opposite ends of the cell.
Prophase and Centrosomes
• As the centrosomes are moving to opposite
ends of the cell (prophase and prometaphase
are now occurring), microtubules grow out
from them and attach to the kinetochore on
each sister chromatid.
• After attachment, each sister chromatid
begins to move toward the pole from which
the microtubule extends.
Centrosomes and Prophase
• The actual movement of the chromatids
is prevented because of the binding of
the microtubule from the opposite end
of the cell.
• A “tug-of-war” now takes place until the
c-somes are aligned at the metaphase
plate.
Aster
• As all of this is happening, a radial array
of short microtubules (called the aster)
is extending outward from each
centromere (moving in the opposite
direction) and attaches to the plasma
membrane.
Mitotic Spindle
• The mitotic spindle is an important part
mitosis because is the organized array
of microtubules that moves the
chromosomes during cell division.
Mitotic Spindle
• In addition to the asters sending
microtubules outward, the microtubules
which have grown and not attached to
the kinetochores interact with other
microtubules (from centrioles at the
opposite end of the cell) that haven’t
attached to the kinetochore forming the
mitotic spindle.
Cytokinesis in Animals
• During cytokinesis, a cleavage furrow forms
and cleaves the cell into 2 new cells.
• The cleavage furrow forms as a contractile
ring of actin microfilaments interacts with
myosin molecules on the cytoplasmic side of
the plasma membrane.
• When this interaction occurs, the ring
contracts pinching the cell in two.
Cytokinesis in Plants
• In plants, cytokinesis is much different. No
cleavage furrow forms. Instead, vesicles
which come from the Golgi migrate along
microtubules to the center of the cell after
division of the cytoplasmic contents.
• The vesicles which collect at the center of the
cell form a cell plate which eventually
becomes a cell wall and two new cells are
formed.
Bacterial Cell Division
• Prokaryotic cells divide by a process
called binary fission.
Stages of the Cell Cycle
• The various stages of the cell cycle are
determined by a variety of cytoplasmic
signals.
• Evidence comes form experiments performed
in the early 1970’s.
• When this happens, a cell replicates its DNA,
the cell elongates and the plasma membrane
grows inward dividing the cell in 2.
Cytoplasmic Signals--Evidence
• Scientists grew cells in culture and
harvested them at different stages of
the cell cycle.
• They then fused them together and
found a variety of interesting things.
Results of Cell Fusion
Experiments
• When cells in the S phase were fused
with those in the G1 phase, the G1
nuclei immediately entered the S phase.
• Likewise, when M-phase cells were
fused with cells in any other phase, the
second cells immediately entered the
M-phase.
What This Means…
• All of the experiments lent support to
the notion that cytoplasmic signals
caused the various stages of the cell
cycle and cell division.
• They are sometimes called a “Cell Cycle
Control System.”
Cell Cycle Control System
• This system cyclically operates and
controls the key events in the cell cycle.
• Each of the phases of the cell cycle
seem to be controlled by a checkpoint
which is where critical stop-and-go
signals regulate the cell cycle.
Cell Cycle Control System
• These signals shut down certain cell
cycles and start them when key cellular
processes have been completed.
• The 3 major checkpoints are G1, G2 and
M.
G1
• For many cells, G1 is important because
cells receive a “go” signal here and will
usually go through S, G2 and M.
• If no go signal is received, the cell
enters G0 which is a phase of nondivision.
G0
• Most cells in the human body are in the
G0 phase and never divide (nerve and
muscle cells).
• Others are in G0, but can be stimulated
to divide when needed (liver).
Regulatory Proteins
• Cyclins and Kinases are the two main
types of regulatory molecules (proteins)
found in cells.
Kinases
• Kinases are enzymes that function by
either activating or inactivating other
proteins by phosphorylation.
• Certain proteins give “go” signals at the
G1 and G2 checkpoints.
Kinases
• Kinases are usually present in constant
concentration within the cell but are
inactive.
• They become active when attached to a
cyclin protein.
• These kinases are called CdKs for cyclin
dependent kinases.
Cyclin-Kinase Interactions
• When cyclins and kinases interact, they
initiate a variety of changes which result
in the division of the cell.
Cancer Cells
• Cancer cells don’t respond normally to
the body’s control mechanisms.
• They don’t heed the normal signals to
regulate the cell cycle.
• Others don’t need growth factors in
their medium to grow and divide, some
may even make their own.
Cancer Cells
• Cancer cell also fails to exhibit what is
known as density dependent inhibition
which is where an external physical
factor regarding cell division stops the
division of crowded cells.
Cancer Cells
• Normal cells usually divide 20-50x in
the process of aging and then die.
• Cancer cells are seemingly immortal
because they will continue to divide
forever if they are given a continuous
supply of nutrients.
He-La Cells
• Henrietta Lacks was a patient from
whom some cancer cells were taken
and cultured in 1951. These cells have
been continually grown in culture since
then.
• The total number of cell divisions of
these cells have far exceeded the
normal 20-50 cell cycles.