Cell Cycle
Honors Genetics
2005
Cell cycle and Mitosis Tutorial
http://www.biology.arizona.edu/cell_bio
/tutorials/cell_cycle/main.html
http://www.cellsalive.com/cell_cycle.ht
m
http://nobelprize.org/medicine/educati
onal/2001/
Play these three animations to help you
learn about the cell cycle and control
Cell Cycle
G1- Highest metabolic activity.
Cytoplasmic contents increase. Protein
synthesis, membrane synthesis
S – replication of DNA. Chromosomes
copied( sister chromatids form)
G2- Preparation for cell division –
Spindle fiber formation
M – Cell division
Cytokinesis – division of the cytoplasm
Chromosome overview
Chromsome structure
Metaphase Structure
Go
Many times a cell will leave the cell cycle, temporarily
or permanently. It exits the cycle at G1 and enters a
stage designated G0 (G zero). A G0 cell is often
called "quiescent", but that is probably more a
reflection of the interests of the scientists studying
the cell cycle than the cell itself. Many G0 cells are
anything but quiescent. They are busy carrying out
their functions in the organism. e.g., secretion,
attacking pathogens.
Often G0 cells are terminally differentiated: they
will never reenter the cell cycle but instead will carry
out their function in the organism until they die.
Cyclins and cell cycle regulation
The passage of a cell
through the cell
cycle is controlled
by proteins in the
cytoplasm.
Among the main
players in animal
cells are:
Cyclins
a G1 cyclin (cyclin D)
S-phase cyclins
(cyclins E and A)
mitotic cyclins
(cyclins B and A)
Cyclind dependent kinases
Cyclin-dependent
kinases (Cdks)
a G1 Cdk (Cdk4)
an S-phase Cdk
((Cdk2)
an M-phase Cdk
(Cdk1)
Their levels depend
upon the stage of
the cell cycle
They add phosphates
to cyclins during the
phases of the cell
cycle
Cyclins - events
A rising level of G1-cyclins bind to their Cdks
and signal the cell to prepare the
chromosomes for replication.
A rising level of S-phase promoting factor
(SPF) — which includes cyclin A bound to Cdk2
— enters the nucleus and prepares the cell to
duplicate its DNA (and its centrosomes).
As DNA replication continues, cyclin E is
destroyed, and the level of mitotic cyclins
begins to rise (in G2).
Checkpoints
DNA damage checkpoints. These sense
DNA damage
Before the cell enters S phase (a G1
checkpoint);
During S phase, and
After DNA replication (a G2
checkpoint).
Oncogenes
All the checkpoints examined require
the services of a complex of proteins.
Mutations in the genes encoding some of
these have been associated with cancer;
that is, they are oncogenes. This should
not be surprising since checkpoint
failures allow the cell to continue
dividing despite damage to its integrity.
Cyclins and cell cycle regulations
Spindle fibers have three
destinations:
Some attach to one kinetochore of a dyad
with those growing from the opposite
centrosome binding to the other kinetochore
of that dyad.
Some bind to the arms of the chromosomes.
Still others continue growing from the two
centrosomes until they extend between each
other in a region of overlap.
How Spindle Fibers Work
Microtubules
Grow at each end by
the polymerization
of tubulin dimers
(powered by the
hydrolysis of GTP),
and
Shrink at each end
by the release of
tubulin dimers
(depolymerization)
All types of spindle fibers
participate in
the assembly of the chromosomes at the metaphase
plate at metaphase. Proposed mechanism (the diagram
shows only 1 and 2):
Microtubules attached to opposite sides of the dyad
shrink or grow until they are of equal length.
Microtubules motors attached to the kinetochores
move them
– toward the minus end of shrinking microtubules (a dynein);
– toward the plus end of lengthening microtubules (a kinesin).
The chromosome arms use a different kinesin to move
to the metaphase plate.
Microtubule Motors
There are two major groups of
microtubule motors:
kinesins (most of these move toward
the plus end of the microtubules) and
dyneins (which move toward the minus
end).
Kinesins and dyneins
The sister kinetochores separate and,
carrying their attached chromatid,
move along the microtubules powered by
minus-end motors, dyneins, while the
microtubules themselves shorten (probably at
both ends).
The overlapping spindle fibers move past each
other (pushing the poles farther apart)
powered by plus-end motors, the "bipolar"
kinesins.
In this way the sister chromatids end up at
opposite poles.
Mitosis/Meiosis promoting factor
M-phase promoting factor (the complex of mitotic
cyclins with the M-phase Cdk) initiates
– assembly of the mitotic spindle
– breakdown of the nuclear envelope
– condensation of the chromosomes
These events take the cell to metaphase of mitosis.
At this point, the M-phase promoting factor activates
the anaphase-promoting complex (APC/C) which
Allows the sister chromatids at the metaphase plate
to separate and move to the poles (= anaphase),
completing mitosis;
Destroys cyclin B. It does this by attaching it to the
protein ubiquitin which targets it for destruction by
proteasomes.
Anaphase promoting complex
The anaphase-promoting complex
(APC). (The APC is also called the
cyclosome, and the complex is often
designated as the APC/C.) The APC/C
Triggers the events leading to
destruction of the cohesins thus
allowing the sister chromatids to
separate;
Degrades the mitotic cyclin B.
Proteosome – Core Particle
The core particle is
made of 2 copies of
each of 14 different
proteins.
These are assembled
in groups of 7
forming a ring.
The 4 rings are
stacked on each
other (like 4 donuts)
Proteosome – Regulatory Particle
There are two identical
RPs, one at each end of
the core particle.
Each is made of 14
different proteins (none
of them the same as
those in the CP).
6 of these are ATPases.
Some of the subunits
have sites that
recognize the small
protein ubiquitin.
Ubiquitin
A small protein (76 amino acids)
Conserved throughout all the kingdoms
of life; that is, virtually identical in
sequence whether in bacteria, yeast, or
mammals.
Used by all these creatures to target
proteins for destruction.
Proteosome and proteins
Are conjugated to a molecule of ubiquitin
which binds to the terminal amino group of a
lysine residue.
Additional molecules of ubiquitin bind to the
first forming a chain.
The complex binds to ubiquitin-recognizing
site(s) on the regulatory particle.
The protein is unfolded by the ATPases using
the energy of ATP
The unfolded protein is translocated into the
central cavity of the core particle.
Proteosomes and Particles(2)
Several active sites on the inner surface of the two
middle "donuts" break various specific peptide bonds
of the chain.
This produces a set of peptides averaging about 8
amino acids long.
These leave the core particle by an unknown route
where
they may be further broken down into individual
amino acids by peptidases in the cytosol or
in mammals, they may be incorporated in a class I
histocompatibility molecule to be presented to the
immune system as a potential antigen [see below].
The regulatory particle releases the ubiquitins for
reuse.
Spindle Checkpoints
Spindle checkpoints. Some of these
that have been discovered
Detect any failure of spindle fibers to
attach to kinetochores and arrest the
cell in metaphase (M checkpoint —
example);
Detect improper alignment of the
spindle itself and block cytokinesis;
Mitosis occurs in somatic cells
http://www.johnkyrk.com/mitosis.html
Check this animation
Mitosis - Prophase
The two centrosomes of the cell, each with
its pair of centrioles, move to opposite "poles"
of the cell.
The mitotic spindle forms. This is an array of
spindle fibers, each containing ~20
microtubules. Microtubules are synthesized
from tubulin monomers in the cytoplasm and
grow out from each centrosome.
The chromosomes become shorter and more
compact.
Mitosis - Prometaphase
The nuclear envelope disintegrates because of the
dissolution of the lamins that stabilize its inner
membrane.
A protein structure, the kinetochore, appears at the
centromere of each chromatid.
With the breakdown of the nuclear envelope, spindle
fibers attach to the kinetochores as well as to the
arms of the chromosomes.
For each dyad, one of the kinetochores is attached to
one pole, the second (or sister) chromatid to the
opposite pole. Failure of a kinetochore to become
attached to a spindle fiber interrupts the process.
Metaphase
At metaphase all the
dyads have reached
an equilibrium
position midway
between the poles
called the
metaphase plate.
The chromosomes
are at their most
compact at this
time.
Anaphase
The sister
kinetochores
suddenly separate
and each moves to
its respective pole
dragging its
attached chromatid
(chromosome)
behind it.
Telophase
The chromosomes
reach the poles
A nuclear envelope
reforms around each
cluster of
These return to
their more extended
form.
Cytokineses
In animal cells, a belt of actin
filaments forms around the
perimeter of the cell, midway
between the poles. The
interaction of actin and a myosin
(not the one found in skeletal
muscle) tightens the belt, and
the cell is pinched into two
daughter cells.
In plant cells, a membranebounded cell plate forms where
the metaphase plate had been.
The cell plate, which is
synthesized by the Golgi
apparatus, supplies the plasma
membrane that will separate the
two daughter cells. Synthesis of
a new cell wall between the
daughter cells also occurs at the
cell plate.
Telomeres and aging
http://runews.rockefeller.edu/index.php
?page=engine&id=138
Telomeres
....TTGGGGTTGGGGTTGGGG
TTGGGGTTGGGGTTGGGGTT
GGGGTT 3'
....AACCCCAACCCCAACCCC
5'
The extra G-T rich
sequences are not present in
telomeres as single stranded
DNA.
G-T rich strands can, in the
test tube, form an unusual
structure in which 4 G's are
paired with one another. A
metal ion (M+) assists the
formation of these G
quartets. They stack on top
of one another. The Tstretches connect the sides
of the quartet.
Intermolecular G quartets
can form.
Telomere repeats
Telomeric facts
At their ends, telomeres can assume special structures, the G
quartets.
G quartets can account for the stability properties of
chromosome ends.
The formation of intermolecular G quartets can account for the
apparent interaction of telomeres (bouqet formation).
The studies using TRF2 suggest that the single strand telomeric
end is not arranged as a doubled-back G-quartet, but rather
invades duplex DNA near the junction of telomeric and nontelomeric DNA.
The analysis of psoralen-treated nuclei suggests that the TRF2facilitated structures probably exist in actual cells. There is no
analogous evidence favoring a biological relevance of G-quartet
structures.