Download 6- The Cell Cycle and Its Regulators

7th WEEK
Cell Cycle
Gihan E-H Gawish, MSc, PhD
Ass. Professor
Molecular Genetics and Clinical Biochemistry
KSU
Genetic and molecular studies in diverse biological systems
have resulted in identification and characterization of the
cell cycle machinery

Cell Cycle
 Introduction to cell cycle
 CYCLINS and cdk’s
 CELL CYCLE CHECKPOINTS

Mechanisms of Carcinogenesis
 TUMOR SUPPRESSOR GENES
 ONCOGENES
Mitotic spindle
Chiasmata
DNA replication
Dynamic instabilty
Cell-cycle control
Maturation-promoting factor
Regulation of Cdc2
Cyclin characterization
Checkpoint control
p53
The mitotic checkpoint
The APC and proteolysis
SCF and F-box proteins
Cdc mutants
The restriction point
Yeast centromeres
Cell-cycle conservation
Replication origins
Retinoblastoma/E2F
Body-plan regulation
A new class of cyclins
CDK inhibitors
Sister-chromatid cohesion
Living cells go through a series of
stages known as the cell cycle. The
cells grow, copy their chromosomes,
and then divide to form new cells.
G1 phase. The cell grows.
S phase. The cell makes copies of
its
chromosomes.
Each
chromosome now consists of two
sister chromatids.
G2 phase. The cell checks the
duplicated chromosomes and gets
ready to divide.
M phase. The cell separates the
copied chromosomes to form two full
sets (mitosis) and the cell divides
into two new cells (cytokinesis).
Animation
Mitosis
Mitosis is used to produce daughter cells
that are genetically identical to the parent
cells. The cell copies - or 'replicates' - its
chromosomes, and then splits the copied
chromosomes equally to make sure that
each daughter cell has a full set
Animation
Meioses
Meiosis is used to make special cells - sperm cells
and egg cells - that have half the normal number of
chromosomes. It reduces the number from 23 pairs
of chromosomes to 23 single chromosomes. The
cell copies its chromosomes, but then separates
the 23 pairs to ensure that each daughter cell has
only one copy of each chromosome. A second
division that divides each daughter cell again to
produce four daughter cells.
Mitosis
is a way of making more cells that
are genetically the same as the parent cell.
It plays an important part in the development of
embryos, and it is important for the growth and
development of our bodies as well.
Mitosis produces new cells, and replaces cells
that are old, lost or damaged.
In mitosis a cell divides to form two identical
daughter cells. It is important that the daughter
cells have a copy of every chromosome, so the
process involves copying the chromosomes first
and then carefully separating the copies to give
each new cell a full set.
Before mitosis, the chromosomes are copied.
They then coil up, and each chromosome looks
like a letter X in the nucleus of the cell. The
chromosomes now consist of two sister
chromatids. Mitosis separates these chromatids,
so that each new cell has a copy of every
chromosome
A diagram of a cell ready for mitosis. The
copied chromosomes consist of two
chromatids joined at the centromere
The process of mitosis involves a
number of different stages
Prophase: The replicated chromosomes condense and the mitotic spindle
begins to assemble outside the nucleus.
Prometaphase: The membrane surrounding the nucleus (nuclear
envelope) breaks down and allows the mitotic spindle to contact the
chromosomes.
Metaphase: All the chromosomes are gathered at the center of the mitotic
spindle
Anaphase: The chromosomes are split apart and pulled to opposite sides
of the cell
Telophase: The nuclear envelope reassembles around the two new sets of
separated chromosomes to form two nuclei.
The
second feature of M phase: It is the time in
which the other components of the cell— membranes,
cytoskeleton, organelles, and soluble proteins—are
distributed to the two daughter cells.
This is the final task that a cell must complete to
finish its reproduction

The cells in our bodies contain 23 pairs of chromosomes - giving us
46 chromosomes in total.

Sperm cells and egg cells contain 23 single chromosomes, half the
normal number, and are made by a special form of cell division
called meiosis.

Meiosis separates the pairs of matching (or 'homologous')
chromosomes, so that sperm cells and egg cells have only one
copy of each.

That way, when an egg cell fuses with a sperm cell, the fertilised
egg has a full set: that is, two copies of every chromosome.

Meiosis involves two cell divisions: Meiosis I and Meiosis II.
Meiosis I separates the
matching
or
'homologous' - pairs of
chromosomes.
•Each
daughter
cell
receives
a
mix
of
chromosomes from the two
sets in the parent cell.
•The chromosomes in
each matching pair swap
some genetic material
before they are parted in a
process called crossing
over.
•These processes produce
new combinations of
genes in the sperm cells
and egg cells.
Mitosis II divides each
chromosome into two
copies
(much
like
mitosis).

CYCLIN DEPENDENT KINASES
(cdk)

CYCLINS

Regulators of CYCLIN/cdk
 Activating Phosphatases
 Inhibitory Kinases
 Non-kinase inhibitors
tyr15-P
thr14-P
P-thr161
- protein kinase
- binds to cyclin
- kinase domain
- regulatory domain
- present throughout cell cycle
e.g. cdk1 (= cdc2)
- no intrinsic enzymatic activity
- binds cdk
- synthesized and degraded each cycle
- essential component for cdk activity
e.g. Cyclin B
tyr15-P
thr14-P
Regulated by:
-tyr15 phosphorylation
• inhibitory kinases
• activating phosphatases
-direct interaction
• inhibitory proteins
•e.g. p21, p27, p57
•p16, p15, p18,p19
P-thr161
cdk1
(cdc2)
cyclin B
cdk1 = G2/M transition



budding yeast (S cerevisiae) = cdc28
fission yeast (S pombe) = cdc2
man = cdc2 homolog; p34
cdk2 = G1/S transition
u
u
u
budding yeast (S cerevisiae) = cdc28
fission yeast (S pombe) = cdc2 (S form)
man = cdk2; p33 - PSALRE motif



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CYCLIN B = G2/M with cdc2; =cdc13 ; =CLB1-4
CYCLIN A = S + G2 with cdk2; cig2 ; =CLB5,6
CYCLIN E = G1/S with cdk2; cig1 ?puc1; =CLN1,2,3
CYCLIN D = G1 with cdk4,5,6; in yeast (early vs late
G1)
CYCLIN C = present through cycle: CTD of RNA pol II
CYCLIN F = peaks like A; cyclin stability and proteolysis
CYCLIN G = induced 3 hr growth stim and by p53:
function
CYCLIN H = part of CAK (with cdk7)
CYCLIN J = sorry, it exists
CYCLIN E / cdk2
S
CYCLIN D / cdk4,5,6
G1
CYCLIN A/ cdk2
G2
M
CYCLIN B/ cdc2
G1 cyclin-CDK complexes
become active to prepare the cell for S phase
promoting the expression of transcription factors that in
turn promote the expression of S cyclins and of enzymes
required for DNA replication.
also promote the degradation of molecules that function
as S phase inhibitors by targeting them for ubiquitination.
Active S cyclin-CDK complexes
phosphorylate proteins that make up the pre-replication
complexes assembled during G1 phase on DNA replication
origins.
The phosphorylation serves two purposes:
to activate each already-assembled pre-replication complex
to prevent new complexes from forming.
This ensures that every portion of the cell's genome will be
replicated once and only once.
Mitotic cyclin-CDK complexes
• synthesized but inactivated during S and G2 phases
•promote the initiation of mitosis by stimulating downstream
proteins involved in chromosome condensation and mitotic
spindle assembly.
A critical complex activated during this process is anaphasepromoting complex (APC):
•promotes degradation of structural proteins associated with the
chromosomal kinetochore
•APC also targets the mitotic cyclins for degradation, ensuring that
telophase and cytokinesis can proceed.
Two families of genes:
prevent the progression of the cell cycle.
 the cip/kip family
 INK4a/ARF
(Inhibitor of Kinase 4/Alternative
Reading Frame)
Because these genes are instrumental in
prevention of tumor formation, they are known as
tumor suppressors.
1 -The cip/kip family
includes the genes p21, p27 and p57.
They halt cell cycle in G1 phase, by binding to, and
inactivating, cyclin-CDK complexes.
p21 is activated by p53
p27 is activated by Transforming Growth Factor β (TGF β),
a growth inhibitor.
2 - The INK4a/ARF family
1. p16INK4a, which binds to CDK4 and arrests the cell cycle
in G1 phase
2. p14arf which prevents p53 degradation. And the amount
of chromosomes are able to double at the same rate as in
S phase .
Three main checkpoints in the cell cycle
•2001 Nobel Prize was awarded to 3 scientists who studied
genes that regulate the cell cycle
1.
1.
Is cell the correct size?
Is DNA damaged?
2. Is DNA fully replicated?
Is DNA damage repaired?
3.
3. Have spindle fibers formed?
Have they attached to
chromosomes correctly?
2.



MITOSIS ENTRY (G2/M)
 Replication Complete
 Growth/ Protein Synthesis adequate
 No DNA Damage
S-PHASE ENTRY (G1/S)
 Mitosis Complete ?signal - cyclin degradation
 Growth/ Protein Synthesis (G1 CYCLINS)
 No DNA Damage
OTHERS
 MITOSIS EXIT: ?coupling to S-phase
 S PHASE : coupling to mitosis
▪ also in response to DNA damage
 G1 sequence of events
▪ signaling from cell surface




Checkpoints are used by the cell to monitor and regulate the progress
of the cell cycle.
Checkpoints prevent cell cycle progression at specific points, allowing
verification of necessary phase processes and repair of DNA damage.
The cell cannot proceed to the next phase until checkpoint
requirements have been met.
Several checkpoints are designed to ensure that damaged or
incomplete DNA is not passed on to daughter cells.
Two main checkpoints exist: the G1/S checkpoint
and the G2/M checkpoint.
G1/S transition is a rate-limiting step in the cell
cycle and is also known as restriction point.
An alternative model of the cell cycle response to
DNA damage
has
also
been
proposed;
postreplication checkpoint.
p53 plays an important role in triggering the control
mechanisms at both G1/S and G2/M checkpoints.

CDKs, CYCLINs and regulators of Cyclin/cdks

Cell cycle events coupled to maintain the correct sequence

G0/G1, G1/S and exit from Mitosis: transitionsthat are often
altered in cell transformation/oncogensis.

DNA Damage leads to rapid cessation of DNA synthesis, and
cycle arrest at G1/S and G2/M

Stable state are G0, S exit, M exit

G2/M transition has greater impact on cell survival after DNA
Damage, relative toG1/S
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What are and what happens during the phases
of the cell cycle?
Which proteins are involved in the regulation
of the cell cycle?
Which cyclins and cyclin-dependent kinases
are most important in individual phases of the
cell cycle?
What are four mechanisms for regulating
cyclin-dependent kinase activity?
What role do p53, p21, and pRb play in the G1
to S transition?
Animation & Quiz
http://highered.mcgrawhill.com/sites/st
udent_/control_of_the_cell_cycle.html
Please, Submit the online Quiz to my email;
[email protected]
In your G0 phase you can play this fun animation,
http://www.cell-action.com/cell_cycle/cell_cycle.html