UNIT 12 Flashcards
Cell cycle checkpoints
The events that take place during different phases of the cell cycle must be __ so that they occur in the appropriate order.
coordinated with one another
The events that take place during different phases of the cell cycle must be coordinated with one another so that they occur in the appropriate order.
This order must be preserved even if
one of these events takes longer than usual.
It is critically important that the cell not begin mitosis
until
DNA replication during S phase has been completed
It is critically important that the cell not begin mitosis
until DNA replication during S phase has been completed. The alternative would be __
a catastrophic cell division, in which the daughter cells fail to inherit complete copies of daughter chromosomes.
It is crucial for cells to double in size during interphase (interphase = G1 + S + G2) before dividing in two during M phase. Otherwise,
cells would get smaller with each division.
order of events: cell cycle
- G1 (cell growth)
- S (DNA replication)
- G2 (cell growth)
- M (mitosis + cytokinesis)
interphase equals which of the 4 cell cycles :
interphase = G1+S+G2
The coordination between different events of the cell cycle is acheived by :
a system of CHECKPOINTS
What do cell-cycle checkpoints do?
PREVENT ENTRY INTO THE NEXT PHASE OF THE CELL CYCLE until the events of the preceeding phase have been completed
How many main checkpoints in the cell cycle?
3
What are the three main checkpoints in the cell cycle:
(1) Step 1 - G1 checkpoint
(2) Step 3 - G2 checkpoint
(3) Step 4 - M phase checkpoint also called the SPINDLE ASSEMBLY CHECKPOINT (SAC)
which of the 4 steps in the cell cycle does NOT have a “main checkpoint”
S stage (DNA replication stage) (to G2 stage
Describe G1 checkpoint regulates from __ to __
G1 regulates the transition from G1 to S phase
G1 monitors:
the environment before allowing the cell cycle to progress
Describe G2 checkpoint regulates:
The transition from G2-to-M phase
The G2 checkpoint monitors:
The G2 checkpoint monitors the DNA for proper replication and the environment
The M-phase checkpoint is also called:
the spindle assembly checkpoint
the m phase checkpoint regulates:
the transition between metaphase and anaphase (in mitosis)
The M-phase checkpoint monitors:
the chromosomes for proper attachment to microtubules and for porper alignment at the metaphase plate
Cells divide when stimulated by signals called
mitogens
Cells divide when stimulated by signals called MITOGENS, this __
sets off a signal transduction pathways that lets the cell know to start the process of growth and division
Describe the “RESTRICTION POINT” in cells ( 3 - part explanation)
(1) Rb protein suppresses the expression of genes whose proteins are needed for S phase
(2) Rb-dependant suppression of gene expression must be relieved for cells to commit to entering the S phase and the rest of the cell cycle
(3) Once these proteins (the ones whose expression was suppressed by Rb) are synthesized, the cell is committed to progressing into S phase and growth factors are no longer needed
Mitogens
Signals that stimulate cell division by initiating a signal transduction pathway
Rb (Retinoblastoma Protein):
A protein that suppresses the expression of genes needed for S phase.
Restriction Point: E2F
Transcription factor inhibited by Rb
Describe what happens at the molecular level of the Restricting point:
-E2F (transcription factor, suppressed) bound by Rb protein
- (CyclinD-Cdk4/6) phosphorlates Rb protein releasing E2F –> Atp -> ADP , free P’s bind to Rb
-E2F can bind to DNA strand and transcribe
What is the actual checkpoint happening at G1
Restriction point
Restriction Point:
The point at which the cell is committed to entering S phase, and growth factors are no longer needed
G0 Phase:
A specialized, non-dividing state that cells enter if they do not commit to S phase. Cyclins and Cdks are not present in G0.
Some cells, like nerve cells (terminally differentiated cells), permanently
exit the cell cycle and can NEVER divide again
liver cells, in comparison to nerve cells can:
go in and out of G0 to divide again
is G0 part of the cell cycle ?
no
G0 is NOT part of the cell cycle, so, __ and __ are not present
cyclins and cdks - two core cell cycle regulators
Cells in G0 can (2):
(1) Remain in this non-dividing state (G0) for long periods of time and can EMERGE FROM IT WHEN SIGNALED ex. liver cells
(2)
nerve cells __ the cell cycle control system and cannot divide
PERMANENTLY DISMANTLE
DNA damage can arrest the cell cycle in (2):
G1 and G2
DNA damage can halt the cell cycle in G1 phase, providing time for repair or triggering cell death if the damage is irreparable. This checkpoint involves (4):
(1) activation of two kinases (ATM and ATR)
(2) ATM and ATR phosphorylate two downstream kinases (Chk1 and Chk2) (3) the two downstream kinases (Chk1 and Chk2) phosphorylate p53 and stabilize it
(4) Phosphorylated p53 can then induce expression og p21 and puma
ATM and ATR
Kinases activated by DNA damage (G1)
Chk1 and Chk2:
Downstream kinases phosphorylated by ATM and ATR (G1)
p53:
A transcription factor phosphorylated and stabilized by Chk1 and Chk2.
p21:
A protein induced by p53 that binds to and inhibits cyclin-Cdk complexes, preventing inactivation of Rb and arresting the cell in G1.
Puma:
A protein induced by p53 that inhibits Bcl2, promoting cell death in response to prolonged or irreparable DNA damage.
Cancer and p53:
p53 is often inactivated in cancer cells, allowing them to bypass DNA damage-mediated cell cycle arrest and cell death.
p53 is inactivated in many different types of cancer.
Thus, cancer cells can
elude the DNA damage-mediated arrest of the cell cycle or cell death.
The unrestrained replication of damaged DNA leads to a high rate of mutation and the production of cells that tend to become cancerous.
DNA Damage Checkpoint in G2 Phase:Similar to the G1 checkpoint, DNA damage or incomplete DNA replication can prevent cells from entering M phase. This involves (3):
(1) DNA activates ATR
(2) activated ATR leads to activation Chk1
(3) Chk1 then phosphorylates and inactivates Cdc25
(Cdc phosphatase is needed to fully activate cyclin B-Cdk1)
summary of how DNA damage can inhibit the cell cycle:
P21 can block the cell cycle at:
multiple stages (recall:p21 can bind to and inhibit the main cellular CDks - 1,2 and 4)
Cells must only undergo __ cycle of DNA replication per cell cycle
one
Cells must only undergo one cycle of DNA replication per cell cycle. This is ensured by a protein called
Geminin
Replication requires numerous proteins including
MCM and Cdt1
MCM and Cdt1. These proteins help form the
pre-replication complex
expression of geminin begins:
During S phase, when DNA is synthesized
geminin binds to __, which:
Cdt1; prevents it from forming new pre- replication complexes.
when is geminin degraded?
anaphase
During anaphase, Geminin is degraded, thus allowing
Cdt1 to participate in another round of DNA replication.
Anaphase
separation of sister chromatids during mitosis
Anaphase (separation of sister chromatids during mitosis) can only happen when (2):
(1) all chromosomes are attached to microtubules (MTs)
(2) properly aligned at the metaphase plate
Anaphase (separation of sister chromatids during mitosis) can only happen when all chromosomes are attached to microtubules (MTs) and properly aligned at the metaphase plate. Otherwise, the daughter cells will
not have the appropriate number of chromosomes.
Anaphase (separation of sister chromatids during mitosis) can only happen when all chromosomes are attached to microtubules (MTs) and properly aligned at the metaphase plate. Otherwise, the daughter cells will not have the appropriate number of chromosomes.
The cell monitors this by
a complex checkpoint called the spindle assembly checkpoint (SAC).
two targets of active APC/C:
** remember APC/C complex is a ubiquitin ligase that adds a small rpotein of 76 amino acids to proteins to target them for proteolysis (degradation)
(1) m-phase cyclin
(2) securin
Anaphase Promoting Complex/Cyclosome (APC/C):
A ubiquitin ligase that, when associated with Cdc20, targets proteins for degradation.
Cdc20:
A protein that associates with APC/C to form a functional ubiquitin ligase.
Ubiquitin Ligase:
An enzyme that adds ubiquitin to proteins, targeting them for proteolysis (degradation).
Cyclin B
An M-phase cyclin targeted for degradation by active APC/C. Degradation of cyclin B inactivates M-phase Cdk (Cdk1).
Securin:
A protein that binds to and inhibits separase.
Separase:
An enzyme that degrades cohesin proteins, allowing sister chromatids to separate during anaphase.
Cohesin:
Proteins that hold sister chromatids together.
Mad1:
A protein bound to kinetochores that converts Mad2 into the closed form (cMad2).
p31comet:
A protein that displaces cMad2 from Cdc20 when all kinetochores are properly attached to microtubules.
securin binds to an enzyme called:
separase
what does separase do?
degrades cohesin proteins
spindle assembly checkpoint:When bound to securin, separase is
inactive and the sister chromatids remain attached to each other.
spindle assembly checkpoint: Upon activation of the APC/C, securin is
degraded and separase is activated. Separase then degrades cohesin allowing the chromosomes to be pulled to opposite spindle poles during anaphase.
inhibited cdc20 =
inhibited anaphase
spindle assembly checkpoint: A protein called Mad2 can exist in either
a closed or open form (cMad2 and oMad2, respectively)
A protein called Mad2 can exist in either a closed or open form (cMad2 and oMad2, respectively). In the closed form it can
bind to and inhibit Cdc20. (no anaphase)
Mad2 is converted into the closed form by
associating with kinetochore- bound Mad1
a single unattached kinetochore is sufficient to produce
enough cMad2 to inhibit all of the cellular Cdc20 and prevent anaphase.
The spindle assembly checkpoint
How is APC/C activated (3 steps)?
(1)When all kinetochores are properly attached to microtubules: Mad1 dissociates from the kinetochore and stops producing cMad2.
(2) A protein called p31comet is then able to displace cMad2 from Cdc20.
(3) ThefreeCdc20canthenassociatewithAPC/Ctoproducetheactive APC/C.
only __ oriented chromosomes inactivate the SAC
bi-oriented
Microtubules can attach in __ different ways to kinetochores
4
Amphitelic
both kinetochores are attached to MTs from the two
spindle poles
Merotelic
only one kinetochore is attached to MTs that come from
both spindle poles
Syntelic
Both kinetochores are attached to MTs coming from one
spindle pole
Monotelic–
nlyonekinetochoreisattachedtoMTs
which of the four attachments is the “correct” type:
amphitelic
tension mechanism: aurora B kinase:
Amphitelic attachment creates PHYSICAL TENSION
This tension moves kinetochores AWAY from Aurora B
When kinetochores are far from Aurora B, key proteins AREN’T phosphorylated
This signals “ALL CLEAR” for cell division
no tension: aurora B kinase:
Aurora B phosphorylates kinetochore proteins (like Ndc80 complex)
This LOOSENS microtubule attachments
Allows incorrect attachments to REFORM correctly
Only bi-oriented chromosomes
inactivate the SAC
once amphiphatic attachments are made, what can inactivate the SAC?
Once amphitelic attachments are made, Mad1 can come off the kinetochore and inactivate the SAC.
