Cell division Flashcards
(20 cards)
What happens in de M-phase?
PMAT and Cytokinesis
The expression levels of different ___________ fluctuate throughout the cell cycle.
cyclins
Different cyclins are expressed during the phases of the cell cycle. These cyclins bind to Cdks, which are expressed throughout the cell cycle, but only become activated when they are bound by the appropriate cyclin.
The slow rise of S cyclin levels throughout G1 phase is due to ___________, and the abrupt decrease is caused by ___________.
transcription; proteolysis
Cyclin levels rise slowly because they are transcribed and translated and it takes time for the proteins to accumulate prior to the onset of their cell-cycle phase. Cyclins are rapidly degraded later in the cell cycle by targeted proteolysis.
At the end of M phase, cells shut down Cdk activity through what mechanism?
deployment of Cdk inhibitors
At the end of M phase, cells shut down Cdk activity through multiple mechanisms, one of which is blocking Cdk activity through Cdk inhibitors. In addition, cyclin levels drop quickly by preventing new cyclin synthesis and targeted degradation of existing cyclins.
Rb is an important protein for controlling cell proliferation by blocking entry into S phase. How does it exert its effect?
In its unphosphorylated state, Rb is active and blocks transcriptional regulators.
Rb blocks the passage from G1 into S phase by inactivating transcription factors required for the expression of genes that allow entry into S phase. Mitogen signaling can activate Cdks to phosphorylate and inactivate Rb, thus lifting the restriction on S-phase entry.
In response to DNA damage, the ___________ protein is phosphorylated and activates the transcription of a Cdk inhibitor to halt cell cycle progression.
p53
p53 is a transcriptional activator that is activated in response to DNA damage. The genes expressed by p53 help the cell respond to the DNA damage by blocking cell-cycle progression.
How does S-Cdk prevent re-replication?
phosphorylation of ORC and Cdc6
S-Cdk prevents re-replication of the genome by phosphorylating the ORC and Cdc6 complexes. This inactivates these proteins from assembling a new pre-replicative complex until the next cell cycle.
M-Cdk is suddenly activated at the end of G2 by
dephosphorylation by Cdc25.
The M-Cdk complex is held in an inactive state by an inhibitory phosphate while it accumulates throughout G2 phase. Cdc25 phosphatase removes this phosphate.
What is the function of condensins?
to coil sister chromatids into a compact form
Condensins function as rings that hold loops of DNA together to compact chromosome structure during mitosis. Cohesins are similar in structure, but hold two sister chromatids together during replication.
In which phase of mitosis does the nuclear envelope break down?
prometaphase
After spindles form and chromosomes condense in the previous mitotic stage, the nuclear envelope breaks down in prometaphase. This allows the spindle microtubules to have access to the chromosomes.
How are spindle microtubules attached to chromosomes?
The microtubules bind to the kinetochore complexes through a connecting protein.
Spindle microtubules bind to the chromosomes through a connecting protein that binds both the plus end of microtubules and the kinetochore complex on the centromere region of the chromosome.
If a chromosome is attached to two microtubules from opposite poles and one of the microtubules is experimentally severed, what occurs?
The chromosome migrates quickly to the pole to which it is still attached.
Chromosomes that are attached to opposite poles are under tension because both microtubules are pulling and pushing on the chromosome. If one microtubule contact is severed, the chromosome will immediately move to the pole to which it remains attached.
Cohesin is cleaved by the enzyme ___________, which is held in an inactive state by ___________ until it is degraded by the APC/C complex.
separase; securin
Separase is the enzyme that cleaves cohesin molecules. The protein securin holds separase in an inactive form until anaphase onset, when securin is cleaved by the APC/C complex.
What drives the reassembly of the nuclear envelope?
dephosphorylation of lamins
Lamins, which are phosphorylated and disassembled by the mitotic Cdk, are dephosphorylated in telophase and can reassemble the nuclear envelope.
What determines the position of the cleavage furrow of the dividing cell?
The interpolar microtubules send signals to form a cleavage furrow between the poles.
The cleavage furrow must form directly between the two spindle poles to ensure that the divided chromosomes are separated into two equivalent daughter cells. The location is determined by a signal emanating from the overlapping sections of the interpolar microtubules to the plasma membrane to mark the center of the spindle.
The contractile ring is composed of
actin and myosin.
The contractile ring is a structure made up of a ring of actin filaments and myosin motors. The myosin powers the sliding of the actin filaments toward each other and leads to a narrowing of the ring to pinch the membrane and split the cell in two.
In response to an apoptotic stimulus, initiator caspases
cleave and activate executioner caspases.
Apoptotic stimuli activate the apoptotic signaling cascade, which is a series of proteolytic cleavages. An initiator caspase is activated by the stimulus, then cleaves an executioner caspase to activate it.
De laatste voorbereidingen voor de celdeling worden getroffen in de
G2
Verschillende Cyclin dependant kinases (Cdk’s) zetten verschillende stadia van de celcyclus aan, omdat:
hun activiteit toeneemt tijdens verschillende stadia van de celcyclus
Het controlesysteem van de celcyclus gebruikt Cdk remmende eiwitten om:
d. de celcyclus op specifieke checkpoints stil te zetten (arrest)
