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Flashcards in Cell Division (LE019) Deck (23):

What is the phase in which centrosomes are duplicated?



What is the Go phase?

When cell exits cell cycle and will (almost) never replicate again, unless artificially stimulated. For cells that are determined like neurons.


What is a master controller?

A 2 protein controller: CDK (cyclin dependent kinases) and cyclin, that drive cell cycle events.
CDKs – affect function and structure of certain machinery that will drive the cell towards the next stage of cell cycle
Cyclin – protein that has 2 functions:
1) regulates and modifies the CDK to do its function
2) brings the CDK towards the machinery that it will modify in order to make the cell go towards its next cycle


Cyclin has 2 functions. What are they?

Cyclins are one of the two proteins of the “master controller” that drive cell cycle events.
1) regulates and modifies the CDK (actives CDK)
2) brings the CDK towards the necessary components to drive the cell towards its next cycle.


What are the levels of cyclins and CDKs during the various cell cycles?

Cyclin levels will rise and fall during various parts of the cell cycle, while CDK levels will remain quite stable. Their activation is determined on the levels of cyclins that will activate them.


What is the one transition of phases in cell cycle that does not utilize a change in level of cyclins?

Metaphase to Anaphase does not utilize the rise in level of a specific cyclin. This transition utilizes APC which is the Anaphase Promoting Complex.


What is the APC?

Anaphase Promoting Complex is responsible for transitioning the cell from Metaphase into Anaphase by:
1) targeting all cyclins it can get its hands on for degradation by adding ubiquitin
2) targeting cohesins for degradation by adding ubiquitin. This will allow for the chromosomes to separate since the cohesins are the things holding the chromosomes together


Which cyclins and CDKs are associated with:

G1 – cyclin D, CDK 4, CDK 6
G1/S – cyclin E, CDK 2
S – cyclin A, CDK 2
Mitosis – cyclin B, CDK 1
Anaphase – APC (no cyclins and CDKs), the APC will mark all cyclins for degradation with ubiquitin.


List the 3 different levels of activating CDKs

1) Cyclin binds to domain on CDK that partially opens pocket to partially expose T-loop
2) Phosphorylation in this active site by CAK (CDK Activating Kinase) that will completely open up that pocket, no loop.
(This is NOT really a point of regulation, because CAK will phosphorylate pretty readily)
3) Wee 1 kinase will add ANOTHER phosphate which is inhibitory so that CDK is now inactive.
CDC25 phosphatase which specifically removes this inhibitory phosphate
^ This is the most important .
When DNA damage or something, then you will make sure to use this regulation here at this stage with Wee 1 and CDC25 to stop moving forward to next cell cycle state.


Positive feedback loop of CDK activation: list the 2 methods of this and its significance.

Review: CDK is activated when
1) cyclin is bound to partially open up that active site
2) CAK phosphorylates at the active site to completely open up that area so that ATP of CDK can readily phosphorylate)
3) CDC25 will remove the inhibiting (second) phosphate that Wee1 kinase will add to inactivate the CDK –cyclin complex.
The activated CDK will now have a positive feedback on its own activation by:
1) inhibiting the Wee1 kinase activity (so less adding of that inhibiting phosphate)
2) activating the CDC25 activity, which removes that specific inhibiting second phosphate from the active site of the CDK
Very rapid increase of the active CDK complex.
Cell wants to ensure that it fully commits to that part of the cell cycle. Don’t want this to be reversible! If cell partially activate the cell cycle, then you’ll have a partially unseparated spindle, partially replicated DNA, etc.
If you have a slow increase of CDK activity (which would happen if you didn’t have positive feedback) then various activities in that cell cycle would happen at different times because not enough CDK is activated, and then you might have reversal if the level of cyclin decreases.
Now that you have the Wee1 (inhibiting) and CDC25 (activating) balance, along with the positive feedback ability of the activated CDK complex: you have more of a switch-like activity. You have 0 activity for a while, because it will equilibrate back and forth with inactive CDK. But then when level of cyclin is high enough, then your activated CDK will become high enough to positive feedback itself and turn a switch on, and the cell must commit to going to next stage.


Explain how the CDK activity is like a switch given a linear increase of cyclin.

“Off” : because there is the Wee1 (inhibiting) and CDC25 (activating) , you need to exceed a certain level of cyclin in order to start developing an increase of CDK activity.
“Switch to On” : because of the positive feedback ability of the activated CDK complex, it will really jump up! IF there wasn’t the positive feedback ability, it would just slowly increase linearly.


How does the activated CDK complex become inactive? What happens to the cyclin and CDK ?

The cyclin will become ubiquitinated, and marked for degradation.
KEY: CDK will not become degraded. The level of CDK remains constant during the cell cycle, the difference is when it is activated and inactivated.


How does the cell ensure that replication only occurs once?

CDK S (for S-phase) and cyclin will come in and start to phosphorylate some of the DNA replication machinery.
Phosphorylating the CDC6 protein, which is necessary for recruiting helicase to unwind the DNA and therefore, trigger S phase) will mark it for degradation.
Phosphorylating the ORC complex (origin of replication complex) will down regulate it. As long as the active CDK – cyclin levels are high (during S phase), ORC will continue to be phosphorylated and downregulated. When the CDK levels drop at the end of S-phase, the ORC will lose its phosphate.


What is CDC6? (in DNA replication)

It recruits helicase to unwind the DNA


What are cohesins?
When are they degraded and by what?

Cohesins are ring-like complexes that tie tether sister chromatids to prevent premature separation of them.
They are ubiquinated by the APC complex (anaphase promoting complex) in anaphase that marks the for degradation so that the sister chromatids can separate in Anaphase


When does the nuclear envelope disintegrate?

During prometaphase


What is Anaphase A? What is Anaphase B? Which mechanisms allow for these to happen?

Anaphase A: when sister chromatids start to separate
Uses the force of depolymerization of the microtubules. Splaying of protofilaments of the microtubules will end up pulling on the kinetichore complex ring-like structure that is a collar around the microtubules.
Note: it is NOT the chromokinesins!!
Anaphase B: when the centrosomes separate even further to opposite sides of the cell and the spindle will elongate
Uses pulling of Dyneins sitting on astral microtubules and pushing of Kinesin-5 on the Interpolar microtubules.


What is Taxol?
How is the depolymerization and repolymerization of microtubules to form spindles a target for cancer treatment?

If you cannot depolymerize the microtubules and repolymerize them to form the spindles, then you cannot undergo cell division.
Taxol is a drug that does just this. It is arrested in mitosis, and eventually will undergo apoptosis


What are the 3 types of microtubules that comprise the mitotic spindle?
And why are all 3 needed?

Astral: eminate to the cell walls. Most of them make contact with the cell membrane.
Interpolar: eminate from opposite centrosomes and then overlap in the middle.
Kinetichore: eminate from the centrosomes and make physical contact with the chromosomes. Involved in that initial step of chromosome separation in anaphase.

Astral and Interpolar help position the centrosomes to the opposite sides of the cell. Remember, during G2, the centrosome is replicated right next to the original one. Cell must physically move the spindle around in the cytoplasm of the cell.

Kinetichore is obvious, it is the one attached to the chromosomes.


What are the motor proteins that are involved in assembling and positioning the spindle in spindle formation of mitosis?

Dynein – Motor protein that is attached to the cell membrane, and wants to walk towards the (-) side of the Astral microtubule. Since it is attached to the membrane, it will end up pulling on the microtubule, pulling the centrosome towards it.

Kinesin-5 – is a tetrameric motor protein that is attached to both Interpolar microtubules, and wants to walk towards both (+) ends of them.

The Dynein and Kinesin-5 will both pull and push the centrosomes apart, respectively.

Kinesin-14 – counter-balances out the Dynein and Kinesin 5, therefore, preventing it from completely going towards the opposite membranes. In Anaphase B, it will go further towards the membranes, but not fully just yet. It does this by having a tail attached to one of the Interpolar microtubules, and a motor end that walks towards the (-) end of the other interpolar.

Dynein + Kinesin-5 vs Kinesin-14 to make it stay in a specific part of the opposite side of the cell.

Chromokinesins – bind chromosomes. Physically interact with chromosomes. Walk towards the (+) end of the Kinetochore microtubules.

*Note: Chromokinesins are specifically only used for this for this stage of the cell cycle – NO other part of the cellular events. Therefore, it is a great target for cancer treatment! Will only affect cells undergoing cell division, rather than all cells that have microtubule activity.


Why is targeting Chromokinesins a better option than using Taxol? (What is taxol, what are chromokinesins)

Chromokinesins – one of the motor proteins in spindle formation. They are attached to chromosomes, and walk towards the (+) end of Kinetichore Microtubules. They are also ONLY used in cells for this particular function.

Taxol is a drug that prevents microtubules from depolymerizing. This is useful for cancer treatment because microtubules must first depolymerize and then repolymerize to form spindles in cell division.

However, Taxol will also affect other cells that are not undergoing cell division, because they are using microtubule depolymerization for other things.

Targeting chromokinesins will only target cells that are undergoing cell division.


In anaphase: how does the cell prevent aneuploidy? (Consider Kinetichores, Cdc20, and APCs)

There must be completely attachment of kinetochores of chromosomes to kinetichore microtubules to the opposite centromeres. ALL of the kinetochores must be attached to microtubules. Even if you throw an extra kinetochore into the cell, cell will arrest there. With long enough arrest, the cell should undergo apoptosis.
An unbound kintichore will INHIBIT activation of CDC20. CDC20 activation is required to activate APC, which allows for cell to go into anaphase by ubiquitinizing 1) all cyclins and 2) Securin, which is the inhibiting protein bound to Separase, preventing Separase from chewing up Cohesins that are binding the chromosomes together.


What is APC and what does it do?

APC – Anaphase Promoting Complex. It is an E3 ubiquitin ligase. When activated (by adding CDC20), this triggers entry into anaphase and separation of the chromosomes by:
1) ubiquitinizing all cyclins it can get its hands on, targeting them for degradation, therefore inactivating all of the other CDKs, allowing the cell to move forward into anaphase.
2) Separates chromosomes, which are held together by the ring cohesins by activating the enzyme “Separase” by ubiquitinizes the inhibiting protein called “Securin”, allowing for Separase to go and chew up the cohesins.

Separase is usually attached to Securin which prevents it from going and chewing up the Cohesins that are rings to bind the chromosomes together.