The Cell Cycle and Mitosis Flashcards
What is cell theory
- Cells arise only by replication & division of a pre-existing cell
- Cells are the smallests units of life
- All organisms are composed of 1 or more cells
Why is cell division required in multicellular organisms
- During development
- In somatic (adult) tissues, millions of cells need to be replaced every second e.g. injury, replenishing worn out cells etc
Many cells are ‘dormant’ for the majority of their life-span, but must retain proliferative potential and respond to appropriate cues
What happens if this goes wrong
Cancer may develop = a disease of uncontrolled cell division/proliferation
What occurs in the G1 phase
A period of growth - RNA and protein synthesis
They then if called to do so, will enter the G0 phase
This takes around 10hrs
What occurs during the G0 phase
It is considered a resting phase
This is what can affect the length of the cell cycle
What happens in the S phase
Synthesis - DNA replication
Allowing DNA to be replicated from one generation to the next
This takes about 7.5hrs
What happens in the G2 phase
Second growth phase
The DNA is checked just before this phase starts
This takes about 4hrs to occur
What occurs after the G2 phase
The M phase - mitosis phase
This takes about an hour
The whole cell cycle takes how long
Around 24hrs as long as there is all nutrients/proteins etc present for this to happen
Why do cells enter the G0 phase
- because they run out of nutrients or space
If a cell undergoing the cell cycle comes into contact with other cells, it will be triggered to go into the G0 phase, as there is not enough space for it to divide
This creates a sigmoidal curve for cell growth - or because they need to differentiate to allow them to specialise
What is mitosis
- Is the final stage of cell division cell cycle
- Transmission of the genome for one ‘generation to the next’
- prequisites are completion of S-phase synthesis, sufficient growth, and genome intact (no DNA damage)
Anything which is not mitosis is
interphase
In G1 phase the chromosomes are held together via a centromere, they are called the sister chromatids
They remain together until after replication (G2)
When do they full seperate
Mitosis
How are the replicated sister chromatids stuck together
Through cohesin
The movement of chromosomes to the centre of the cell in mitosis is called
Congression
What are the centrosomes
Are where the microtubules are generated
(Known as the spindle poles during mitosis)
It is they cytoskeletal components of the mitotic spindle
What is the centromere
It is the button-like structure at which chromosomes are held together until Anaphase
What is the Kinetochore
Is where the cytoskeleton elements of the microtubules are captured by the chromosomes in order to be moved around the cell
Chromatin condenses around histones to form
Chromosomes
Which contains rightly wound DNA
What can be observed during prophase?
- You can see the chromosomes as spaghetti-like structures
- Chromosomes condense, packing into short rods called chromatids
- Cohesion is lost between the arms of the chromatids, which remain glued together only at the centromere
- The nuclear envelope disintegrates - chromosomes are liberated
- Centrosomes separate to the opposite sides of the nucleus and nucleate microtubules, with Eg5 responsible (Kinesin)
What is the motorprotein responsible for allowing the chromosomes to seperate during prophase
Eg5 (its a type of kinesin 5 proteins)
It binds to one microtubule and uses the other microtububle as cargo
Allowing the sliding apart of the microtubules
If you don’t have Eg5 present during mitosis, what occurs instead
The formation of a monopolar spindle
Chromosomes are attached to just one pole with two centrosomes (as they have not managed to seperate apart)
The cell would arrest as the chromosomes have not been aligned at the right places
Chromosome movement to the centre of the cell depends on the mitotic spindle, a structure composed of microtubules
Microtububles can grow and strink from the same end, how?
There is GTP bound to B-tubulin at the end of a microtubule, the microtubules will continue to grow
If the GDP is hydrolysed faster than the tubulin addition, this will cause rapid shrinkages (catastrophy)
During Chromosome Congression, what is ‘search and Capture’
- Capture of the chromosomes starts as a stochastic process (random)
- Errors/inappropriate connections are made on 1st, 2nd… attempts, which need to be corrected.
- All chromosomes must be attached and aligned for anaphase to occur, otherwise, you will not get an equal amount of chromosomes within each cell
- The chromosomes then become aligned at the centre of the cells on a mitotic spindle composed of microtubules (two sister chromatids joined at the centrosome
- This occurs in a prometaphase stage
What occurs in Metaphase
- All cell have all chromosomes aligned at the cell centre which remain condensed and cohesed by the mitotic spindle
- Information is being transferred through the microtubules to allow the initation of the next stage - anaphase. They maintain a symmetrical bipolar spindle
- Cells WAIT until a signal tells them to separate the chromosomes (spindle checkpoint)
The two proteins involved within the Spindle Assembly Checkpoint (SAC) are Mad2 and BubR1, what are their roles?
Mad2 - this monitors microtubule attachment to the kinetochore
BubR1 - this monitors tension across the centromere
It is when these two circumstances are met, will be mean there is a correct confirguration for anaphase to occur
What is the function of cyclin-dependent kinase 1 (cdk1) in the metaphase-anaphase transition (mitotic exit) with the use of cyclin B
It will phosphorylate things when cyclin B is present.
This allows SAC and cyclin B expression to regulate the activity of cdk1
The drop of cyclin B, stimulates cdk1 levels to drop and hence stimulating anaphase to occur
What is the use of the protein Securin
Allows a protein called seperase to break the cohesion at the centromere
It can only cleave the centromere if the the protein securin is degrades
It is vital for the onset of anaphase
What happens in anaphase (A)
- Cohesion is lost from sister chromatids
- Chromosomes move to opposite spindle poles (centrosomes) from the shortening of the microtubules
- Centrosomes remian same distance apart
- ckd1 is inactivated
What happens in anaphase (B)
- Chromosomes continue to move to opposite poles
- The Centrosome-centrosome distance increases
- Midzone microtubules slide past each other in the centre of the cell
- Cleavage plane determined (where the contractoral ring will develop which will result in cell dividing)
What occurs in Telophase
- A bit of anaphase B is still occuring at this point
- Chromosomes at opposite sends of the cell start to decondense
- Nuclear envelope starts to reform
- Thick bundles of interdigitating microtubules between the chromatin masses
- Cleavage furrow formed and ingressing
What occurs during cytokinesis
Chromosomes decondense and are packaged into nucleu (this is the start of the next G1)
Cells constrict into two identical daughter cells - through a Actin-Myosin II contraction which also leads to its own disassembly (same mechanism as a muscle contraction)
The plasma membrane then needs to be pinched off
What stages are includes within early Mitosis
Prophase
Prometaphase
Metaphase
The regulation of the metaphase-anaphase transition is called
proteolysis
What stages are includes within late mitosis
Anaphase
Late Anaphase/Telophase
Cytokinesis
Name all stage of the cell cycle in order
G1 (then possibly to rest phase G0)
S
G2
M - mitosis
Cdk are a family of Kinases. We have already considered cyclin B and CDK1
What other cyclins and hence CDK are there
Cyclin D - CDK4
Cyclin D - CDK6
Cyclin A - CDK2
CDK activity peaks sequentially in accordance with the cylical expression of their partner cyclin
What is the function of Cyclin D and CDK4 in the cell cycle
It is used to call a cell to continue in the cell cycle if they have entered G0 phase
How does an increase in the activity of CDK 4 feed into the activity of CDK 2
With a number of factors in between, it causes the increase in the production of cyclin E, which associates with the activity of CDK2, which allows cells to enter S phase of the cell cycle
The extression of the Cyclins is regulated by…
Proteolysis
This means Cyclin are destroyed at the point they are no longer needed - this means the cell cycle is always driven in a forwards direction
How do Cyclins activate CDK
Through the binding of CDK to Cyclin
Proteolysis is a ubiquitin dependent mechanism
What is the use of the Anaphase promoting complex/cyclosome (APC/C)
It is an E3 ubiquitin ligase
It is held in the inactive form by subunits
Upon its activation by Cdc20, it will cause the ubiquitination of cyclin leading to a polyubiquitination chain tail
This stimulates cyclins degradation by the proteasome
Once the Spindle Assembly Checkpoint (SAC) is complete, the Mad2 and the BubR1 come away from the Kinetochore
What effect does this have
Cdc20 is left to activate the anaphase-promoting complex
This leads to the activation of the APC and consequently the inactivation of Cdk1
Overall leading to the exit of the cell cycle
G0 cells can enter a state of terminal differentiation
What does this mean?
Once fully differentiated cells cannot return to the cell division cycle and proliferate, and therefore remain ad infinintely in G0.
How does cancer relate to proliferative potential
cancer only develops in cells with proliferative potential, ie. Those that are able to resume the cell cycle,
thus Most cells in an adult body will be in G0
If you withdraw nutrients from the growth medium of cells, what will happen?
This condition is mantained by
A quiescent digression
Cell will move into the G0 phase
The condition is maintain by an inhibitory growth factos called transforming growth-facto-beta (TGF-beta)
How do cells re-enter the cell cycle after being in the G0 (quiescent) phase
This requires a signal from a growth factor
This will initiate transcription and translation again, which causes the expression of immediate early genes (competence phase) which are transcription factors which instruct the expression of cyclin-D and Cyclin-E (G1 Re-entry)
This allows the crossing of the restriction point and re-entering of the cell cycle
After this point the cycle is growth factor independent
What controls the extression and degradation of cyclin D1
Transcription factors activated by growth factos (fos/myc)
The D cyclins the activate the Cdk4
This drives the cells through the rest of G1
Increased expression of Cdk4 causes what
Stimulates the expression of cyclin E
This in turn activates Cdk2 enabling transit into S-phase
Once the cross of the restriction point has occured and Cdk2 is activated
What happens now?
Cdk2 phosphorylates pRB (retinoblastoma protein) which is a tumour suppressor protein allowing passage and the rest of the cell cycle to occur
This level of phosphorylation is maximal until the end of the cell cycle where it drops again to a hypophosphorylation state
What does pRB do (retinoblastoma protein)
pRB inhibts S-phase promoting transcription Factors called E2Fs
pRB does this by binding the E2Fs so they cannot carry out their function
What are Oncogenes
- These are genes which promote tumour formation - grow
- Positive regulators of the cell cycle
- Encoded by genes required for normal cell cycle progression - ‘proto-oncogene’
- When proto-oncogenes no longer wait for a signal to become activated (e.g. mutation) they become oncogenic (they are V-fos and v-myc)
What are Tumour Suppressors
- Negative regulators of the cell cycle
- Prevent progression in different ways
- Transcription factos or regulators of TFs
- pRB (tumour supressor) is mutated in nearly all cancers
- Tumor Supressors are activated at checkpoints before triggers the proceeding phases in the cell cycle
P53 is an example of a tumour supressor in the cell cycle
What is its function
It is intrinsically unstable and does not do anything unless there is a fault
Dubbed the guardian of the genome, will not allow the moving on to S phase from G1 (using cyclin E & cdk2) and instead will causes Apoptosis OR arrest which will reduce concentrations of cyclin E and cdk2
