The cell cycle I Flashcards
What are 5 causes of cancer?
What are examples?
1) Environmental/carcinogens
2) Viral infections (Rous Sarcoma virus)
3) Inherited factors (Retinoblastoma)
4) Genetic instability (Oncogenes and tumour suppressor genes)
5) Defects in the machinery that regulate normal cell growth/cell death (genes promoting/blocking cell division)
What causes cells to proliferate?
EXTRINSIC factors
What 2 things can cause the halt of cell proliferation?
1) The ABSENCE of the extrinsic factors due to blockage by other signalling pathways
2) Induction of a POST-MITOTIC, differentiated state where no proliferation occurs
What is the ‘cell cycle clock’?
Master governor that operates in the nucleus and and makes the major decision of whether to: proliferate, be quiescent or differentiate
Network of interacting proteins (CDKs and cyclins)
How does the cell cycle clock make the decision on if to proliferate, be quiescent or differentiate?
Receives and integrates signals from the OUTSIDE and the INSIDE of the cell
What happens if the cell cycle clock makes the decision to proliferate?
Cell goes through cell cycle of growth and division
What happens if the cell cycle clock makes the decision to quiesence?
Cells go into a non-proliferative state (G0)
What must occur between neighbouring cells when cells decide to grow or become quiescent?
Why?
The cells must ‘consult’ neighbouring cells
There are many different types of cells in a tissue - proliferation must be coordinated between the cells
How do cells communicate to their neighbours to tell them if they are proliferating?
Through GROWTH FACTORS
What are growth factors?
Small proteins that are RELEASED by cells
Travel through the extracellular matrix
Convey messages to OTHER cells
What are growth factors also called?
Why?
Mitogens
This name indicates the ability of the GFs to be able to induce cells to proliferate
What do virtually ALL cells need in order to grow and divide?
Which cells are the exceptions? How?
GF signals
Exceptions: embryonic stem cells
Mouse ES cells shown to drive their own proliferation through INTERNALLY GENERATED signals
What are the ONLY examples of WT cells that can induce a benign tumour in an adult organism?
What is this cancer called?
ES cells
Cancer - teratoma
Why are the external signals required when a cell is proliferating?
To maintain the precise STRUCTURE and INTEGRITY of the tissue in which the cells are located
What does uncontrolled proliferation lead to?
The formation of a tumour
What happens to the cell cycle clock in cancer?
It becomes influenced by cancer-associated proteins (oncogenes, tumour supressor genes)
Which DISRUPT the NORMAL control mechanisms
And lead to SUSTAINED cell proliferation
What are 5 ways in which sustained proliferation can occur?
1) Production of GF by themselves (autocrine stimulation)
2) Signals to SURROUNDING cells to produce MORE GF
3) DeREGULATION of downstream GF receptors
4) Constitutive ACTIVATION of signalling downstream of GF receptors
5) Disruption of the NEGATIVE FEEDBACK mechanisms that normally reduce proliferative signalling
What are GF produced by?
The STROMA around cells
What does the deregulation of downstream GF receptors cause?
Elevated level of receptors
Ligand-independant firing
What does the constitutive activation of signalling downstream of growth factors cause?
GFR become independant of the the ligand in the environment
What does the disruption of negative feedback mechanisms cause?
Increase in proliferative signalling (negative feedback normally acts to reduce the signalling)
What is the major difference between the growth of normal cells and the growth of cancer cells?
Normal cells ONLY grow in the presence of GF
Cancer cells can grow either in the presence or absence of GF
What happens to the cells when they proliferate?
Go through the cell cycle and division
What is G0?
Where do cells enter G0 from?
The NON-GROWING (quiescent) state of the cell cycle
Cells enter from G1
Why do cells enter G0?
Due to the ABSENCE of GF
OR
The PRESENCE of growth INHIBITORS
What is an example of a growth inhibitor?
TGFb
When cells are in G0, what 2 things can happen?
Describe these
Stay REVERSIBLY - Mitogens can re-stimulate cell proliferation
Stay IRREVERSIBLY - Post-mitotic cells
Where does the decision of the cell to undergo proliferation or quiescence occur?
In the G1 phase of the cell cycle
What happens the cell decides to proliferate?
What is this process called? What is this different to?
DOUBLING of the cells macromolecular constituents - ACCUMULATION oft the cellular constituents
Called CELL GROWTH - Different to cell DIVISION
What happens in S phase?
DNA is REPLICATED
What is the length of S phase determined by?
The AMOUNT of DNA needed to be replicated
Which cells have a shorter S phase?
Cells that proliferate A LOT (lympocytes, embryonic cells)
What occurs in G2 phase of the cell cycle?
Cell growth
Preparation for mitosis
What is interphase of the cell cycle?
G1-G2 phase (G1, S, G2)
How long does G1 take?
8-10 hours
How long does mitosis take?
1 hour
What are the subphases of mitosis?
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
What happens during prophase?
- Chromosome CONDENSATION
- Spindle fibres EMERGE from the centrosomes
- Nuclear envelope BREAKS DOWN
- Centrosomes move towards OPPOSITE poles of the cell
What happens during metaphase?
- Chromosomes line up along the metaphase plate
- Each sister chromatid is ATTACHED to the spindle fibre that originates from the OPPOSITE pole
What is lining up of the chromatids on the metaphase plate crucial for?
The 50/50 split of DNA into each of the daughter cells
What happens during anaphase?
- Sister chromatids are PULLED towards OPPOSITE poles of the cell
- Spindle fibres (that are not attached to chromatids) begin to elongate the cell
What happens during telophase?
- Chromosomes arrive at the opposite poles of the cell
- Nuclear envelop reforms around each set of chromosomes
- Mitotic spindle breaks down
- Some spindle fibres push the poles apart
What is cytokinesis?
Division of the cytoplasm of the mother cell into 2 daughter cells
How is cell cycle progression monitored?
Describe this process
Using FLOW-CYTOMETRY:
- Cells are treated with FLUORESCENT DYE - labels DNA quantitatively (more fluorescence = more dye)
- As in the S phase, the DNA of cells DOUBLES - double the intensity of fluorescence
- Cells in G0 or G1 (before S phase) have HALF fluorescence than cells in G2 or M (after S phase)
Describe the graph of flow cytometry when looking at the cell cycle
- Small peak (G0, G1)
- Slow INCREASE (S phase - begin to increase DNA)
- Second peak that is DOUBLE the first peak (G2,M)
What happens is perform flow cytometry on cells that are mainly quiescent?
First peak (G0/G1) is LARGER than the second peak (G2/M)
What are the advantages of flow cytometry?
- Graphs produces with precise numbers
- THOUSANDS of cells can be analysed in minutes
- Outcome is very quantitive - detects small differences in cell proliferation
How else can MITOSIS be monitored?
Why?
Immunofluorescence and epiflourescence microscopy
- Can stain the nucleus of cells and label the microtubules
- There are massive changes in the organisation of DNA and microtubules (different morphologies) in the different stages of mitosis
- Can observe the morphologies of the microtubules and the DNA and see what stage of mitosis the cell is in
What is the disadvantage of using immunofluorescence to observe mitosis?
NOT very QUANTITATIVE
When is using immunofluorescence to observe mitosis advantageous?
When designing DRUGS that interfere with mitosis - can see WHERE and HOW the drug is acting
Where in G1 is the growth v/s quiescence decision made?
What happens before this stage?
About an hour before the G1/S transition, when the cells have completed 80-90% of G1
Before this stage - cells consult the extracellular environment
How was the timing of the decision making in G1 discovered?
Using cells in culture:
- If remove GF BEFORE 80-90% of G1 has occurred = cells FAIL to proceed further –> revert BACK to G0
- Removed in the FINAL HOUR of G1 = PROCEED to S, G2 and M phase
When does TGFb have an effect on the cells in G1?
What affect do they have?
During the window just before the G1/S transition
Have an INHIBITORY effect
What is the stage where the cell makes a decision just before the G1/S transition called?
The RESTRICTION (R) point
When do cells pass the R point?
What happens if the cells pass the R point?
They have decided to PROLIFERATE
When pass - they are COMMITTED to complete the WHOLE of the cell cycle REGARDLESS of the presence of GF
What happens to the R point in cancer?
Why?
The decision machinery is DEREGULATED
So that cell proliferation can occur REGARDLESS of any other inputs
What parts of the cell cycle are SIMILAR between norma cells and cancer cells?
Late G1-M (after the R point)
Hows was the discovery of how the cell cycle is regulated made?
Fusion experiments - mixing nuclei together in the SAME cytoplasm to determine whether they influence each other
What happened in fusion experiments when S + G1 nuclei were mixed together?
What did this conclude?
G1 nuclei started S phase
Concluded that the S phase nuclei contains a DIFFUSIBLE factor that will introduce REPLICATION (entry into S phase)
What happened in fusion experiments when S + G2 nuclei were mixed together?
What did this conclude?
Nothing happened
Concluded that the G2 nucleus is RESISTANT to the S phase promoting factor
What happened in fusion experiments when G1 + G2 nuclei were mixed together?
What did this conclude?
Nothing happened
Concluded that G1 and G2 nuclei do NOT influence each other
What happened in fusion experiments when interphase + M nuclei were mixed together?
What did this conclude?
Interphase nuclei undergoes mitosis
Concluded that mitotic nuclei release MITOSIS-PROMOTING FACTOR that affects ALL interphase nuclei
What are the components of the ‘cell cycle clock’?
Cyclins and CDKs
What residues do CDKs phosphorylate when they are active?
Serine/threonine residues
What are CDKs dependant on?
Accessory proteins (CYCLINS)
What are cyclin/CDK complexes responsible for?
How?
Sending out signals from the CELL CYCLE CLOCK to the PROTEINS that carry out the work to move the cells through the cell cycle
By PHOSPHORYLATION of the proteins
How do kinases amplify the signal?
What does this allow?
ONE kinase can phosphorylate MANY downstream targets
Causing MANY intracellular pathways to be switched on/off at the same time
What 2 things do cyclins help CDKs?
1) Activate the CATALYTIC activity of their CDK partners
2) Help with SUBSTRATE RECOGNITION - are required for the CDKs to phosphorylate the correct targets in the cell
What are the 3 model systems for studying the control of the cell cycle?
Why?
3 model systems:
1) Genetic manipulations in YEAST
2) BIOCHEMICAL ANALYSIS of early embryonic cell division in frog eggs - Big dividing embryos, easy to perform
3) Sea urchins - MANY embryos
What did Paul Nurse and Leland Hartwell do?
How?
Identified:
1) Cell division cycle (cdc) mutants
2) Wee mutants
Using temperature sensitive (ts) mutants
What was the idea behind the experiments of Nurse and Hartwell?
Thought that if they could find a mutation that arrests the cell cycle
Dependant on where the cell cycle arrested - the non-mutant is specifically required to go through that stage of the cycle
What is the process of using ts mutants?
At permissive temperatures (25 degrees) the mutant will behave as a WT
At restrictive temperatures (37 degrees) ts-cdc mutants can grow BUT cannot divide - cells arrest the cell cycle here
What did experiments by Masui lead to?
How?
The discovery of the maturation promoting factor (MPF)
Injection of a substance from the cytoplasm of a MATURE egg into an ooctye –> entry into the M-phase, therefore:
- Must be something in the cytoplasm that induces the M phase
- MPF
What are the 2 names for the MPF?
MATURATION promoting factor/MITOSIS promoting factor
What does MPF do?
Induce mitosis in ALL eukaryotic cells
What is MPF linked to?
Protein kinase activity that oscillates during the cell cycle
(Cyclin levels increase and decrease, cyclin/Cdc2 kinase increase when cyclin levels do)
What did experiments by Tim Hunt lead to?
How?
The identification of the CYCLINS
- Dunked sea urchin eggs in soapy water –> spontaneously start dividing
- Labelled eggs with radioactive methionine and ran the proteins on the cell
- One specific protein accumulated as the cells PREPARED to DIVIDE and then DISAPPEARED as the cells split
- —->CYCLIN
What does injection of cyclin into a frog oocyte cause?
Induction of maturation and the activation of MFP
What are the 2 activities of MFP?
1) Cause the binding of cyclin to Cdc2 kinase
2) Phosphorylation of Cdc2
