Cell Replication Flashcards
What is the cell cycle?
An orderly sequence of events in which the cell duplicates its contents and divides in two
Duplication
Division
Co-ordination
What stages of the cell cycle make up interphase?
G1 + S + G2
What factors do different rates of mitosis depend on?
- How complex the system is
- The need for renewal
- Age
- Tumour = out of sync replication
- State of differentiation (as some cells never divide= terminally differentiated)
Which cells never divide?
Neurones and cardiac myocytes
What does pre-mature, abnormal mitosis result in?
Cell death
What happens during the cell cycle that are major events?
Replicate 3 billion bp DNA
Double in size
Tear itself apart in a controlled fashion
Why is mitosis the most vulnerable period of the cell cycle?
DNA damage cannot be repaired, gene transcription is silenced, cell metabolism is low, cells are killed more easily
What is G0?
The quiescent phase
When do cells enter the G0 phase?
In the absence of stimulus
What state are the cells in when they are in G0?
The cells are not dormant, but non-dividing
What is the role of the cell cycle?
Monitoring of external environment
- nutrients
- growth factors
If not
- DNA repair
- Undergo apoptosis
Control at multiple checkpoints guards against disastrous progression through the cycle
What is the G1 checkpoint?
Is the environment favourable?
What is the G1 checkpoint?
Is all DNA replicated?
Is all DNA damage repaired?
What is the M checkpoint?
Are all chromosomes properly attached to the mitotic spindle?
If everything ok, pull duplicated chromosomes apart
How/ Why do cells ever leave G0?
Signalling cascades:
Response to extracellular factors
- Growth factors stimulate entry from G0 to G1 phase
Signal amplification
Signal integration/ modulation by other pathways
Ras/ Raf/ MEK/ ERK
What does the centrosome consist of?
Two centrioles at 90 degrees to one another
What is a centriole?
Barrels of 9 triplet microtubules which form the mitotic spindle
Where do the microtubules grow from on the centrosome?
Microtubules grow from the nucleating site on the centrosome
Microtubules are polymers of what?
Alpha and beta tubulin dimers
What is a kinetochore?
Protein complexes that assemble at the centromere of a chromosome and function to connect the chromosome to the microtubules during anaphase
In what phase of mitosis does the spindle attach to the kinetochore?
During metaphase
What happens to the microtubules during anaphase?
They get shorter as they pull chromosomes apart
In which phase d the spindle microtubules start to form?
Prophase
What occurs in the G1 phase of the cell cycle?
The cell makes mRNA and proteins in preparation for the next steps
What is aneuploidy?
An abnormal number of chromosomes
What is meant by syntelic attachment of the spindle?
When both kinetochores attach to spindle from one spindle pole, so the whole chromosome is pulled to one pole
What is meant by merotelic attachment?
When spindle from two poles attach to one kinetochore
What occurs during the S phase?
The synthesis phase - organelle replication and protein synthesis
What happens during the G2 phase?
Period of rapid cell growth in preparation for mitosis
What happens to cohesin during anaphase?
Cohesin breaks down
What occurs during the telophase?
The daughter chromosomes arrive at the spindle and nuclear envelopes reassemble at each pore
What happens to chromatin during the prophase?
Chromatin condenses
What is the function of a MTOC?
Microtubule Organising Center - forms the spindle fibers
How do we get cell growth? (Leaving G0 to G1)
Growth factors bind to tyrosine kinase receptors, which triggers an intracellular signalling pathway which ultimately leads to protein synthesis increasing and protein degradation decreasing, stimulating cell growth
What type of molecule is c-Myc?
A transcription factor
What is a trnascription factor?
Stimulates the expression of cell cycle genes
What induces the expression of c-Myc?
Growth factor signalling pathways
What does c-Myc promote?
G0 to G1 transition
Which oncogene is over expressed in many tumours?
c-Myc
What is the order of the cell cylce?
S–> G2–> M–> G1
what two things can occur if something goes wrong with cel replication?
- Cell cycle arrest - can be temporary while damage is being fixed
- Programmed cell death = apoptosis
What happens to the cell when the DNA damage is too great and cannot be repaired?
Programmed cell death = apoptosis
In the absence of a stimuli to progress into the next stage of replication, what happens to the cell?
Cells go into G0 phase (quiscent phase)
What does the exit from G0 phase require?
Growth factors and intracellular signalling cascades
What forms when cyclins bind to cyclin dependant kinases?
They form an activated cyclin-CDK complex
When are cyclin dependent kinases (Cdks) active?
Only when a cyclin is bound
Where are cyclin dependent kinases present?
in proliferating cells
Why are cyclins called cyclins?
Because their concentrations within the cell fluctuate or cycle
Which Cdk’s and cyclins mediate the transition from G1 to S phase?
Cdk2-Cyclin E complex
Which Cdk’s and cyclins mediate the transition from S to G2 phase?
Cdk2-Cyclin A
Cell cycle entry requires what Cdks and cyclin complex, and how does it work?
Cdk4/6- cyclin D complex
growth factor–> C-Myc–> Cyclin D—> Cyclin D/ Cdk 4/6 complex
What checkpoints can arrest the cell?
G1= damaged DNA and unfavourable extracellular environment
S= damaged or incompletely replicated DNA
G2= damaged or incompletely replicated DNA
M= chromosome improperly attached to mitotic spindle
How are cyclins expressed through the cell cycle?
expressed transiently
How are cyclins switched off during the cell cycle?
They are made inactive by ubiquitination - this is where tags are added to destroy the cyclin
Describe how Cyclin E helps the cell move from G1 to S phase
Cyclin E -> Binds to Cdk2 -> a lot of complexes are formed -> This leads to the phosphorylation of Rb -> Means Rb releases E2F transcription factor, so cell can continue to proliferate
How does retinoblastoma protein work?
It acts as a brake on cell proliferation
Active Rb sequesters a transcription factor in an inactive form e.g., E2F family of TFs
The TFs cannot turn on genes needed for cell cycle progression
e.g., DNA polymerase, Thymidine kinase
How does retinoblastomas occur?
Retinoblastoma protein is missing or inactive
What is the retinoblastoma protein?
A tumour supressor, abundant in all nucleated cells
What are mitogens?
Anything that stimulates cell division (growth factors)
What drives protein synthesis?
intracellular signalling pathways
During the cell cycle is protein degradation inhibited?
Yes= net increase in protein synthesis
What phosphorylates Rb?
Activated cdk-cyclin complexes
What does phosphorylation of Rb do?
releases the brake…
Proliferating cell
Activation of intracellular signalling leads to production of
G1-Cdk and G1/S–Cdk complexes
They can phosphorylate Rb inducing the inactivation of Rb and release of the TF.
Target genes such as DNA polymerase and thymidine kinase can now be activated.
What does release of E2F do?
Allows cell cycle progression
Describe how p53 works as a tumour suppressor?
- p53 recognises damage to DNA
- p53 phosphorylated = activated
- Active p53 binds to p21 gene
- Makes p21 protein
- p21 is a cdk inhibitor so no cdk complexes can form to phosphorylate Rb so no progression into next phase
What does p21 do?
it inactivates Cdk-cyclin complexes meaning they cannot go and phosphorylate Rb
When does Cdk activity peak?
During mitosis
What does c-Myc induce the expression of?
Cyclin D
Why is cyclin D so important?
entry to the cell cycle requires Cyclin D
What do protein kinase cascades lead to?
Signal amplification, diversification and an opportunity for regulation
Does each kinase have one or multiple targets?
multiple
What are all the cyclin-dependent kinases that can be found and how is there activity regulated?
Cdk1, Cdk2, cdk4, Cdk6
present in proliferating cells throughout cell cycle
activity is regulated by:
- interaction with cyclins
- phosphorylation
What are all the cyclins?
Cyclin A/B/D/E
Transiently expressed at specific points in the cell cycle
Regulated at level of expression
Synthesised, then degraded
What is the protein kinase cascade?
when a kinase is turned on via phosphorylation which then leads to another kinase to be turned on by phosphorylation
- phosphorylation by kinases
Then the final kinase turned on switches off, by phosphorylation being reversed by phosphatases
this then leads to the other kinases being turned off
Why are protein kinases useful for cell cycles?
Since the protein kinases can be turned on and off, it is very helpful in regulating the progression through the cell cycle
What actually activates Cdks?
After cyclin has bound, the Cdk complex has to be sequentially phosphorylated with inhibitory and activating phosphates. Once the inhibitory phosphate has been removed by protein kinases, the complex is active
What removes inhibitory phosphates from the Cdk complexes in order to make it inactive?
Phosphatase
How does positive feedback in the Cdk-cyclin system work?
The active Cdk complexes then activate more phosphatases so the inhibitory phosphates can be removed from the inactive complexes, leading to more activation of complexes
During ubiquitination, what do cyclins get degraded into?
Amino acids
How does ubiquitylation of cyclin occur?
ubiquitin attaches to it, then leading to destruction of the cyclin
Why do cyclins have cyclical activation?
they are susceptible to degradation
What do cyclins give to the cell cycle?
direction and timing to cycle
How might over expression of c-Myc lead to aberrant cell cycling and cancer?
Inappropriate entry into G1 – S phase
Why do cyclin dependent kinases have multiple phosphorylation points?
there are inhibitory phosphates and activating phosphates
therefore it is possible to phosphorylate at multiple points
How are Cdks rendered active to allow progression to the next phase of the cell cycle?
- Binding of cyclins
- Phosphorylation
- Dephosphorylation
How are active Cdk-cyclin complexes rendered inactive to allow orderly progression to the next phase of the cell cycle?
Degradation of cyclins
Do you understand the diagram of positive feedback of the cell cycle and Cdks?
How can p53 rapidly respond to DNA damage?
p53 protein is continuously made and degraded
What is the oncogene EGFR/ HER2?
Mutationally activated of over expressed in breast cancers. Herceptin antibody for the treatment of HER2+ metastatic breast cancer.
What is the oncogene Ras?
Mutationally activated in many cancers
What is the oncogene Cyclin D1?
over expressed in 50% of breast cancers
What is the oncogene C-Myc?
Over expressed in many tumours
What is the tumour supressor Rb?
loss of function in 80% of small cell lung cancers
What is the tumour supressor p53?
loss of function mutations in over 50% of all human cancers
