Cell Replication Flashcards
What is the cell cycle?
An orderly sequence of events which a cell duplicates its contents and divides in time
At what rates do cell divide?
Embryonic (vs adult) = fast Low complexity (vs high complexity) = fast High turnover needed = fast Terminally differentiated cells e.g. neurones = don't divide Tumour cells = fast
What are the different phases of the cell cycle?
Interphase: G1 and S (DNA replication) and G2
Mitosis: M , most vulnerable phase off the cycle. Cells more easily killed, DNA damage can’t be repaired, gene transcription silences
What are things a cell must achieve within a cell cycle?
Replicate 3 billion bp
Double in size
Tear itself apart in a controlled fashion
What is the G0 phase?
The quiescent phase
In the absence of a stimulus, cells go into this phase
Cells are not dormant however are non-dividing e.g. skeletal muscle, neurones
What are cell cycle checkpoints?
Cells need to monitor the external environment e.g. nutrients
The cell may need to pause e.g. for DNA repair or to undergo apoptosis
What are the different checkpoints for different phases?
G1: is the environment favourable?
G2: Is the DNA replicated? Is all DNA damage repaired?
M: Are all the chromosomes properly attached to the mitotic spindle?
How do cells leave G0?
There is a signalling cascade
Growth factors stimulate entry from G0 to G1
There is a signal amplification and then signal integration/ modulation by other pathways
What is the key role of C-MYC?
Growth factor signalling pathways induce the expression of C-MYC
C-MYC promotes G0 to G1 transition
C-MYC is an oncogene- its over expressed in many tumours
What is the role of cyclin dependent kinases? (CDKs)
The stimulate the synthesis of genes needed for next phase
They are important in key signalling events
Serine/Theanine/Tyrosine can be phosphorylated
CDKs are present in proliferating cells throughout the cell cycle
They’re only active when a cyclin is bound
What are the different cyclins that bind to CDKs?
Cyclin A, B, D and E
They’re expressed at different specific points in the cycle
They’re synthesised then degraded
Their concentration fluctuates within the cell cycle
How are different cyclins activated?
1.Growth factor induces C-MYC
2.C-MYC induces cyclin D
3. Cyclin D binds to CDK 4/6
and stimulates expression of cyclin E
4. Cyclin E binds to CDK 2 and stimulated expression od cyclin A
5. Cyclin A binds to CDK2 and stimulates expression of cyclin B
6. Cyclin B binds to CDK1 and induces mitosis
What is a protein kinase cascade?
Frequently the protein regulated by a kinase is another kinase and so on. This leads to:
signal amplification
diversification
opportunity for regulation
What is cell cycle control based on?
Cyclically activated and expressed proteins e.g. CDKs
How is CDK activity regulated?
By interaction with cyclins and phosphorylation
Phosphorylation and dephosphorylation control a lot of the activity of the kinases
How is a CDK activated by phosphorylation?
- Cyclin is produced and binds to CDK to give cyclin-CDK complex
This complex is inactive - Variety of protein kinases will phosphorylate the CDK
- Activating protein phosphatase will remove the inhibitory phosphate and lead to an active cyclin- CDK complex
How does positive feedback drive the cell through the cell cycle?
- Phosphorylation of M-CDK by CDK activating kinase (CAK) and CDK-inhibitory kinase (Wee1)
- Removal of inhibitory phosphate by phosphatase gives active M-CDK which drives cell through M phase
- Active M-CDK can phosphorylate cdc25 which activates production of more M-CDKs
- This is positive feedback and drives cell from G2 to M phase
What 3 steps are needed to drive CDK progression?
- phosphorylation of CDK
- Dephosphorylaton of CDK
- Cyclin binding
How are cyclins turned off?
Cyclins are turned off by ubiquitination
Cells stick ubiquitin molecules onto proteins it wants to degrade
Cyclin gets degraded into amino acids
What is retinoblastoma (Rb) ?
A tumour suppressor and a molecular break
If Rb is inactive then there are problems with cell cycle progression
Its found in al nucleated cells
How does retinoblastoma affect cell proliferation?
Active Rb sequesters a TF (transcription factor) in an active form
The TFs cannot turn on genes needed for cell cycle progression e.g. DNA polymerase
How is retinoblastoma inactivated?
Activation of intracellular signalling leads to production of G1-CDK and G1/S-CDK complexes
They can phosphorylate Rb causing its inactivation and release of TF
Target genes such as DNA polymerase and thymidine kinase can now be activated
What is the role of P53?
P53 arrests cells with damaged DNA in G1
X-rays induce a break in dsDNA
P53 recognises the break and activates protein kinases that phosphorylate p53 and activates it- P53 is stablised
Active P53 binds to regulatory region of P21 gene which leads to transcription of p21 mRNA
What happens to P53 when its not needed?
P53 is degraded in absence of DNA damage
This means when damage is detected it can be very rapidly repaired
What is the role of P21?
P21s are inhibitors of cyclin-CDK complexes
This breaks the cell cycle
What are some examples of oncogenes?
EGFR/ HER2: mutationally activated and over expressed in breast cancers
Herceptin is used for treatment of HER2+ metastatic breast cancer
Ras: mutationally activated in many cancers
Cyclin D1: over expressed in 50% of breast cancers
C-MYC: over expressed in many tumours
What are examples of tumour suppressors?
Rb: loss of function mutations in 80% of small cell lung cancers
P53: loss of function mutations in over 50% of all human cancers
