2.8 - Cell Replication Flashcards
1
Q
The cell cycle
A
- orderly sequence of events in which a cell duplicates its contents and divides in two
- duplication, division, coordination
- G1 + S + G2 = interphase
- M = mitosis
- cell cycle is incredibly busy as cell must replicate 3 billion bp DNA, double in size, tear itself apart in a controlled fashion
2
Q
Cell division in specific cells
A
Different cells divide at different rates:
- embryonic vs adult cells
- complexity of system
- necessity for renewal
- state of differentiation (some cells never divide e.g. neurons and cardiac myocytes)
- tumour cells?
3
Q
G0
A
- G0 = quiescent phase
- in the absence of stimulus, cells go into G0
- most cells in the body which are differentiated to perform specific functions
- if a cell is damaged
- if cells are not dormant, but non-dividing e.g. neurons, skeletal muscle, hepatocytes
4
Q
Cell cycle checkpoints
A
- control at multiple checkpoints guards against disastrous progression through the cycle
- monitoring of external environment - is it favourable?
- in terms of nutrients, growth factors
- DNA can undergo repair, or the cell can undergo apoptosis
- G1 checkpoint - before entry into S phase - is environment favourable? is there damaged DNA?
- G2 checkpoint - before entry into mitotic phase - is all DNA replicated? is all DNA damage repaired?
- mitosis checkpoint - before anaphase - are all chromosomes properly attached to the mitotic spindle?
- a variety of checkpoints can arrest the cell cycle - if checkpoint not met
5
Q
How / why do cells leave G0?
A
Signalling cascades:
- response to extracellular factors - growth factors stimulate entry from G0 into G1
- signal amplification
- signal integration / modulation by other pathways
- Ras / Raf / MEK / ERK
- growth factor –> growth factor receptor –> intracellular signalling pathway –> protein synthesis increased + protein degradation decreased –> cell growth
6
Q
Cell cycle entry - role of c-Myc
A
- growth factor stimulation of signalling pathways promotes G0 to G1 transition
- growth factor signalling pathways induces the expression of c-Myc
- c-Myc is a transcription factor that promotes G0 to G1 transition
- stimulates the expression of cell cycle genes
- oncogene that is overexpressed in many tumours
7
Q
Cyclin dependent kinases (Cdks)
A
- cell cycle control based on cyclically activated and expressed proteins
- require cyclin to function
- levels of Cdks are fairly stable - it is the levels of cyclin that drive the active process
- present in proliferating cells throughout the cell cycle, BUT only active when a cyclin is bound
- activity is regulated by: interaction with cyclins, phosphorylation
- serine / threonine / tyrosine residues phosphorylated/dephosphorylated by Cdks
- become sequentially active and stimulate synthesis of genes required for next phase
- growth factor –> C-Myc –> cyclin D –> cyclin D:Cdk4/6 complex (cyclin bound to Cdk) –> cyclin E = gives direction and timing to the cycle
- e.g: Cdk1, Cdk2, Cdk4, Cdk6
- cell cycle entry requires Cdk 4/6:cyclin D complex
8
Q
Cyclins
A
- named because their concentrations within the cell fluctuate/cycle
- transiently expressed at specific points in the cell cycle
- regulated at level of expression
- produced/synthesised, then degraded (cyclic nature)
- only the cyclin:Cdk complex is active during mitosis
- cyclins susceptible to degradation, hence cyclical activation
- e.g: CyclinA, CyclinB, CyclinD, CyclinE
9
Q
Protein kinase cascades
A
- frequently, the protein regulated by a kinase is another kinase
- leads to signal amplification, diversification and opportunity for regulation
- regulation = dephosphorylation
- phosphorylation by kinases (activation)
- phosphorylation reversed by phosphatases (deactivation)
10
Q
Sequential phosphorylation and dephosphorylation activates Cdks
A
- cyclin binds to Cdk, forming an inactive cyclin-Cdk complex (needs phosphorylation) - has an inhibitory phosphate and an activating phosphate
- protein kinase activates activating phosphate
- activating protein phosphatase dephosphorylates inhibitory phosphate
- active cyclin-Cdk complex
11
Q
Active Cdks
A
- S-Cdk = controls S phase (when active S-Cdk is present, S phase is triggered, S cyclin falls as M phase begins)
- M-Cdk = controls M phase (when active M-Cdk is present, mitosis is triggered, M cyclin falls before G1 begins)
- positive feedback drives the cell cycle forwards - more active M-Cdk = more phosphatase (Cdc25) produced, reinforcing production of active M-Cdk
- cyclins are turned off by ubiquitination - method of degrading proteins
- active cyclin-Cdk complex –> ubiquitination of cyclin (ubiquitin attaches) –> destruction of cyclin –> inactive Cdk
12
Q
Retinoblastoma
A
- a molecular ‘brake’
- first identified through studies of a childhood eye tumour
- retinoblastoma protein (Rb) - essential for sequential progression - either missing or inactive
- tumour suppressor - inhibits cell proliferation and tumour development + often lost/inactivated in tumours, removing negative regulation of proliferation –> tumour
- abundant in all nucleated cells
- resting cell - active Rb is tightly bound/sequesters a transition factor in an inactive form (e.g. E2F family of TFs) –> TFs cannot turn on genes needed for cell cycle progression e.g. DNA polymerase, thymidine kinase
- in a proliferating cell, activation of intracellular signalling leads to production of G1-Cdk and G1/S-Cdk complexes, which can phosphorylate Rb inducing the inactivation of Rb and release of TF = target genes now activated
- E2F family members regulate the expression of several genes needed for cell cycle progression
13
Q
Growth factor (mitogen) signalling
A
- for the cell cycle to progress, the cell needs to double in size
- intracellular signalling pathways drive protein synthesis
- also protein degradation is inhibited
- net increase in protein synthesis
14
Q
P53
A
- p53 (transcription factor + tumour suppressor) arrests cells with damaged DNA in G1
- active p53 induces the p21 gene: stable active p53 binds to regulatory region of p21 gene –> p21 mRNA by transcription
- p21 family members are inhibitors of cyclin:Cdk complexes - p21 codes for inhibitors which bind to Cdks and inactivate them
- in absence of DNA damage, p53 is degraded in proteasomes
15
Q
Cell cycle regulatory proteins and cancer - oncogenes
A
- oncogenes are genes with the potential to cause cancer
- overexpression leads to cancer
- EGFR/HER2 - mutationally activated / overexpressed in breast cancers - Herceptin antibody for the treatment of HER2+ metastatic breast cancer
- Ras - proto-oncogene mutationally activated in many cancers
- Cyclin D1 - overexpressed in 50% of breast cancers
- C-Myc - overexpressed in many tumours
