Lecture 3 + 4 Flashcards
What is Cytokinesis ?
Cytokinesis is the division of the cytoplasm with the resultant generation of the 2 daughter cells.
What phase does DNA replication occur ?
DNA replication occurs during S phase
Which growth factor stimulate entry into G1 phase
Growth factors such as EGF (epidermal growth factor) can stimulate entry into G1 phase
How to p53 levels increase
p53 levels increase in response to DNA damage and halt cell cycle progression
What occurs when the cell detects DNA damage
If the cell detects DNA damage it will halt cell cycle progression, during which time it will try to repair the DNA damage and if successful re-enter the programme for cell cycle division
What happens at the spindle checkpoint
Spindle checkpoint ensures that the chromosomes are properly aligned on the spindle during metaphase
Where do cell checkpoints exist
Cell cycle checkpoints exist at G1/S and G2/M
What gene factor is involved control of G1 phase
RB is involved in teh control of the G1 phase of the cell cycle
What is p53 and what does it do
p53 is a Tumour Suppressor portein that has an important role during the G1/S checkpoint and is mutated in a high percentage of ovarian cancers
How does CDK become active
CDKs depend on associated regulatory subunits, the cyclin proteins for proper functioning
How is familial retinoblastoma developed
In familial retinoblastoma one of the 2 required Rb gene mutations has been passed on through the germline from a parent to the fertilised egg. Any one of the retinal cells needs to sustain only a single somatic mutation knocking out the still wild type allele in order to develop retinoblastoma
How is sporadic retinoblastoma developed
In sporadic retinoblastoma the zygote is genetically wild type at the Rb locus, and retinoblastoma development requires 2 successive somatic mutations stricking the 2 copies of the Rb gene carried by a lineage of retinal precursor cells
Where are mutation in APC seen
Mutations in APC (adenomatous polyposis coli) are frequently seen in sporadic colorectal cancers, mutations are also found in a form of familial colon cancer
What is cell division used for
- Maintenance: In the adult there is a constant loss of cells (skin, gut epithelium) which needs to be replaced
- Repair (wound healing etc)
- Selective advantage
5 Phases of cell cycle
G1 (first Gap Phase) -make decisions about growth versus quiescence
S Phase (synthesis) period of DNA synthesis
G2 (second Gap Phase)
M Phase (2 new cells -division of the genetic material -mitosis (prophase, prometaphase, metaphase, anaphase, telophase) and cytokinesis.
G0 -resting, nonproliferative state of
cells that have withdrawn from the active cell cycle -cells can remain in Go for days or even Years.
3 cell checkpoints
- G1/S -is the cell ready for division-DNA synthesis begin?
- Intra-S and G2/M - respond to DNA damage-prevent the replication of damaged DNA until repaired
- Spindle Checkpoint - Are all chromosomes aligned correctly to spindle?
What are early and mid G1 cells responsive to
early and mid-G1 cells are responsive
to mitogenic (growth factors) as well as to inhibitory signals such as TGFb.
How does G1 phase progress
Progression to S depends on the presence of growth promoting signals in the G1 phase up until the R or restriction point.
What occurs at R point
At the R point a cell must make the decision
of whether to proceed to S or to enter Go
(resting).
What happens while Cyclin D1 is present in cell cycle
While Cyclin D1 is present in cell cycle phases other than G1, it is exported from the nucleus after the G1/S transition and cannot therefore affect cell cycle.
What are Ubiquitin Ligases are responsible for
Ubiquitin Ligases are responsible for cyclin degradation.
Give examples of Growth Factors which can be Broad Specificity
Epidermal Growth Factor (EGF)
* Platelet-Derived Growth Factor (PDGF)
* Insulin Like Growth Factor (IGF)
* Transforming Growth Factor-b (TGF-b)
* Fibroblast Growth Factor (FGF)
What blocks THE ACTIVITY OF CYCLIN/CDKs
THE ACTION OF CYCLIN DEPENDENT KINASE INHIBITORS
(CDKIs)
What are the INK4/CDKN2 family of proteins and their function
p16, p15, p18 and p19
The INK4 proteins bind to CDK4/6 and interfere with the binding of CDK4/6 to cyclin D.
What are THE CIP/KIP/CDKN1 family members and their function
p21cip1 and p27kip1
inhibit all of the other cyclin-CDK complexes. These CKIs block
the ATP binding site.
What drives cells to divide pt 1
- Mitogenic signals (GFs) activate signalling pathwyas which lead to actuvation of transcription factors (TFs) which regulate expression of key cyclins eg Cyclin D
- D-cyclins once synthesised move to the nucleus and associate with CDK4 and CDK6.
- Cyclin D/CDK4 or 6 are phosphorylated by CAK and once activated this then phosphorylates Rb.
- CyclinE/CDK2 further phosphorylates Rb.
- Hyperphosphorylation of Rb then leads to the release of E2F which is a key transcription factor of S phase genes.
What drives cells to divide pt 2
- As cyclin D-CDK4/6 complexes accumulate in mid G1, p27 and p21 are redistributed from cyclinE-CDK2 to cyclin D-CDK4/6 complexes, this leads to the activation of cyclinE-CDK2.
- At the same time expression of cyclin E is occurring (E2F regulated).
- Active cyclinE-CDK2 phosphorylates p27 which targets it for degradation. This allows for more active cylinE-CDK2 which leads to hyperphosphorylation of Rb.
- Hyperphosphorylation of Rb releases E2F which transcribes late G1 phase genes as well as S phase genes.
- S phase gene expression leads to synthesis of S-phase machinery, DNA polymerase, topoisomerase, ligase etc. As well as production of structural proteins
What are involved in G2/M Phase Progression
- During G2 CDK1 is kept in an inactive state by the kinases WEE1 and MYT1.
- As M phase approaches the phosphatase CDC25 is activated activating the CDK1/Cyclin B complex.
- The activation of CDK1 at the G2/M boundary includes two independent events;
- one is CAK- mediated phosphorylation which is necessary for CDK1/cyclin-B complex formation, and
- the other involves CDC25-mediated dephosphorylation.
What does CDC25 phosphatases and their target
CDC25 phosphatases (isoforms A.B, & C) dephosphorylate specific tyrosine and threonine residues on CDKs (which inhibit CDK function)
targets of the checkpoint machinery during DNA damage. They are inactivated or degraded to stop the cell cycle.
What prevents phosphorylation of Rb
TGFb prevents phosphorylation of Rb-blocks cell cycle progression
What are CDC25 involved in
CDC25s are involved in G1-S and G2-M transition
What is Akt and function
Akt, a serine/threonine kinase is activated downstream of Growth factor Receptors
- Functions include
-inhibition of CDK Inhibitors and promoting progression of cell
cycle by blocking the actions of GSK3b.
What happens when Akt phosphorylates p21 and p27,
leads to their translocation from the nucleus to the cytoplasm-degraded by proteosome
What happens when Akt also phosphorylates GSK3b
inactivation of GSK3b targets of GSK3b include MYC and CyclinD1 (GSK3b inhibitory action on both MYC and CyclinD1)
Is the expression levels altered or the activity altered of proteins such as Akt, RAS or MYC in Cancer -are they different from normal cells?
RAS and Akt are abnormally activated and MYC amplification is a frequent event in breast tumors
