Chapter 5: The cell cycle (Lecture) Flashcards
What is the cell cycle?
The process that cells progress through in order to proliferate. It is key to development and homeostasis.
What can dysfunction in cell cycle control lead to?
Uncontrolled cell proliferation or errors during the cell cycle (–> mutations).
What is meant by “omnis cellula e cellula”?
All cells arise only from pre-existing cells (there’s also a spontaneous generation theory)
What is the main goal of the cell cycle?
To generate two identical daughter cells out of a single progenitor cell.
How is this main goal of generating identical daughter cells out of a single progenitor achieved?
Through DNA replication (step 1) and partitioning of the duplicated genome into two new cells (step 2).
Describe the cell cycle in short.
- Most cells are in the G0-phase (quiescent state). From here cells can enter the cell cycle at G1.
- In G1-phase (gap phase) there’s preparation and control (whether there’s the right metabolic conditions).
- In S-phase there’s DNA replication (1 sister chromatide -> pair of sister chromatides)
- In G2-phase there’s preparation and control that prior processes have went well.
- In M-phase there’s mitosis -> cell and chromatide partitioning.
What happens in the S-phase?
Duplication of each chromosome by DNA replication. Machinery ensure error free replication, at the right time.
What is normally a signal for cells in the G0-phase to re-enter the cell cycle?
The presence of mitogens
What is meant by the interphase?
All the phases except mitoses. So if a cell is in interphase, it just means it’s not in the M-phase.
How are sister chromatides held together after the S-phase?
Bij ring-like structures called cohesins
How is chromosome partioning achieved in mitosis?
Chromosomes are physically moved and segregated by a large, dynamic machine made up of microtubule polymers, the mitotic spindle.
Name the phases in mitosis.
Prophase - Prometaphase - Metaphase - Anaphase - Telophase/Cytokinesis
What happens in the prophase?
Individual chromosomes slowly become visible and the mitotic spindle is starting to form.
What happens in the prometaphase?
The mitotic spindle is visibly forming and the chromosomes are still scattered around.
What happens in the metaphase?
The chromosomes are lined in the middle of the cell.
What happens in the anaphase?
The chromosomes are separated by the mitotic spindle.
What happens in the telophase/cytokinesis?
Two cells are starting to form which will seperate.
What is the first and second grow/gap phase (Grow Phase 1 and 2)?
- Grow phase 1 happens in the G1-phase, here mitogenic signals are being sensed and nucleotides being made.
- Grow phase 2 happens in the G2-phase.
How is order established (i.e. what are the engines that drive the cycle?
Key to cell cycle control are CDKs (Cyclin-dependent Kinases).
What is the function/influence of CDKs?
CDKs phosphorylate substrates, these define where the cell cycle cells are. Fluctuating activities of these kinases thus drive cell cycle transitions and processes.
CDKs need another protein to fulfill their kinase function. What protein?
Cyclins, different CDKs associate with defined Cyclins and are needed for specific cell cycle phases.
What CDKs are needed for the G1-phase? Also name CDKs needed for S-phase and G2/M-phase.
- G1-phase -> CDK4/6
- S-phase -> CDK2
- G2/M-phase -> CDK1
Cyclin levels fluctuate during the cell cycle. What determines these levels?
Transcriptional activity (cyclin availability) as well as proteolytic degradation (ubiquitin-mediated proteolysis).
During the cell cycle different cyclins are present. Name these.
Cyclin E, A, D and B.
During the cell cycle different cyclins are present at different levels. Describe the levels of these different cyclins as the cell cycle phases progress.
- Cyclin D is present during all the phases (highest in S-phase)
- Cyclin E is present during the transition from G1- to S-phase.
- Cyclin A is present from G1 to beginning of mitosis (highest during G2-phase)
- Cyclin B is present from S-phase to in the middle of mitosis.
What are two ways to make Cyclin-CDK complexes inactive?
Association with CDK-inhibitors (CDKi’s) (like p16 and p21) or through phosphoregulation.
What is phosphoregulation?
Regulation of the phosphorylation levels on Cyclin-CDK complexes. There are certain kinases (Wee1/Myt1 or CAK) and phosphatases (Cdc25) that can phosphorylate these complexes.
What “p-proteins” can interfere with Cyclin binding?
(members of the family of) p16, 15, 18 and 19.
What “p-proteins” can interfere with ATP binding and kinase function?
(members of the family of) p57, 27 and 21.
The “p-proteins” that interfere with cyclin binding, can only inhibit certain CDKs. Which?
CDK4/6
The “p-proteins” that interfere with ATP-binding and kinase function, can only inhibit certain CDKs. Which?
two complexes of CDK2 (A-CDK2 and E-CDK2) and two complexes of CDK1 (A-CDK1 and B-CDK1)
There are certain checkpoints for control of transitions. When are these checkpoints?
During mitosis, G1-restriction point and inbetween G1-S phase and G2-M phase.
What is controlled during these checkpoints?
- During mitosis -> correct chromosome segregation? - G1-restriction point –> favorable conditions for S-phase? - Inbetween G1-S phase –> DNA integrity? - Inbetween G2-M phase –> DNA integrity?
What transcription factor drives certain programs required for S-phase?
E2F
What is the target of E2F?
Cyclin E and A and S-phase genes (thymidylate synthase, and dihydrofolate reductase)
How is E2F inhibited?
Retinoblastoma (Rb) protein is an inhibitor of E2F and binds with it. It also recruits HDAC (which prevents release of E2F).
What can phosphorylate Rb? And what happens after phosphorylation of Rb?
Rb is phosphorylated by Cyclin D/CDK4 complex, which leads to the release of HDAC and subsequent transcription of cyclin E.
How are S-phase genes stimulated to be transcribed?
When cyclin E is transcribed and froms a complex with CDK2, it can again phosphorylate Rb. This promotes full activation of E2F, where E2F can than stimulate transcription of S-phase genes.
How is S-phase activation prevented when cells experience genotoxic stress?
Through a p53-p21-dependent pathway, which prevents Cyclin E/CDK2 activation.
What are key components of G2-M checkpoint?
ATM and ATR kinases
What is the eventual downstream effect of these kinases (ATM and ATR)?
Inhibition of CDK activitiy (Cyclin B/CDK1), via phosphorylation.
What is the mitotic checkpoint?
Checkpoint if all chromosomes are properly attached to the mitotic spindle.
What happens if not all chromosomes are properly attached to the mitotic spindle during the mitotic checkpoint?
Aurora kinase ubiquitinates and tags Cyclin B for degradation.
In what phase will cells arrest if not all chromosomes are properly attached during the mitotic checkpoint?
Metaphase
Many cancer exhibit defect in checkpoint function. In what checkpoint can the most defects be found?
In G1-S checkpoint
In what cancers is cyclin D upregulated?
Breast, head & neck, oesophagus
In what cancers is there loss of p16?
Melanomas, bladder, head & neck, lung
In what cancer is there loss of Rb?
Retinoblastoma osteosarcoma, prostate, breast, bladder, lung
In what cancer is there loss of p53?
In most tumours.
ATM kinase can mutate which can result in ataxia-telangiectasia. What is this?
A neurodegenerative disease that increases the risk of cancer (lymphoma, leukemia)
p53 can mutate which can result in Li-Fraumeni syndrome. What is this?
A syndrome that increases cancer risk of carcoma, breast cancer and leukemia.
Aurora kinase can mutate or amplificate. What does this result in?
Mitotic defects and aneuploidy. Increases the risk for breast, colon and pancreatic cancer.
What is aneuploidy?
The incomplete segregation of chromosomes, caused by errors in chromosome segregation in mitosis.
Are there often defects/mutations in the mitotic checkpoint?
No, they are rarely seen.
Pfizer’s CDK4/6 inhibitor is approved for what kind of cancer?
Advanced breast cancer
How do many classical chemotherapeutic approaches work?
By activating cell cycle checkpoints, as such inhibiting cellullar proliferation.
What is a microtubule drug that can activate mitotic checkpoint?
Taxol, it induces mitotic arrest.
What is the function of doxorubucin or PARP-inhibitors?
They trigger DNA damage which activates DNA integrity checkpoints.
What is a more recent approach of targeting the cell cycle?
To target specific mutations and characteristics that define cell cyle progression (like vulnerabilities that are present in mutated situations or exposed by aneuploidy).
In breast carcinoma there’s Cyclin E overexpression. How can this overexpression be regulated?
By inhibiting PKMyt1 (kinase for phosphoregulation).
- What are the basic machinery that drive the cell cycle?
- What are agents that stop the cell cycle in case of defects?
- What are specific vulnerabilities created by the inherent mutational landscape seen in cancer cells?
- CDKs
- Taxol, DNA damaging agents
- Gene amplification, aneuploidie
Cyclin-CDK complexes are key drivers of the cell cycle. Consider the following statements: Cyclin-CDK complex activities are controlled by:
- Cell cycle-regulated transcription of CDK genes
- Ubiquitin-mediated proteolysis of Cyclin proteins
- Phosphorylation/dephosphorylation of Cyclin-CDK complexes
What is correct?
A. 1., 2., and 3. are correct
B. Only 2. is correct
C. 1. and 3. are correct
D. 2. and 3. are correct
D. 2. and 3. are correct
