The Cell Cycle & Its Control Flashcards
Cells divide at different rates depending on various factors - what are these?
- Adult vs. embryonic cells
• embryo cells divides much FASTER - Complexity of system
• e.g. yeast cells ~1.5-3hrs - Necessity for self-renewal
• e.g. intestinal epithelial cells ~20hrs - State of differentiation
• e.g. neurones & cardiac myocytes never divide - Tumour cells?
Relevance of the appropriate regulation of cell division?
• Cell death
- premature, aberrant mitosis leads to this
• Aneuploidy (abnormal chr no. & content)
- in solid tumours due to mutations in oncogenes and TSGs
• Chromosome instability
- as loose & gain chr. during cell division
• Contact inhibition of growth
• Perturbation of protein levels of cell cycle regulations
- due to abnormal mitosis
• Anti-cancer strategies
- aimed at attacking machinery that regulates chr. segregation
What is the cell cycle?
Orderly sequence of events in which a cell duplicates its contents and divides in two
• duplication
• division
• co-ordination
What is involved in the M-phase?
Mitosis (division)
• nuclear division
• cytokinesis
What is involved in interphase?
Duplication
• DNA
• organelles
• protein synthesis
Which stage of the cell cycle is the most vulnerable and why?
MITOSIS • cells more easily killed (irradiation, heat shock, chemicals) • DNA damage can NOT be repaired • Gene transcription is silenced • Metabolism?
What are the vague stages of the eukaryotic cell cycle?
M phase - mitosis
Interphase: • Go - cell cycle machinery dismantled • G1 - decision point • S - synthesis of DNA/protein • G2 - decision point
Describe the S phase
Replication stage preparing for division in mitosis
• DNA replication
• Protein synthesis
- initiation of synthesis is increased along with ability to do so (capacity)
• Replication of organelles
- e.g. golgi, mitochondria (will coordinate with mitochondrial DNA for this)
Define centrosomes
Consists of two CENTRIOLES
• barrels of 9 triplet microtubules
What is the function of centrosomes
Form the
• MTOC (microtubule organising centre)
AND
• the MITOTIC SPINDLE
6 different phases of mitosis?
Prophase Prometaphase Metaphase Anaphase Telophase
Cytokinesis
(technically not mitosis)
Main actions in prophase?
• Chromosones condense
• Duplicated centrosomes migrate to opposite sides of cell
- and become MTOC
• Mitotic spindles form between 2 centrosomes
Describe the condensed chromosomes formed in prophase?
Consist of 2 sister chromatids
• each have a KINETOCHORE
• provides site of attachments for spindles
Explain how the chromosomes condense in prophase?
- 2nm DNA
- 11nm chromatin string
- 30nm chromatin fibre
- 300-700nm sacffold-associated form
- 1400nm chromosome
Describe the spindle formation during prophase?
- Radial microtubule arrays (ASTERS) form around each centrosome
• go on to form MTOC - Radial arrays meet
- Polar microtubules meet
This is a DYNAMIC ENVIRON. with polar microtubules constantly forming & breaking
What occurs during metaphase?
Chromosomes align at the equator of the spindle
What in unique about metaphase to the other stages?
Split into 2 phases:
• early prometaphase
• late prometaphase
What happens during early prometaphase?
- Breakdown of nuclear membrane
- Spindle formation completion
- Chromosome attachment via. spindles to kinetochores
What happens during late prometaphase?
- Microtubule from opposite pole is captures by sister kinetochore
- Chromosomes attached to each pole come to the middle
- Chromosomes slides rapidly towards center along microtubules
What is CENP-E?
Centromere Protein E
• kinetochore tension sensing protein
What happens during anaphase?
Paired chromatids separate into daughter chromosomes
• with cohesin holding sister chromatids together
What in unique about anaphase compared to the other stages?
Separated into 2 phases
• anaphase A
• anaphase B
What occurs during anaphase A?
• Cohesin is broken down
- Microtubules shorten
- Daughter chromosomes pulled towards opposite spindle poles
What occurs during anaphase B?
- Daughter chromsomes migrate towards poles
* Centrosomes migrate further apart
What happens during telophase?
- Daughter chromsomes arrive at spindle
- Nuclear envelope reassembles at each pole
• Contractile ring forms
- made up of actin & myosin
What happens during cytokinesis?
The contractile ring forms
• it contracts, leaving behind a mid-body
What is the spindle assembly checkpoint?
Transition out of metaphase - monitors:
• chromosome alignment
AND
• the spindle assembly
HENCE monitors kinetochore activity
• an unattached kinetochore generates a checkpoint signal = will NOT let system advance into anaphase
What is required for the spindle assembly checkpoint?
CENP-E
BUB protein kinases
• these dissociate from the kinetochore when the chromosome properly attaches to the spindle
(when all dissociated, anaphase proceeds)
What two things can lead to aneuploidy?
(a)
mis-attachment of microtubules to kinetochores
(b)
aberrant centrosome/DNA duplication
Explain how (a) leads to aneuploidy
Amphelic
• normal
Monotelic
• only ONE kinetochore bound
Merotelic
• BOTH centrosomes to same kinetochores
• sister chromosome lost during cytokinesis
Syntelic
• same centrosome to both kinetochores
• will put BOTH sister chromosomes to one side
(ONENOTE for picture!!)
How does (b) lead to aneuploidy?
DNA and centrosome duplication
• leads to too many centrosomes (often 4) AND • a multipolar spindle = • aberrant cytokinesis
Main principle anti-cancer therapy is based on?
Induces gross chromosome mis-segregations
Inhibits attachment
• error-correction mechanism
How does inhibiting attachment help in anti-cancer therapy?
Chekpoint kinase (CHKE1 & CHKE2)
- serine theronine kinase activation holds cells in G2 phase until all is ready
- inhibition leads to untimely cell transition to mitosis
What can be used for breast & ovarian cancer?
Taxanes & vinca alkaloids
• alters microtubule dynamics
• produces unattached kinetochores
• causes long-term mitotic arrest
If something goes wrong during the cell cycle, what are the 2 things the cell can do?
- Cell Cycle ARREST
• at check points (G1 & spindle check)
• can be temporary (i.e. following DNA repair) - APOPTOSIS
• DNA damage too great so cannot be repaired
• Chromosomal abnormalities
• Toxic agents
What are the 3 checkpoints in the cell cycle and at which points?
- During G1
• GFs required to exit G0 and into G1 - BEFORE mitosis (G2 checkpoint)
• to check for DNA damage - Metaphase checkpoint
• check for sister chromatid alignment
What triggers a cell to enter the cell cycle and divide?
In the absence of stimulus:
• cells go into the G0 stage (quiescent phase)
• cells are NOT dormant but are NON-dividing
Exit from G0 is highly regulated
• requires GFs & IC signalling cascades
How to tumours affect the cell cycle and de-regulate it?
Tumours ignore ALL the checkpoints
ALSO
Tumours block the ability of the cell to exit the cycle into G0
• thus cell CONTINUES to DIVIDE
What does the signalling cascade in the cell involve?
- Response to EC factors
- Signal amplification
- Signal integration
- Modulation (via. other pathways)
- Regulation of divergent responses
(look at ONENOTE picture!!)
Explain signalling by peptide GFs
Ligand binds and activates the receptor!
e.g. EGF (epidermal) & PDGF (platelet-derived)
Receptors are MONOMERIC in their INACTIVE state - when ligand binds
• receptors form dimers = kinase cascade
• kinases are PHOSPHOSRYLATED = binding of adapted proteins
Explain the role of phosphorylation in the signalling cascade
• Phosphate transfer from ATP - to a hydroxyl group - examples of side chains that can be phosphorylated include • serine • THREONINE • tyrosine
The added phosphate can alter the protein function by:
• change in conformation –> change in activity
• creating a docking site for another protein
Overall:
what happens in the presence of ligands
AND
what does receptor activation trigger?
In presence of ligands
(a) receptors form dimers
(b) are activated by phosphorylation
Receptor activation triggers:
(a) kinase cascades
(b) binding of adapted proteins
Describe the protein kinase cascase
Normally, the protein regulated by a kinase is ANOTHER kinase (and so on)……
i.e. activation of one kinase activates another to activate another
This leads to
• signal amplification
• signal diversification
• opportunity for regulation
What is phosphorylation reversed by?
Phosphates
