Cancer 4 - The cell cycle and its regulation Flashcards
Different cells divide at different rates. 5 factors influencing this
- Embryonic vs adult cells
- Complexity of system
- Need for renewal
- State of differentiation (neurons and cardiac myocytes cannot divide)
- Tumour cells = very fast
Tumour cells have lost contact inhibition. What is contact inhibition?
Whereby cells recognise when to stop dividing by sensing their surrounding environment
What does premature, aberrant mitosis lead to?
Cell death
Most solid tumour cells are aneuploid, meaning?
That they have abnormal chromosome number and content
Many cancer cell lines show chromosome instability, meaning?
They can lose and gain whole chromosomes during cell division
What happens in the M-phase?
M for mitosis
- Nuclear division
- Cell division
What happens in interphase
Duplication of:
- DNA
- Organelles
- Protein synthesis
During which period are cells most vulnerable?
M-phase
Cells more easily killed (irradiation, heat shock, chemicals)
DNA damage can’t be repaired, Gene transcription silencing, etc
Which part of the eukaryotic cells is the steady state of the cells (i.e. just doing their business like secretion, etc)
G0 - cell cycle machinery dismantled
What are the stages of interphase in eukaryotes
G0 = cell cycle machinery dismantled
G1 (gap) - decision point - do I need to divide?
S phase - synthesis of DNA/protein
G2 (gap) - decision point - have I acquired sufficient materials to divide?
What occurs in S-phase?
- DNA replication
- Protein synthesis - initiation of translation and elongation, with increased capacity
- Organelle replication - e.g. centrosomes, Golgi, mitochondria, etc
Mitochondrial DNA replication must be coordinated with cell DNA replication
From what organelle do spindle fibres develop in mitosis?
Centrosome
Describe the structure centrosome
- consists of 2 centrioles - centriole = barrel like structure made up of 9 triplet microtubules
- Centrioles lie perpendicular to each other - mother and daughter centrioles
What is the function of the centrosome
- Contain MTOC (Microtubule Organising Centre) - to organise microtubule network
- Contain mitotic spindle
How does centrosome duplication work?
- In G1 phase, mother and daughter centrioles separate and start to duplicate
- Duplication occurs in the S-phase - mother produces daughter and daughter produces mother
- Clouds of protein complexes containing Nucleating sites (for microtubules) are formed
- When cell needs to mitose, arrays of Microtubules emerge from nucleating sites
What are the 6 stages of the M-phase
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
How are chromosomes condensed in prophase
(They were duplicated in S-phase of interphase)
Chromatin compacted and wraps around histones to become 30nm wide fibres.
30nm fibres are then extended as a scaffold - compacted to 300nm wide fibre.
Fibres further wrapped until chromosome formed
Each duplicated, condensed chromosome exist as sister chromatids. Explain their structure
Centromere in the middle - acts like a belt - constriction around chromosomes
Centromere contains many protein complexes that form the kinetochore
What does the kinetochore do?
Protein complex that regulates the processes in cell cycle
What happens in late prophase/early prometaphase?
- Nuclear envelope breaks down - causes chromosomes to come out into cytoplasm
- Centrosomes migrate to opposite sides and begins to form and organise mitotic spindle
Explain the process of spindle formation
- Radial microtubule arrays (ASTERS) form around each centrosome
- Radial arrays meet
- Polar microtubules form
This is a dynamic process
Metaphase is when chromosomes are aligned at the equator of the cell. Metaphase consists of 2 subphases which are?
Early prometaphase and late prometaphase
What happens in early prometaphase
- Breakdown of nuclear membrane
2. Chromosomes attach to spindle fibres via kinetochores (one on each side)
What happens in late prometaphase?
- Microtubule on opposite pole is captured by sister kinetochore
- Chromosomes attached on each pole congress to middle by sliding rapidly along microtubules
Overall, in anaphase, what happens?
Anaphase A + B
Paired chromatids separate to form daughter chromatids
What happens in Anaphase A
- Cohesin is broken down
- Microtubules get shorter
- Daughter chromosomes are pulled towards opposite spindle poles
What happens in Anaphase B
- Daughter chromosomes finish migrating to poles
2. Spindle poles (aka centrosomes) migrate apart to allow space for the incoming daughter chromosomes
What protein is responsible for sensing tension in kinetochores
CENP-E
What happens in telophase
- Daughter chromosomes arrive at spindle
- Nuclear envelope reassembles at each pole
- Condensation of material where cells are going to split
- Assembly of a contractile ring consisting of actin and myosin filaments
- Contractile ring squeezes to begin forming 2 daughter cells
What is the cleavage furrow
Where cells are going to be cleaved
What happens in cytokinesis
- Insertion of new membrane at cleavage furrow
Mid-body = where actin-myosin ring is formed
What is the Spindle Assembly Checkpoint
When the cell wants to exit metaphase and enter anaphase, the cell ensures completion of chromosome alignment and spindle assembly.
How does Spindle Assembly checkpoint happen
Kinetochore emits a signal when it is not attached to a microtubule - so wait till kinetochores stop sending signals to proceed to anaphase
Which proteins are involved in the signalling process of the Spindle Assembly unit?
CENP-E and BUB protein kinases
BUBS dissociate from kinetochore when chromosomes are properly attached to the spindle
Anaphase occurs when all BUBS have dissociated
How can mitotic checkpoint deficiency lead to aneuploidy
Anaphase occurs before spindle fibres have attached properly
What are the 3 types of mis-attachment of microtubules that can cause aneuploidy
- Syntelic attachment - both kinetochores are attached by (2) microtubule arrays from the SAME centrosome
- Merotelic attachment - more than one microtubule array is attached to the same kinetochore - hence one of the chromatids is being pulled in 2 different directions
- Monotelic attachment - only one kinetochore is attached to a microtubule, the other is unattached
How can problems with DNA and centrosome duplication cause aberrant mitosis?
Incorrect duplication of centrosomes - may lead to 4 centrosomes instead of 2.
Leads to abnormal attachment of microtubules to kinetochores and so abnormal cytokineses
Describe 2 anti-cancer therapies achieved through chromosome mis-segregations
- Checkpoint kinase inhibitor - inhibit kinetochore signalling so cell incorrectly proceeds to anaphase
- Taxanes and vinca alkaloids - alters microtubule dynamics which produces unattached kinetochores - causing long term mitotic arrest
What happens if something goes wrong during the cell cycle?
- Cell cycle arrest - occurs at checkpoint (e.g. G1 / spindle check point). May be temporary (i.e. cell cycle resumes after necessary DNA repair)
- Programmed cell death (apoptosis)
- too much DNA damage to repair
- Chromosomal abnormalities preent
- toxic agent
All cause cell cycle progression to be aborted and cell to apoptose
Which 3 checkpoints are tumours able to bypass
- G1 checkpoint - growth factors
- G2 checkpoint - DNA damage can occur
- Metaphase-anaphase checkpoint - sister chromatid alignment occurs here
How can tumours deregulate the cell cycle?
Usually, when cells come out of G1 - they enter G0.
Tumours block the ability of the cell to leave the cell cycle and enter G0 - they continue to undergo cell divisions
What triggers a cell to enter the cell cycle and divide?
Growth factors - it is a highly regulated pathway with many intracellular signalling cascades
How is signalling by peptide growth factors achieved?
When the ligand binds and activates the receptor
- EGF/PDGF binds to monomeric receptors. Upon binding - the receptors become dimeric
(2. Receptor Protein Tyrosine Kinase (RPTK) involved. ) - Dimeric receptors are activated by phosphorylation in the presence of a ligand.
- The phosphorylated tyrosine residues can now cause kinase cascades and act as docking sites to allow binding of adapter proteins
What does protein kinase cascade lead to?
- Signal amplification
- Diversification
- Opportunity for regulation
