Cell cycle Flashcards
What occurs during mitosis and interphase?
Mitosis = PMAT, Prophase, Anaphase, Metaphase, Telophase (and cytokinesis)
Interphase = G1, S phase (DNA replication), G2 the Mitosis
G1 and G2 are gaps between the M and S phases, where growth usually occurs
What are post-translational modifications and what 2 things can they do?
What is the most important type of PTM?
Covalent modifications of amino acids
Either finishes off the protein OR acts as a reversible switch
Protein phosphorylation (adding a phosphate group) is the most important type of PTM
What is phosphorylation exactly (include which 3 residues can be involved)?
Addition of a gamma-phosphoryl group from an ATP (end phosphate group) to a serine, threonine, or tyrosine residue
(forms a phosphoserine, phosphothreonine, or phosphotyrosine)
What do kinases do and what are the 2 types of kinases?
What is a consensus sequence?
Kinases transfer the gamma phosphoryl group from the ATP to the target protein (serine, threonine, or tyrosine)
2 types: Serine/threonine kinases and tyrosine kinases
Consensus sequence = Kinase-specific recognition sequence next to the residue that gets phosphorylated (next to the Ser, Thr, or Tyr)
What do Phosphatases do?
Reverse phosphorylation (take away the gamma phosphoryl group)
What 2 things can phosphorylation of a protein change and what 2 thing can these changes promote/inhibit?
Phosphorylation can change:
- Protein conformation (secondary/tertiary structure)
- Surface shape/charge
These changes can inhibit/promote:
- Enzyme activity
- Protein-protein interactions
What are CDKs and what do they do?
What are the 4 types of proteins regulated by CDKs?
CDKs = regulator of the cell cycle, only active when bound to cyclin
Regulates other protein kinases and regulates other proteins directly
Proteins regulated:
- Chromosomal proteins
- Nuclear Lamins (nuclear envelope disassembly)
- Microtubule-associated proteins (mitotic spindle assembly)
- Other proteins involved in cytokinesis
When is CDK activity high, what controls this, and how is cyclin concentration cyclical?
CDK activity is high during mitosis, when cyclin concentrations are high
Cyclin concentrations increase during interphase, are high during mitosis and then decreases after mitosis (see diagram in jotter)
How is CDK activity controlled exactly? (5)
Regulated by phosphorylation
- Inhibitory phosphorylation keeps CDK inactive, even when bound to cyclin
- Phosphatase Cdc25 removes the inhibitory phosphorylation
- This activates the CDK, and the active CDK then phosphorylates the Cdc25 to make Cdc25 more active (positive feedback)
- So a little bit of active CDK leads to further activation.
- Once cyclin levels reach a threshold, CDK activity quickly increases, as when cyclin accumulates it promotes the phosphatases to activate more CDKs
Look in jotter for diagram
What is the stage in mitosis that is the point of no return and why?
How does the cell exit M-phase?
Anaphase, as this is when sister chromatid separation occurs
Cyclin levels need to decrease, and so cyclin is degraded through proteolysis (destruction of cyclin proteins) via another type of PTM called Ubiquitylation
What is ubiquitylation, how can polyubiquitylation occur, and what is an Isopeptide bond?
Ubiquitylation = ubiquitin forms an isopeptide bond with a lysine side chain in the target protein (diagram in jotter)
Isopeptide bond = bond between 2 side chains
Polyubiquitylation = The lysine on ubiquitin itself can be joined onto the lysine of another ubiquitin to form a chain
What are the 3 steps involved in the proteolysis of cyclin?
- Chromosome alignment in metaphase spindle leads to activation of a multiprotein ubiquitin ligase called Anaphase promoting complex (APC)
- APC ubiquitylates/polyubiquitylates cyclin
- The ubiquitylated cyclin is then recognised by the proteosome and degraded - ubiquitylated cyclin is like a tag for the proteasome to recognise that the cyclin needs to be degraded
Diagram in jotter
How are cohesion rings attaching sister chromatids destroyed during chromatid separation? (2)
- Active APC activates a proteolytic enzyme called seperase, by destroying the inhibitory protein securin which is attached to seperase via ubiquitylation
- The activated seperase cleaves and destroys the cohesion rings to allow the sister chromatids to separate
Diagram in jotter
What technique can be used to measure the length of the different phases in the cell cycle and describe how it works?
Flow cytometry
Cell are stained with a fluorescent dye which integrates onto the DNA
If the DNA has replicated it will have double the fluorescence, as it will have double the amount of DNA (as it has replicated)
(Has 2-4N when in the S phase (while replicating), and has 4n after the S phase is completed (once fully replicated))
If 36% of cells are in the S-phase, then the S-phase occupies 36% of the cell cycle (differs in different cells, just an example number)
See diagram in jotter
What are the 3 types of CDKs involved in the cell cycle?
- G1/S-cyclin CDK
- S-cyclin CDK
- M-cyclin CDK
How does replication work in terms of ORCs, preRCs, S-CDKs, M-CDKs and APCs and at what stage in the cell cycle does each occur?
Diagram in jotter
- ORC binds to replication origin (G1)
- preRC formed at ORC (G1)
- S-phase CDK activates replication at preRCs - ensures “once and only once” replication (S)
- M-CDK activation causes the chromosomes to separate (M)
- CDK inactivates and APC (Anaphase promoting complex) activation causes assembly of new preRCs at origins (G1)
What are the 3 main cell cycle checkpoint and what point in the cell cycle they occur? (just names)
- DNA damage checkpoint at G2/M phase
- Chromosome alignment checkpoint at the metaphase-anaphase transition of mitosis
- Late G1 checkpoint
What are the 4 steps involved in the DNA damage checkpoint?
- Protein kinases detect DNA damage and activate other “checkpoint” kinases
- These “checkpoint” kinases phosphorylate the phosphatase Cdc25 to cause Cdc25 inactivation.
- When Cdc25 is inactive, CDK can’t be dephosphorylated (can’t be activated) at its inhibitory phosphorylation sites and mitosis can’t progress - cell cycle arrests at G2/M phase
- When DNA damage is repaired the “checkpoint” kinases stop signalling, allowing CDK-cyclin to become active again and for mitosis to occur
What are the 5 steps involved in the metaphase-anaphase checkpoint?
- Unattached kinetochores (chromosomes unattached from mitotic spindle) generate a “wait anaphase” signal
- The “wait anaphase” signal inhibits Anaphase Promoting Complex (APC)
- This prevents cyclin degradation and cleavage of cohesion rings
- After all chromosomes are bioriented (aligned), the “wait anaphase” signal is destroyed.
- The APC is then activated, cyclin is degraded, and cohesion rings are cleaved allowing the sister chromatids to separate
In what 3 scenarios do cells need to divide?
- Development of an organism = differentiation into different cell/tissue types
- Renewing cells/tissues that turn over rapidly (need replaced)
- Repairing damaged tissues
What are 3 alternative names for the late G1 checkpoint and what 4 things can occur at this checkpoint?
What type of cyclin does this involve?
Alternative names = Commitment, restriction, or start point
Once cells get past this point they enter the S phase and are committed to cell division and completing the cell cycle
At this checkpoint the cell can:
- Proceed to S phase
- Pause
- Withdraw to G0 ( a non-dividing state)
- Withdraw permanently from the cell cycle
Involves the G1/S CDK-cyclins (don’t need to know why)
What are mitogens, what are they also known as, what do they do and what does mitogenic mean?
Mitogens = outside signals which bind to a receptor and promote proliferation via positive signals
Mitogenic = causes mitosis
Mitogens also known as growth factors
What is contact inhibition and what is different about this in cancer cells?
When a neighbouring cell comes into contact with the cell, it prevents proliferation of the cell (signal that it is getting too big)
Cancer cells don’t experience these signals, and so keep dividing
How are cells limited to a certain number of cell divisions (2), how do cells that need to constantly divide overcome this?
- Telomere DNA sequences on chromosomes shorten in each cycle of DNA replication (diagram in jotter)
- Once telomere length decreases past a certain point, the cell senses this and stops the cell from dividing (makes senescent) or kills the cell to prevent further division
Telomere length can be restored by a mechanism involving the enzyme telomerase in cell that need to divide constantly like stem cells
What is tumorigenesis?
What is metastasis?
Tumorigenesis = abnormal, uncontrolled cell proliferation
Metastasis = spread of cancer cells to other parts of the body
What 4 situations cause uncontrolled proliferation in cancer cells?
What 2 molecules can cancer mutations occur in and what is each molecule involved in?
- Inappropriate expression of “positive signals” (overexpression)
- “think” they are receiving positive signals when they’re not
- Failure to produce/respond to negative signals
- Failure to senesce (can re-express telomerase)
Mutations can be in the “activating ras molecule or the “inactivating” Rb molecule
What is p53 and what can mutations in p53 lead to?
How does p53 work and what 3 outcomes can it lead to?
What are the 4 types of stressors that can induce this?
p53 = protects cells from damage and the consequences from damage. It’s a transcription factor which binds to DNA.
Mutations = cancer
p53 is usually unstable (ubiquitylated)
After various stressors, it becomes stabilised by phosphorylation which causes: cell-cycle arrest, senescence, or apoptosis
Stressors:
- Hyperproliferative signals
- DNA damage
- Telomere shortening
- Hypoxia
See diagram in jotter
