1
Q
What are the two major consequences of cell cycle dysregulation?
A
- Genome instability (i.e. the complement of chromosomes becomes unstable)
- Chromosome loss, duplication and rearrangement (aneuploidy).
- Defective DNA repair: mutagenesis.
Activation of proto-oncogenes.
Inactivation of tumour suppressor genes.
- Too much cell proliferation –> tumour development
- Due to genome instability = inactivation of tumour suppressor genes)
2
Q
What are the two major phases of the cell cycle?
(hint - don’f forget what happens between them!)
A
- M phase = mitosis and cytokinesis
- S phase = DNA synthesis
- G phase = Gap phases
- G1 (in between M and S)
- G2 (in between S and M)
3
Q
What is interphase?
A
Interphase = everything apart from M phase (i.e.G1 + S + G2)
- Cells grow in interphase
- Organelles duplicate/multiply.
- Durations vary with species, cell type, conditions etc.
4
Q
Define post-mitotic and G0
A
- Post-mitotic = cells that have come out of the cell cycle
- G0 = quiescent
*Neither of these states are permanent *
5
Q
What are the 6 stages of mitosis?
A
interphase
Prophase
Pro-metaphase
Metaphase
Anaphase
Telophase
6
Q
Outline the first stage of mitosis
A
Phase 1 = interphase, G2
- Nucleus is full of DNA in its uncondensed form
7
Q
Outline the second stage of mitosis
A
Phase 2 = prophase
- Sister chromatids form
8
Q
Outline the third stage of mitosis
A
Phase 3 = pro-metaphase
- Nuclear envelope begins to breakdown
- Pro-metaphase chromosomes appear
- Two kinds of microtubules present kinetochore microtubules and non-kinetochore microtubules
9
Q
Outline the fourth stage of mitosis
A
Phase 4 = metaphase
- Fully condensed chromosomes
- Nuclear envelope has completely degraded
10
Q
Outline the fifth stage of mitosis
A
Phase 5 = anaphase
- Chromosomes are pulled apart to the spindle poles (by the kinetochores)
11
Q
Outline the sixth stage of mitosis
A
Phase 6 = telophase
- Nuceloles begins to form
- Nuclear envelope re forms
- Cytokinesis begins
12
Q
What are the two types of microtubules?
(hint - these appear at pro-metaphase)
A
- Kinetochore = handles of the chromosomes
- Non-kinetochore = attach to each other, not the chromosome
13
Q
What is cytokinesis?
A
Cytokinesis – the process of creating two daughter cells from one cell
14
Q
At what stage (of mitosis) do spindles form?
A
During metaphase (phase 4)
15
Q
What does the primary spindle apparatus consist of?
A
Centrosomes and Microtubules
16
Q
What is the purpose of the centrosome?
A
Organelle which serves as the primary microtubule organizing centre (MTOC)
17
Q
Describe the structure of centrosomes
A
- Consists of two centrioles (barrels of nine triplet microtubules)
- Fibres are made of tubulin forming a barrel-like structure
18
Q
What are the spindles made of?
A
tubulin + associated proteins
19
Q
Tubulin spindles form ASTERS - what is this?
A
Radial microtubule array around the centrosome
(the spindles radiate out from the MTOC)
20
Q
What two structures can form after the radial tubulin arrays meet?
A
Form either a kinetochore microtubules or non-kinetochore microtubules after the radial arrays meet
21
Q
What must be degraded for metaphase –> anaphase?
A
Cohesin (molecular glue)
22
Q
What is the result of anaphase?
A
Appearance of daughter chromosomes
23
Q
During anaphase, how do the chromatids separate forming daughers?
A
Daughter chromosomes pulled toward opposite spindle poles - the shortening of the microtubules is what is pulling the sister chromatids (i.e. daughter chromosomes) from each other
24
Q
What is the spindle assembly checkpoint?
A
Prevents anaphase from starting too early - ensures that anaphase does not begin until the sister chromatids are correctly attached to the spindles
25
What is the consequence of incorrect spindle attachemnt
**Aneuploidy** - incorrect chromosome number within the cell
## Footnote
26
List the various _incorrect_ spindle attachments and their consequences
* ******Syntelic attachment** – both daughter chromosomes will go to the same cell and the other cell will not have the chromosome (i.e. it will only have one parental copy whereas the other cell would have three parental copies)
* **Monotelic attachment** – one daughter chromosome is not attached to a spindle therefore will not be incorporated into a future daughter cell
* **Merotelic attachment** – one daughter chromosome is simultaneously attached to both spindles therefore it is pulled in half
27
What is the stimulus for the start of telophase?
Daughter chromosomes arriving at their appropriate poles
28
What assists in the formation of new nuclei?
Actin molecule constricts between the two nuclei pinching the one cell into two daughter cells
29
What major events occur during S phase?
S phase (synthesis i.e. DNA replication) - phase of the cell cycle when DNA packaged into chromosomes is replicated (i.e. transcription & translation)
30
What is a replication origin?
Replication orgin = sequence in genome where replication is initiated
* Chromosomes have multiple DNA replication origins
* Replication origins are always upstream of the replication bubble
* Multiple origins fire at varying times during S phase however each origin must fire once per S phase
* The S phase cannot be repeated until the cell has passed through an M phase (i.e. the same replication origin cannot fire again until the cell has gone through a complete M phase)
31
What kind of proteins control the cell cycle?
Cyclically activated protein kinases
32
What are Cyclin-Dependent Kinases (Cdks)?
Cdks are an example of the cyclically activated protein kinases
33
What is the major function of Cdks?
Phosphorylate proteins (serine or threonine) to effect cell cycle progression
34
Where are Cdks found?
Cdks are present in proliferating cells throughout cell cycle
## Footnote
35
What regulates Cdk activity?
* Interactions with **cyclins** (type of protein)
* Phosphorylation
36
What are cyclins?
Proteins transiently expressed at specific points in the cell cycle involved in Cdk regulation (therefore cell cycle regulation)
## Footnote
37
How is cyclin activity regulated?
* Regulated at the level of expression – this creates a wave-like pattern of expression throughout the cell cycle
* Synthesised, then degraded
38
How do Cdks and Cyclins associate?
Cdk + Cyclin form a heterodimer
Humans have multiple cyclin molecules therefore there are multiple potential heterodimers
39
What is MPF?
An important cyclin molecule = mitotic cyclin which when in a heterodimer with cdk forms mitosis promoting factor (or MPF)
40
What major reaction type is involved in the activation of MPF (and other cyclin/Cdk heterodimers)?
Phosphorylation
41
Outline the activation of MPF
1. CDK1 + Cyclin --\> MPF (inactive)
2. CAK + Wee1 phosphorylate MPF --\> MPF-Pi-Pi
3. Dephosphorylation (of MPF-Pi-Pi) by CDC-25 --\> Active MPF (MPF-Pi)
42
What activates the two MPF co-factors CDC and Wee?
* Wee = dephosphorylation
* CDC = phosphorylation
43
When are the levels of cyclin expression at their (a) highest and (b) lowest
* Peaks occur during mitosis where formation of MPFs are highest
* Lowest levels (i.e. the most degradation) is during interphase
44
What are the two major types of CKIs?
CKIs = Cdk inhibitors
* **INK4** – G1 phase CKIs, inhibit Cdk4/6 (either alone or bound)
* **CIP/KIP** – S phase, inhibit all bound cdks
45
How is mitotic cyclin degraded?
**Anaphase Promotion Complex/Cyclosome (APC/C)** is responsible for degradation of mitotic cyclins
* APC/C is key for the metaphase-anaphase transition (i.e. the degradation of molecular glue - cohesin)
* Key point – _ubiquitination_
* CDK activates its own destruction
46
What are cell cycle checkpoints?
Cell cycle checkpoints are mechanisms that ensure the different events of the cell cycle occur on cue and in the correct order.
47
What role does APC/C have in cell cycle control?
APC/C also promotes anaphase (i.e. it is what starts anaphase) by _targeting securin for degradation = cohesion degradation_
1. APC/C targets securing
2. Securin bound to separase – acts as lock mechanism preventing the degradation of cohesin by separase
3. Separase – degrades cohesin (when active)
48
What two factors make up the spindle assembly checkpoint? What is their role?
**Mad2/BubR1** detect unattached kinetochores and inhibit APC/C
49
Explain how the spindle assembly checkpoint works
If kinetochores are attached then Mad2/BubR1 are inactive and allow APC/C to perform its function (as above)
If the kinetochores are not correctly attached at the end of metaphase, Mad2/BubR1 inhibits APC/C so that it cannot degrade securin --\> separase --\> cohesin therefore anaphase will not commense
50
How are cyclins involved in cell cycle control?
Different cyclins and different Cdks are required at different stages of the cell cycle (so they also have a role in the progression of the cell cycle)
51
How are Cdks involved in cell cycle control?
Each Cdk phosphorylates different substances at different stages of the cell cycle
52
Which cyclins and Cdks assosiate to deactivate retinoblastoma (Rb)?
Cdk2 forms heterodimer with G1 cyclin to phosphorylate retinoblastoma (Rb) protein at the start of the cell cycle
53
What is the role of Rb?
Rb (retinoblastoma) ats as a brake on the cell cycle
## Footnote
54
What function does Rb play in the cell? What would the consequences of its dysregulation be?
Rb = tumour suppressor
(therefore duysregulation of the cell cycle = too much phosphorylation = less tumour suppression = tumour formation)
55
What action do Cdks have on Rb?
Cdks phosphorylate Rb = deactivate it
56
What two factors are requried to keep a cell dividing (instead of entering G0)?
growth factors and intracellular signalling cascades.
57
What is Myc?
Trigger for G1 (i.e. preparation for divison) - levels increased by binding of growth factors to cells
58
What is EGF?
epidermal growth factor - a key receptor in the signal transduction process for cell divison stimulation
59
What two potential fates are there if there is a problem during the cell cycle?
If there is an issue during the cell cycle there are two options – **cell cycle arrest** or **apoptosis**
## Footnote
60
Outline the process of cell cycle arrest
* Cell cycle arrest happens at the check points (G1 and spindle check point)
* This can be temporary, as if the cell can repair the DNA damage the cycle will continue.
* If not, cell cycle progression is aborted and the cell is destroyed.
61
At what point would apoptosis be initiated?
Apoptosis occurs if DNA damage is too great and cannot be repaired
* Examples - are chromosomal abnormalities, or if there are toxic agents.
* Cell cycle progression is aborted and the cell is destroyed.
MCD
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