Cell Cycle Flashcards
1
Q
What 2 things must cells be able to do
A
Multiple and differentiate
2
Q
What did Virchow say
When
A
“Omnis cellula e cellular”
1858
3
Q
What is used to mark when observing DNA
A
Histones
4
Q
Cancer is a disease of hyperplasia
True or false
A
True
Therefore many treatments inhibit cell division
5
Q
Other than cancer, what diseases affect the cell cycle
A
Some Viruses cause hyperplasia and even cancer
Cylcomodulins allow pathogenic bacteria to control the host cell cycle
Rare genetic diseases eg microcephaly can be caused by cell cycle mutations
6
Q
What are the phases of the cell cycle
A
M -> G1 -> S -> G2
⬆️ ⬇️ ⬆️⬅️⬅️⬅️⬅️⬅️⬅️
7
Q
What is S phase
A
Synthesis phase (DNA replication)
Making a perfect copy of the genome
8
Q
What is M phase
A
Mitosis and cytokenesis
Nuclear division - splitting DNA between 2 daughter cells
9
Q
What is cytokinesis
A
Cytoplasmic division
10
Q
What happens in the gap phases
A
Critics for maintaining cell size
Cell growth
Also where cell responds to cues
11
Q
What is G0 phase
When is it
A
Prolonged exit from cell cycle
Early G1 phase
12
Q
Which cycle phase must cancers overcome to induce hyperplasia
A
G0
13
Q
When do cells only go through half the cycle
A
Never - it is an all or nothing process
14
Q
What keeps the cell cycle as an all or nothing process
A
Check points - you can proceed unless the previous phase is complete
15
Q
What are the cell cycle checkpoints
A
Spindle assembly checkpoint (SAC)
START (G1/S checkpoint) - commit to entire cycle
G2/M checkpoint - is replication complete
16
Q
How do you build a molecular switch
A
Enzyme is only active when bound to a cofactor which then inhibits the inhibitor of the enzyme creating a bistable switch
17
Q
What is the graph of Cofactors concentration vs enzyme activity if the enzyme requires the cofactors to be active
A
Hyperbolic relationship - increases until enzyme is saturated then plateau
18
Q
What is the graph of a bistable switch
A
Cofactors concentration vs enzyme activity
Shallow positive gradient (off), then v steep gradient, then shallow (on)
19
Q
Explain bistable switch phases
A
Off- when inhibitor concentration is high and cofactor concentration is low - inhibitor is winning so enzyme is “off”
Steep- after a critical point when enough enzyme is activated to inhibit the inhibitor - positive feedback loop - rapid enzyme activation
On- all enzymes are on
20
Q
What do bistable switches ensure
A
You can only go one way and the phase is completed in full
21
Q
What is the master regulator in the cell cycle
A
Cyclin Dependent Kinase (CDK)
22
Q
What do kinases do
A
Transfer phosphates usually from ATP to a substrate
23
Q
How does CDK act on its own
A
It doesn’t it is inactive by itself
It must be activated by binding to different cyclins in different phases of the cell cycle
24
Q
Which cyclin is present in G1
What does it activate
A
Cyclin E
CDK2
25
Describe how the cyclins change in concentration throughout the cell cycle
Cyclin E is high in G1
Cyclin A begins to rise after the START checkpoint and remains high throughout S and G2. Cyclin A then drops at G2/M checkpoint, falling to 0 by the middle of M phase
Cyclin B begins to rise at the end of S phase, rises throughout G2 and plateaus in the first half of M phase and drops to 0 after the SAC checkpoint
26
What does cyclin A activate
CDK2
27
What does cyclin B activate
CDK1
28
When do cyclin D levels rise
What does it activate
G1
CDK4/6
29
Which cyclin and CDK is necessary for replication of genome (S phase)
Cyclin A
| CDK2
30
Which cyclin and CDK is necessary for mitosis
Cyclin B
| CDK 1
31
How are cyclin levels regulated
Cyclin transcription is tightly controlled within the cell cycle
Cyclin levels are tightly controlled by ubiquitin mediated degradation
Inhibitory phosphorylation
CDK inhibitor proteins
32
What are the transcription factors that regulate cyclin transcription
E2F transcription factors
(1-5)
1,2,3 are activators
4,5 are repressors
33
What are the co repressors of the E2F transcription factors
Rb family (repress activator E2F)
p107 and p130 exacerbate repressor E2F proteins
34
What was the first cloned tumour suppressor protein
Rb
All cases of retinoblastoma are caused by Rb mutations
35
What does Rb do in G0 phase
Binds to E2F proteins and inactivates them
36
How do repressor E2F proteins work
Give an example and how it can be exacerbated
They are bound to DNA and inhibit cyclin transcription
Eg E2F4 inhibits cyclin E gene transcription
This is augmented by p107 binding to E2F4
Turn off these genes so cell cycle can’t start
37
How do you start the cell cycle
Mitogens activate cyclin D-CDK4, this deactivates p107 and Rb to inhibit the repressors
38
What does the phosphorylation of Rb by cyclin D-CDK4 do
What about phosphorylation of p107
Releases Rb from E2F1, allowing E2F1 to bind to the target promoters
No longer bound to E2F4, so doesn’t help inhibition
Now cyclin E can be transcribed
39
How does ubiquitin degrade substrates
A chain of ubiquitin bound to a substrate is a signal to the proteasome to destroy the substrate
40
What binds ubiquitin to a substrate
Ubiquitin ligases
41
Name an important ubiquitin ligase in the cell cycle
Cyclosome (APC/C)
Or
Anaphase promoting complex
42
When is APC/C active
When bound to an activator
Either Cdc20 or Cdh1
43
What does APC/C - cdc20 degrade
Cyclin B
| Securin
44
What does the APC/C - cdh1 complex degrade
Cyclin A
45
When is APC-cdc20 active
After mitosis
46
When is APC-cdh1 active
After G0
47
How do cyclins affect APC-cdh1
What does this result in
Cyclin E- CDK2 and cyclin A-CDK2 both inhibit APC-cdh1
Bistable switch
48
How do cyclins affect APC-cdc20
Cyclin B - CDK1 activate APC-cdc20
| It activates its own inhibitor
49
What causes inhibitory phosphorylation of cyclin CDK complexes
Why is this called this
Wee1 kinase
Without Wee1 there is not enough inactive cyclin CDK so cells go through the cycle too quickly and end up being v small
50
What acts antagonistically to Wee1
Cdc25 phosphatase
| Which activates cyclin-CDK complexes
51
When are CDK inhibitor proteins active
In G0 and G1 if you want to be quiescent and you want CDK to be fully inhibited
52
What transition must be overcome to cause hyperplasia
G0 to G1 transition
53
What is a mitogen
Give an example
What does this example bind to
A (usually)extracellular signal that tells a cell to divide
Eg EGF binds to EGFR (a tyrosine kinase receptor)
54
What molecule is almost always involved in starting the cell cycle after the mitogen binds to a receptor
What does this activate
RAS
(Active RAS is bound to GTP, it is inactive when bound to GDP)
MAPK
55
What is the important thing that MAPK activates for the cell cycle
Transcription eg Transcription of MYC
56
What does Myc do for the cell cycle
It is a transcription factor that leads to Cyclin D - CDK4 expression
Also inhibits inhibitors (p16 INK4a) of Cyclin D CDK4
57
How common is EGFR mutated in MSC lung carcinomas
What does this mutation do
20%
Makes it independent of mitogen
58
How common are Ras mutations in human tumours
20-25% of all human tumours
59
How often is Myc altered in cancer
4% of all cancers
60
Do you get multiple mutations in different parts of the cascade leading to entry to the cell cycle in cancer ?
No
| Only one part will be mutated, there is no benefit of having more
61
How do cancer viruses cause cancer
Inhibiting Rb
62
How does HPV inhibit Rb
What about Adenovirus
What about SV40
KSHV
E7 is expressed which inhibits Rb
E1A is expressed
T-antigen
vCyclin
63
What does cCyclin do
Expressed due to KSHV
vCyclin is v similar to Cyclin D2, which binds to CDK6, driving the cell into cell cycle
64
What do you call signals that tell cells to stop dividing
Give an example
Antimitogens
TGF-β
65
How does TGF-β work
Activates SMADS
66
How do SMADS prevent cell cycle progression
They are a co repressor of p107-E2F4 complex - add extra layer of repression
Also Activate p15 INK4b which inhibits Cyclin D-CDK4
67
How are anti mitogens involved in cancer
Give an example
SMADS are deleted
SMAD4 is mutated/ deleted in 55% of pancreatic cancers
68
Talk about the Belgian blue cow
They have mutations in the anti-mitogen myostatin which stops proliferation of myoblasts so end up 2x amount of muscle cells
69
What are the problems with DNA replication
Accuracy and rate
70
How do cells over come the rate of replication problem
Multiple origins
Human cells have roughly 50,000 origins
So replication can occur multiple times in S phase
71
What is the problem that arises from having multiple origins
They must all fire in a coordinated manner
72
How do we coordinate the cell cycle
Replication initiation is a 2 step process
Step 1: licensing
Step 2: activation of helicase and recruitment of polymerases
73
What is licensing
When does this happen
Helicase loading - separating 2 strands
G1 phase
74
Is the cell ready to enter the cell cycle after licensing?
No
| Licensing leads to an inactive complex
75
When does step 2 of replication initiation occur
S phase
76
What stops the 2 steps of replication initiation happening simultaneously
They are mutually exclusive
77
Why are step 1 and step 2 of replication initiation mutually exclusive
Cyclin A -CDK2 is required for step 2
APC-cdh1 is active in step 1
The high [APC- cdh1] rapidly degrades Cyclin A-CDK2 so step 2 cannot be initiated
Also inhibits Geminin which is needed for step 2
Equally, cyclin A - CDK2 inhibits APC-cdh1, and absence of APC - cdh1 allows geminin to to accumulate and thus helicase loading is inhibited. This prevents the previous step occurring at the same time a
78
What does geminin prevent
Loading of helicase
CDK2 also inhibits this
79
What do you want from the 2 daughter chromosomes during mitosis
You want the 2 different chromosomes together at either side of the cell
80
How do cells ensure the 2 sisters don’t end up together in mitosis
2 sisters are bound together and align in the centre so they can be pulled apart to opposite sides of the cell
81
How is DNA supercoiling resolved during mitosis
What is this called
The action of topoisomerases which rotate the chromosome, leading to the sisters becoming intertwined
Topological linkage
82
What is a type 1 topoisomerase
Causes ssDNA nicks
83
What is Type 2 topoisomerase
Causes dsDNA breaks
84
Other that topoisomerase enzymes, what enzyme class can resolve intertwines in DNA
NONE
| Only topoisomerases can
85
Other than topologically, how does replication result in sister chromosomes being held together
Cohesion of the sisters by cohesin
86
describe the structure of cohesin
A ring formed by 2 dimers bound at the top by each dimer’s head and a gap at the opposite end, which is closed by a protein called Scc1
87
What does cohesin do
Wraps around 2 daughters after replication in S phase
88
Where is the last place cohesin is lost from sister chromosomes
The centromere
89
How are sisters aligned in the middle of the cell
Mitotic spindles
90
What are spindles made of
Tubulin which make up polymers called microtubules
91
Other than for spindles what are mircotubules needed for
The structure of the cell
92
What is the kinetochore
The point of attachment of microtubules to the chromosome
93
What is the centromere
The region of the chromosome that assembles the kinetochore
94
What is the centrosome
The microtubule organising centre
95
What are astral microtubules
Emanate away from spindle to the cell vortex to position the spindle in the middle of the cell
96
How do spindles arrange the chromosomes in the centre of the cell
Microtubules released from the centrosome on one side bind to the chromosome, but only when spindles from both sides bind will tension be generated
The tension will be equal in each microtubule when the bound chromosome is in the centre of the cell
97
What happens if the microtubules bind incorrectly to the kinetochore
Low tension
If only one microtubule binds, there will be low tension
If two microtubules from the same side bind to 2 kinetochores on the same chromosome, there will be low tension.
This is how Two centrosomes ensure the chromosomes are aligned in the centre of the cell
98
Which molecule is involved in splitting the sisters after they’re aligned in the cell
What does it do
APC-cdc20
Degrades cyclin B and securin
99
What does securin do
Secured the cohesin ring by binding to and inhibiting separase
100
What does separase do
When is it active
What can happen now
Cleaves cohesin ring by cleaving scc1 gate
When APC-cdc20 is present as it degrades cyclin B (which phosphorylates and inhibits separase) and securin (which inhibits separase)
Spindles can pull sisters apart
101
What does chromatin mean
What about mitosen
Stainable
Thread
102
What are the phases of mitosis
Prophase➡️ prometaphase ➡️ metaphase ➡️ anaphase ➡️ telophase ➡️ cytokinesis
103
What happens in the first phase of prophase
Chromosomes condense in the nucleus
Centrosomes separate in the cytoplasm
104
What happens in prometaphase
Nuclear envelope breakdown and kinetochores start to attach to microtubules
105
What happens in metaphase
Chromosomes align in the centre in the centre of the spindle
After this APC - cdc20 is turned in
106
What happens in anaphase
Sister chromosomes synchronously separate
107
What happens in telophase
Two nuclear envelopes reassemble
108
What is the final stage of mitosis
What happens here
Cytokinesis
Division of the cytoplasm
109
Which cyclin / CDK is needed for centrosome duplication
Cyclin E - CDK2
110
Which cyclin / CDK is needed for genome duplication
Cyclin A
| CDK2
111
What are the 2 important events that cyclin E - CDK2 initiate
```
Centrosome duplication
Genome duplication (via activation of Cyclin A -CDK2
```
112
Where is cyclin B-CDK1 found initially
Why
Cytoplasm
It controls the movement of centrosomes to opposite ends of the cell
And
Controls cell rounding
113
What is cell rounding
Changing the shape of the cell - if the cell is attached to another cell it must detach, go through mitosis, then reattach
114
How does cyclin B CDK1 enter the nucleus
What does it do once it’s in there
Why is this a badly phrased question
By breaking down the nuclear membrane
Inhibits spindle elongation and activates APC -cdc20
Cyclin B CDK1 breaks down the nuclear membrane so there is no nucleus so it can’t be inside the nucleus
115
5 things Cyclin B - CDK1 does to prepare for mitosis
```
Centrosome movement
Cell rounding
Nuclear membrane breakdown
Inhibits spindle elongation
Activates APC-cdc20
```
116
Why is cyclin B CDK1’s inhibition of spindle elongation important
Prevents spindle pulling apart too early and stops anaphase happening too early
117
What is the SAC
Spindle assembly checkpoint
| Checks all chromosomes are attached to the mitotic spindle before anaphase can occur
118
What checkpoint ensures replication is complete
How
G2/M
Use of check point kinases
119
Which check point kinases are used at the G2/M checkpoint
ATM and ATR which then activate check 1 and check 2 (both kinases)
120
What do checkpoint kinases do specifically
What does this do ultimately
Inhibit activation of Cyclin B-CDK1 by inhibiting cdc25 phosphatase and activating Wee1 kinase
Prevents progression into mitosis
121
What tells the The cell not to activate APC yet as kinetochores are not fully attached
SAC - a signal from the unattached kinetochores - the MCC (mitotic checkpoint complex)
This sequesters and inhibits the APC by inhibiting cdc20 subunit
cdc20 is released when all kinetochores are attached and anaphase is allowed
122
How many cancer mutations are caused by random mutations during replication according to which 2017 study
66%
Tomasetti and Vogelstein
123
Most solid cancers are aneuploid. True or false?
True
124
How do cancers become aneuploid
They have more than 2 centrosomes, giving a random distribution of chromosomes
125
How do you get multiple (>2) centrosomes
Mutations in cyclin D-CDK4 pathway (eg of Rb) leads to too much cyclin E and too many centrosomes (as cyclin E leads to centrosome duplication)
126
How was nitrogen mustard discovered to be a treatment for cancer
In WWII when it was used as mustard gas
| Victims were seen to have no RBCs so nitrogen mustard must inhibit replication (RBCs are rapidly dividing)
127
Where do taxane drugs come from
The Pacific yew tree
128
Why may chemotherapy lead to further cancerous evolution
Selection pressure
And
Anything that messes with the cell cycle will result in mutations, which cancers thrive off, and chemotherapy tackles cancer by inhibiting DNA replication or mitosis (both parts of the cell cycle)
129
What kind of cancer drug are Astra Zeneca interested in
What kind of mutant does this attack
PARP inhibitors
BRCA
130
Easy way to remember to remember the phases of mitosis
```
Please (prophase)
Pass (prometaphase)
Me (metaphase)
A (anaphase)
Taco (telophase)
Chief (cytokinesis)
```
