Lecture 22 Cell Cycle I Flashcards
1
Q
What are the two steps of cell reproduction?
A
Duplicate contents of cell
Cytokinesis
2
Q
What are the three major functional aspects of the cell cycle?
A
Cell growth and chromosome replication
Segregation of chromosomes
Cell division
3
Q
How many mistakes are made in one cell division?
A
6
mistake rate 1 x 10 ^-9
6.4 X 10^9 base pairs in diploid genome
4
Q
How many cells in the body
A
3x10^13
5
Q
How many cell divisions in one lifetime?
A
10^16
6
Q
Liver cell cycle =
A
1 year
7
Q
How many RBC?
A
25 trillion
8
Q
How many RBC produced per second?
A
2.4 million
9
Q
What is a disease of excess cell proliferation?
A
cancer
10
Q
What is the cell-cycle control system?
A
Complex network of regulatory proteins
Ordered series of biochemical switches
11
Q
What initiates main events of cell cycle?
A
Reg proteins
12
Q
What coordinates events in the cell cycle so that they occur at the appropriate time?
A
proteins
13
Q
When are chromosomes duplicated in the cell cycle?
A
S phase
14
Q
Chromosome segregation and cell division occurs during what phase of the cell cycle?
A
M
15
Q
Prophase:
A
Chromosomes condense into rigid rods called sister chromatids
16
Q
Metaphase:
A
Line up at equator; attach to spindles
17
Q
Anaphase:
A
Sister chromatids become daughter chromosomes; pulled apart
18
Q
Telophase:
A
Spindle disassembles, separate nuclei, cytokinesis
19
Q
GAP phases:
A
cells have extra gap pahses to allow more time for growth
G1 phase between M and S
G2 phase between S and M
20
Q
How many phases to the cell cycle?
A
4
21
Q
What makes up interphase cycle?
A
G1, S G2
22
Q
M phase last how long?
A
1 hour
23
Q
What is an important switch point?
A
G1 - decide to go into cell cycle or stay resting
24
Q
Checkpoint I:
A
START-cell commits to cell cycle entry and chromosome duplication (also called restriction point)
Going from G1 to S
Qs: is the environment favorable
25
Checkpoint II:
G2/M - chromosome alignment on spindle in metaphase
| Q?: is all DNA replicated, is the environment favorable
26
Checkpoint III:
metaphase - to - anaphase transition - trigger sister chromatid separation and cytokinesis
Qs: are all chromosomes attached to the spindle
27
What is the model organism to study cell cycle?
Yeast
| Animal embryos
28
Fission yeast
Grows by elongation at ends; division occurs when septum or cell plate forms midway along rod-shaped cell
29
Budding yeast
Oval yeast that divides by forming a bud; the bud first appears at G1 and grows until mitosis phase
30
Genes identified in yeast encode components of what?
cell-cycle control system = Cdc genes: cell-division cycle genes
31
What are mamalian cell lines that can be used to study cell-cycle?
fibroblasts
| However, these cells stop dividing in culture after a # of cell cycles
32
Immortalized cell lines:
Cell lines grow forever
Murine erythroleukemia cells
Useful for studying erythroid cell development and the generation of RBC
Also HEL cells
33
If conditions are unfavorable for cell division, the cell cycle control system will do what?
Signal to block progression through START
34
What does the cell cycle control system use to turn on various steps of the cell cycle?
Series of biochemical switches made of cyclin dependent kinases (Cdks) - phosphorylate proteins to activate them
35
What is the heart of the cell-cycle control system?
Cdks
| Activities of Cdks rise and fall during cell cycle
36
What do Cdks cause?
cyclical changes in phosphorylation of substrates that regulate cell cycle events
37
What are proteins that regulate Cdks?
cyclins
| The level of cyclins vary and cycles during the cell cycle - therefor their name
38
What are Cdks dependent on?
Cyclins - must be bound to cyclin to have activity
39
Cyclin levels or Cdks levels are constant?
Cdks
40
G1/S cyclins:
Activates Cdks in late G1
Helps trigger progression through START
commitment made to cell cycle entry
Levels drop in S phase
41
S cyclins
Bind Cdks after progression through START
Helps stimulate chromosome duplication
S-cyclin levels remain high until mitosis
42
M-cyclins
Activate Cdks that stimulate entry into mitosis at G2/M checkpoint
M-cyclins removed mid-mitosis
43
G1 cyclins
Govern activity of G1/S cyclins (control progression through START checkpoint)
44
What are the 4 Cdks in vertebrates?
Two interact with G1 cyclins: G1-Cdk
One with G1/S and S cyclins: G1/S-Cdk and S-Cdk
One with M-cyclins: M-Cdk
45
Without cyclin bound (inactive state), the active site of Cdk is blocked by what?
a region of the protein called the T loop
46
PHosphorylation of Cdk caused by CAK causes what?
phosphorylation of Cdk at T-loop fully activates enzyme
(cave site)
For Cdk to be completely activated it needs cyclin and CAK
47
Cdk activity determined primarily by rise and fall of cyclin levels, however a different phosporylation of Cdk inhibits activity of cyclin-Cdk complex. Describe what occurs in the roof site of Cdk
Wee1: inhibits Cdk activity by phosphorylating the roof site
Cdc25: phosphatase that dephosphorylates roof site to increase Cdk activity
48
CKI proteins:
Causes Cdk-cyclin to be inhibited
A CKI (p27) binds to both Cdk and cyclin to inactivate
This is primarily used for control of G1/S-Cdks + S-Cdks early in cell cycle
49
INK4A gene causes what?
inhibitory effects on Cdk
It is a CKI involved in the G1 phase of cell cycle
Mutation occurs in this gene in hereditary melanoma
Cannot control cell cycle and cells grwo uncontrollably and you get cancer
50
What is a major tumor suppressor?
p53
| p53 influences the expression of many genes
51
How is p21 created and what is its function?
It is a gene that is up-regulated by p53.
p53 is a major tumor suppressor
p21 is a CKI to stop division
Without it the cells will divide uncontrollably
52
G1/S-Cdk makes what cyclin?
Makes S cyclin
| *G1/S-Cdk is important for movement through START*
53
S-Cdk is important to start what?
DNA replication
54
How are CKI's destroyed so that Cdks like s-Cdks can be activated?
p27 is a CKI that binds to both cyclin and Cdk (CKI)
Proteolysis of CKI's can turn on the Cdk.
Ubiquitin system stimulates destruction of regulatory proteins
Use protein called SCF-ubiquitin ligase to add ubiquitin
Target for proteasomes
*SCR activity depends on F-Box proteins (help SCF recognize target)
55
Why is there lots of primed M-Cdk around by the end of G2?
It is formed in its inactive formed
Phosphorylated by CAK to activate it but Wee1 holds M-Cdk in inactive state
When ready to be activated it is dephosphorylated by Cdc25
*note* when M-Cdk is finally acitvated, it can cause positive feedback on Cdc25 or Wee1, so promotes and suppresses. Good to regulate and prevent cancer
56
Progression from metaphase to anaphase is triggered not by protein phosphorylation but by what instead?
Protein destruction
| Key regulator is APC/C: anaphase-promoting complex (or cyclosome)
57
What is APC/C?
A member of ubiquitin ligase family of enzymes
Catalyzes addition of ubiquitin to proteins to cause destruction
2 major proteins it affects are cohesin and securin in that order
Levels rise mid-mitosis
58
What is Cohesin?
Sister chromatids are glued together along their length by this protein
Members of SMC proteins
Forms rings around sister chromatids
Protected by securin
59
What is Secruin?
Protects cohesin protein linages that holds sister chromatid pairs together in early mitosis
Does so by inhibiting a protein called separase that cleaves cohesin
60
Describe how APC/C allows sister-chromatids to separate and have anaphase begin
APC/C levels rise mid-mitosis
Add ubiquitin on securin
activates separase
Separase breaks down cohesin
61
What does increased levels of APC/C do to Cdk-cyclin complexes?
Adds ubiquitin on cyclins and Cdks are inactivated
62
What cyclins are the major target of APC/C?
S and M-cyclins
| Dont want to duplicate chromosomes again
63
How is APC/C activated?
By binding to Cdc20
64
What stimulates genes making G1/S-cyclin and S-Cyclin?
G1-Cdk
65
What triggers prophase, prometaphase and metaphase at G2/M stage?
Increase of M-Cdk activity
* assembly of mitotic spindle
* attachment to sister chromatids
66
What complex is critical in order for chromosomes to be duplicated only once?
PRE-RC and for it to be only present during G1 phase
No new PRE-RC are made during S and M phases
Between G1-G1 NO MORE!!!!!
67
What inhibits assembly of PRE-RC?
Cdk activity
MAJOR POINT!!!!
When S-Cdk and M-Cdk are high during S and M stage NO PRE-RC IS FORMED
68
What is the assembly of PRE-RC stimulated by?
APC/C
| !!!!! So PRE-RC assembly occurs in late mitosis and early G1 when Cdk activity is low and APC/C activity is high
69
How is the PRE-RC complex created?
```
In G1
Two proteins (Cdc6 and Cdt1) bind ORC and help load 6 proteins (Mcm proteins) - creates helicase to unwind DNA
```
The Cdc6, Cdt1 bound to ORC and the 6 proteins creates the PRE-RC
70
What happens to the PRE-RC during the cell cycle
Once the cell cycle begins, S-Cdk triggers assembly of pre initiation protein complex.
S-Cdks triggers disassembly of the Cdc6 (part of PRE-RC complex) by phosphorylating it
Also phosphorylates ORC to inhibit function
Cdt1 inhibited by geminin
= preinitiation complex can bind to DNA, unwinds, and begins DNA replication
71
What is a KEY STEP in the control of initiation of DNA replication?
S-Cdks blocks formation again of pre-RC by causing destruction of Cdc and inactivation of ORC by phosphorylation
72
How are PRE-RC components reformed after M phase?
APC/C causes components to be dephosphorylated
In late mitosis and early G1, APC/C triggers destruction of geminin, which had inhibited Cdt1
Cdt1 activated now allows PRE-RC assembly for next cell cycle
73
What is a key point about the components of PRE-RC?
Cdc6, Cdt1, Mcm helicase cannot form a new PRE-RC until M-Cdk is inactivated and APC/C is activated at the end of mitosis
