Cell Cycle 1 Flashcards
In which phase does chromosome duplication occur?
S Phase
In which phase does chromosome segregation + cell division occur?
M Phase
What is cytokinesis?
Cell Division
What happens in prophase?
Chromosomes condense into rigid rods called sister chromatids (become attached to mitotic spindle: a bipolar array of microtubules).
What happens in metaphase?
Sister-chromatids line up at equator of cell attached to opposite poles of spindle.
What happens in anaphase?
Sister chromatids become daughter chromosomes and are pulled to opposite poles of spindle.
What happens in telophase?
Spindle disassembles, chromosomes packaged into separate nuclei, cytokinesis occurs.
What is the GAP phases?
Cells have extra gap phases to allow more time for growth:
> G1 phase between M and S
> G2 phase between S and M
Which phases of the cell cycle make up interphase?
G1, S, and G2
How long does the M phase last?
1 Hour
What is checkpoint I of the cell cycle?
START: G1 to S - cell commits to cell cycle entry and chromosome duplication (also called restriction point).
What is checkpoint II of the cell cycle?
G2 to M - chromosome alignment on spindle in metaphase.
What is checkpoint III of the cell cycle?
Metaphase to anaphase transition - trigger sister chromatid separation and cytokinesis.
Fission Yeast
> grows by elongation at ends
> division occurs when septum or cell plaste form midway along rod-shaped cell
Budding Yeast
> oval yeast that divides by forming a bud
> the bud first appears at G1 and grows until mitosis phase
What are Cdc genes?
Cell-Division Cycle genes
What did mutant yeast allow the discovery of?
Cdc genes
Give an experimental design using yeast to look at the START (G1/S) (commit to progress into cell cycle) of the cell cycle?
> Before Start: cells placed in poor nutrient medium will grow slowly and cell cycle entry is delayed.
> After Start: cells will continue into the cell cycle even if transferred to poor medium (because they already committed).
Frog embryo
Fertilized egg divides many times without growth and results in 4,096 cells from 1 cell filling same space.
In frog embryo the S phase is 15 minutes and the M phase is 15 minutes without a detectable G1 or G2 phase.
Cell Lines
Fibroblasts are a mammalian cell line that can be used to study cell-cycle.
However, these cells stop dividing in culture after a # of cell cycles - cease dividing after 25-40 divisions.
Immortalized cell lines
These cell lines grow forever.
For example, murine erythroleukemia cells (MEL cells).
Useful for studying erythroid cell development and the generation of red blood cells.
Can induce erythropoeisis in these cells with growth factors.
Also HEL cells (human erythroleukemia cell line).
The cell cycle control system uses a series of biochemical switches made of what, that turn on various steps of the cell cycle?
Cyclin Dependent Kinases (Cdks)
Phosphorylate proteins to activate them
Which protein is the heart of cell-cycle control?
cyclin-dependent kinase (Cdk)
the cell cycle is governed by Cdks
What do Cdks do?
Cause cyclical changes in phosphorylation of substrates that regulate cell cycle events.
Example: increased Cdk activity can increase activity of proteins controlling chromosome activation.
You know that Cdks govern the cell cycle, but which protein regulates Cdks?
cyclins
Cdks are dependent on cyclins - MUST be bound to cyclin to have protein kinase activity.
During the cell cycle how do the levels of cyclin and Cdk vary throughout?
> Cyclin levels vary according to the point of time in cell cycle.
> Cdk levels are constant.
What does the expression of varies types of cyclin proteins control?
Cyclin expression in cells control what step cell is in the cell cycle.
What are the 4 classes of cyclins?
> G1/S cyclins
S cyclins
M cyclins
G1 cyclins
What is the function of G1/S cyclins?
- activates Cdks in late G1
- helps trigger progression through START
- commitment made to cell cycle entry
- levels drop in S phase
What is the function of S cyclins?
- bind Cdks after progression through START
- helps stimulate chromosome duplication
- S-cyclin levels remain high until mitosis
What is the function of M cyclins?
- activate Cdks that stimulate entry into mitosis at G2/M checkpoint
- M-cyclins removed at mid-mitosis
What is the function of G1 cyclins?
- govern activity of G1/S cyclins (control progression through start checkpoint)
Vertebrates have 4 Cdks. Which ones interact with different cyclins?
> 2 interact with G1 cyclins.
> 1 interacts with G1/S and S cyclins.
> 1 interacts with M cyclins.
What is formed when cyclins interact with their repective Cdk?
cyclin-Cdk complexes
> G1-Cdk
G1/S-Cdk
S-Cdk
M-Cdk
What happens to Cdk activity after mitosis?
After mitosis cyclin levels fall and MUST fall!!
Thus, resulting in NO Cdk activity.
What protein initiates metaphase to anaphase transition?
APC/C
Without cyclin bound to Cdk it is in its inactive state. What blocks the active site of Cdk?
A region of the protein called the T loop.
What does the binding of cyclin to Cdk cause?
Causes T-loop to move out of active site (Cdk partly active now).
The binding of cyclin cause the T-loop to move out of the active site of Cdk, resulting in Cdk partly being activated. What fully activates Cdk enzyme?
Phosphorylation of Cdk at T-loop (“cave site”).
phosphorylated by CAK (Cdk activating kinase)
What protein phosphorylates Cdk at the T-loop?
CAK (Cdk activating kinase)
The level of which protein control the changes in cell cycle?
Level of Cyclins
Besides the level of cyclins controlling the activity of Cdks, which two other control mechanisms are involved?
Inhibition and Proteolysis
What protein phosphorylates the roof site of Cdk and inhibits activity of cyclin-Cdk complex?
Wee1
What protein dephosphorylates the roof site of Cdk to increase Cdk activity?
Cdc25
What protein causes Cdk-cyclin to be inhibited by binding to both Cdk and cyclin?
CKI proteins (Cdk Inhibitory Proteins).
Example would be p27.
NOTE: not inhibited by phosphorylation like Wee1 does.
At which point in the cell cycle are CKI proteins primarily used to control Cdk-cyclin complex activity?
Primarily used for control of:
> G1?S-Cdks + S-Cdks early in cell cycle.
What is INK4A?
A CKI protein involved in the G1 phase of cell cycle.
> Mutation occurs in this gene inhereditry melanoma.
Cannot control cell cycle and cells grow uncontrollably and you get cancer.
What is p53 and what gene does it up-regulate?
> p53 is a major tumor suppressor.
p53 influences the expression of many genes.
p53 up-regulates p21.
p21 is a CKI to stop division.
p21 transcription is a target of p53.
If p53 fails in its function - there will be lower p21 expression and without this CKI cells divide uncontrollably.
What is S-Cdk important to start?
DNA replication
When are S-cyclins produced?
As cell cycle advances through G1 into S.
What is G1/S-Cdk important for movement through?
START - makes S cyclin
We have discussed that a CKI protine (e.g., p27) can inhibit cyclin-Cdk complexes by binding to both of them. How can CKI proteins be destroyed so that cyclin-Cdk complex can be turned on again?
PROTEOLYSIS of CKI’s can turn on cyclin-Cdk activity.
> Protein called SCF-ubiquitin ligase to add ubiquiting.
Transfer ubiquitin to target proteins marking them for destruction by proteasomes.
What does SCF protein do?
SCF protein adds ubiquitin to CKI targeting it for destruction.
Adds ubiquitin to CKI proteins in G1 and so activates S-Cdks (and DNA replication) - restores activity of S-Cdk complex.
What does the activity of SCF depend on?
F-Box proteins (help SCF recognize target proteins).
What protein complex controls mitosis?
M-Cdk
Is M-Cdk an inactive or active form?
inactive form
What protein phosphorylates M-Cdk to activate it?
CAK (Cdk activating kinase).
What does Wee1 kinase do to M-Cdk?
Holds M-Cdk in inactive state.
Thus, by the end of G2 there is lots of primed M-Cdk in inactive state waiting to be activated to control entry into mitosis (M phase).
Reversed by Cdc25 phophatase
Activation of Primed M-Cdk with Double-Circuit Positive Feedback.
> Activation of primed M-Cdk is by Cdc25 protein phosphatase - removes inhibitory phosphates from M-Cdk (+ positive feedback).
> Also Wee1 kinase is suppressed.
> Cdc25 removes inhibitory phosphate from roof site that was added by Wee1 kinase.
- this causes a positive feedback to signal more Cdc25 to remove phosphates from other M-Cdk’s, thus activating them.
- production of active M-Cdk also causes positive feedback to inhibit Wee1 kinase (Cdk-inhibitory kinase).
Is the progression from metaphase to anaphase triggered by protein phosphorylation or protein destruction?
Protein Destruction
What is the key regulatory protein in progression from metaphase to anaphase?
APC/C (anaphase-promoting complex or cyclosome).
APC/C is a member of ubiquitin ligase family of enzymes.
What does APC/C do?
Catalyzes addition of ubiquitin to proteins to cause destruction.
> Targets two major proteins: COHESIN + SECURIN
What does cohesin do?
Sister chromatids are glued together along their length by this protein.
What does securin do?
Protects cohesin protein linkages that hold sister chromatid pairs together in early mitosis.
How do securin proteins protect cohesins?
By inhibiting a protein called SEPARASE that cleaves cohesin.
Mitosis and APC/C
APC/C levels rise in mid-mitosis
APC/C adds ubiquitin on securin (inhibitor of separase) to destroy.
Separase (protease) cleaves cohesin.
Sister chromatids come apart.
At which point in the cell-cycle does APC/C destroy securin?
At metaphase-to-anaphase transition.
> Destruction of inhibitor securin by APC/C
> Activates separase (protease) that separates sister-chromatids
> Anaphase begins.
When do APC/C levels start to rise?
mid-mitosis
Besides destroying securin, what other targets does APC/C add ubiquitin to?
APC/C –> destroy cyclins of Cdk-cyclin complexes.
Thus, NO cyclins and no complexes left once anaphase starts.
Which cyclins are the major target of APC/C?
S-cyclins and M-cyclins.
What binds to inactive APC/C to activate it?
Cdc20
Why is it so important that S-cyclin and M-cyclin get destroyed by APC/C when moving into anaphase?
Don’t want more DNA and chromosomes!!
What type of structural damage can inhibit G1/S-Cdk, S-Cdk, and M-Cdk?
DNA Damage
M-Cdk drives expression through what checkpoint?
G2/M checkpoint
At which point in the cell cycles does APC/C + Cdc20 trigger destruction of securing and cyclins?
Metaphase to Anaphase Transition.
At the beginning of M phase, what problem that occur during S and G2 phase can inhibit M-Cdk?
unreplicated DNA
At the start of M phase, what process from S and G2 phases does M-Cdk inhibit?
DNA rereplication
In controlling the activation of APC/C to enter into anaphase, what error can inhibit the activation of APC/C?
chromosome unattached to spindle
For mitosis, cell-cycle regulation occurs in 2 parts, what are they?
1) Increase of M-Cdk activity at G2/M triggers prophase, prometaphase, and metaphase.
- assembly of mitotic spindle
- attachment to sister chromatids
2) Metaphase-to-anaphase transition: APC/C triggers destruction of secruin - this is an inhibitor of a protease (separase) that cleaves cohesin.
- APC/C also triggers destruction of cyclins - leads to Cdk inactivation and dephosphorylation of Cdk targets.
What 2 problems must the cell solve during DNA replication?
1) replication of DNA with complete accuracy to prevent mutations.
2) every nucleotide is copied ONCE to prevent amplification.
If multiple points of duplication on new replicated DNA = spaghetti.
DNA replication in cell cycle involves 2 distinct steps which prevents DNA replication occurring more than once. What are they?
1) At G1 phase in cell cycle prereplicative complex or PRE-RC assembles at origins of replication….Critical!
2) At S phase of cell cycle replication forks are created: sites of DNA replication.
chromosome is duplicated and then mitosis occurs
True of False:
During mitosis, new PRE-RC are made.
False - during mitosis NO new PRE-RC’s are made.
True or False:
Between G1 and next G1 - no new PRE-RC formed.
True
Once is enough!!
What inhibits the assembly of PRE-RC?
Cdk activity
Major point -> while S-Cdk and M-Cdk high during S amd M stage no PRE-RC is formed.
When is Pre-RC formed?
Formed at G1.
In controlling the initiation of DNA replication, what is the role of S-Cdk’s?
> S-Cdk triggers disassembly of PRE-RC.
> S-Cdk triggers assembly of replication forks.
Components of the PRE-RC cannot form a new PRE-RC until?
M-Cdk is inactivated and APC/C is activated at end of mitosis.
APC/C causes -> PRE-RC components to be dephosphorylated.
> In late mitosis G1 - the pre-RC is formed.
