Lecture 21: Cell Cycle-I Flashcards
Name 2 ways to regulate cyclin-Cdk complexes
- Phoshorylation of the Cdk
- Binding of a CKI
- Proteolysis of Cyclins
- Ubiquination of proteins
Describe the order of events inolving APC/C, Cdc20, securin, separase, and cohesion in sister chromatid separation
- APC/C
- initiates metaphase to anaphase transition
- Cohesin
- Sister chromatids are glued together along their length by this protein
- Securin:
- Protects cohesin protein linkages that hold sister chromatid pairs together in early mitosis
- Separase
- cleaves cohesin (blocked by securin)
- APC/C leves rise in mid-mitosis and adds ubiquitin on targets to destroy proteins —> Destruction of Securin (inhibitor of Separase) —> Separase (protease) —> cleaves cohesin —> sister chromatids come apart
Describe the 3 forces driving chromosome movement in cell division
- Depolymerization
- A major force pulls the kinetochore and chromsome toward the spindle pole
- Depolymerization of the plus end fo the microtubule drives the pulling of the kinetochore poleward
- Microtubule flux
- Microtubules are moved toward spindle poles while being dismantled at minus ends
- Polar ejection force
- Kinesin 4,10 motors on chromsomes interact with microtubules and transport chromosomes from poles
*
- Kinesin 4,10 motors on chromsomes interact with microtubules and transport chromosomes from poles
Describe 3 checkpoints in the cell cycle
- Checkpoint I:
- START- cell commits to cell cycle entry and chromosome duplicaiton (also called restriciton point)
- Checkpoint II:
- G2/M- Chromosome alignment on spindle in metaphase
- Checkpoint III:
- Metaphase-to-anaphase transition- trigger sister chromatid separation and cytokinesis
The cycle of dupication and division in a cell is called the
cell cycle
What are the three major functional aspects of the cell cycle
- Cell Growth and Chromosome Replication
- Chromosome Segregation
- Cell Division
There are about ___ mistakes per cell division
6
Liver cell cylce is ___ per year
1
Intestinal cells live only ___ to ___ days and must be constantly replaced
3 to 4
Red blood cells live ___ days
120 days
Humans produce ___ million red blood cellls per second
2.4 milion red blood cells
Cancer is a disease of
excess cell proliferation
____ initiate main events of cell cycle (e.g. chromosome duplication and segregation)
Regulatory proteins
Cells must coordinate ____ with _____ to maintain size
- growth with cell division
Major chromosomal Events in the cell cycle
- Chromosome duplication is in S phase (DNA synthesis phase)
- Chromosome segregation + cell division occur during M phase (Mitosis)
- Cytokinesis (cell division)
G1 is located between what two phases
M and S: allows for growth
G2 is located between what two phases of the cell cycle
S phase and M phase: thus allowing for growth before division
M phase takes about ___ hour(s)
1
What are the 3 major transition checkpoints of the cell cycle
- Start: G1 to S
- G2 to M
- In M phase: anaphase and cytokinesis
Checkpoint 1 of cell cycle
- Also called restriction point
- cell commits to cell cycle entry and chromosome duplicaiton
- G1 to S
Checkpoint II of cell cycle
- G2/M
- Chromsome alignment on spindle in metaphase
Checkpoint III of cell cycle
- Metaphase-to-anaphase transition
- trigger sister chromatid separation and cytokinesis
Fission yeast gorws by
elongation at ends; division occurs when septum or cell plate form midway along rod-shaped cell
Budding yeast is an oval yeast that divides by
by forming a bud; the bud first appears at G1 and grows until mitosis phase
Do frog embryo s have a detectable G1 or G2 phase
NO
_____ are mammalian cell line that can be used to study cell-cylce but these cells stop diving in culture after ______ cell cycles
Fibroblasts, 25-40
Example of an immortalized cell line
- MEL (Murine erythroleukemia cells)
- HEL cells (human erythroleukemia cell line)
The cell cycle control system uses a series of biochemical switches made of ______ that turn on various steps of the cell cycle- phosphorylate proteins to activat them
Cyclin dependent kinases (Cdks)
The cell cycle is governed by ____ and they are the heart of the cell-cycle control system
Cdks
Cdk levels are constant in the cell cycle so how do they control the cycle
- Cyclin levels vary according to the point of time in the cycle and Cdks are dependent on cyclin to become active
What are the 4 classes of cyclins
- G1/S cyclins
- activates Cdks in late G1
- Helps trigger progression through START
- Commitment made to cell cycle entry
- Levels drop in S phase
- S Cyclins
- Bind Cdks after progression through START
- Helps stimulate chromosome duplication
- S-cyclin levels remain high until mitosis
- M cyclins
- Activate Cdks that stimulate entry into mitosis at G2/M checkpoint
- M cylins removed at Mid-mitosis
- G1 cyclins
- Govern activity of G1/S cyclins (control progression through start checkpoint)
How many Cdks are there
- 4
- Two that interact with G1 cyclins
- One with G1/S and S-cyclins
- One with M-cyclins
Without cyclin boud (inactive state), The active site of Cdk is blocked by a region of the protein called the ______
T loop (note binding of cyclin causes T-loop to move out of active site (Cdk partly active)
Phosphorylation of the T loop fully activates the enzyme
Bind of cyclin to Cdk cause Cdk to be partly active but what makes it fully active
phosphorylation at T loop by CAK (Cdk activating kinase)
What is INK4A
inhibitor of Ckd in the G1 phase of cell cycle, mutation in this gene causes hereditary melanoma because cannot control cell cycle and cells grow uncontrollably and you get cancer
p53
- Is a major tumor suppressor
- influences the expression of many genes (e.g. up-regulates p21)
p21 funciton
- is a CKI to stop division
If p53 fails in its funciton there will be lower ____ expression and without this CKI cells divid uncontrollably
p21
what is the protein that adds ubiquitin to S-Cdk to remove CKI inhibition
SCF-ubiquitin ligase
SCF activity depends on _____ proteins (which help SCF recognize target proteins)
F-box
What controls mitosis
M-Cdk
Inactive APC/C activated by binding to
Cdc20
Progression form metaphase to anaphase is triggered not by protein phosphorylation but by
protein destruction (regulated by APC/C which is part of the ubiquitin ligase)
___ and ___ are major targets of APC/C
S-cyclins and M cyclins
Cell must solve what 2 problems for DNA replication
- Replication of DNA with complete accuracy to prevent mutations
- every nucleotide is copied once to prevent amplification
APC/C + Cdc20 function
- Cdc20 activates APC/C
- activated APC/C triggers destruction of securin and cyclins at metaphase-to-anaphase transition
Steps in DNA replication
- 2
- At G1 phase in cell cycle prereplicative complex or PRE-RC assembles at origins of replication
- At S phase of cell cylce replication forks are created:
- sites of replication
Between G1 and next G1 are any new PRE-RCs made
No
What cyclin tirggers disassembly of PRE-RC
S-Cdks
_____ triggers assembly of replication forks
S-Cdks
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
What triggers the assemby of the mitotic spindle
M-Cdk (M-cyclin-Cdk complex)
What are the three types of microtubules that make up the spindle
- Kinetochore Microtubules
- Attach each chromosome to spindle pole
- Interpolar microtubules
- Hold two halves of spindle together
- Astral microtubules
- Interact with cell cortex
Motor proteins of spindle
- Dyneins
- Tends to move to center of cell
- minus end directed microtubule
- Kinesins
- Tend to move to periphery of cell
- Walks toward plus ends of microtubules
- Has 2 globular heads and elongated coil-coil tails
- plays important role in chromsome separation
Kinesin 5
- Two motor domains that interact with plus end of anti-parallel microtubule
- moves these two anti-parallel microtubules past each other to force or push the spindle poles (centrosomes) apart
Kinesin-14
- Minus oriented motor with single motor domain
- walks toward minus end
- pulls poles together
Kinesin-4,10
- Also called chromokinesins
- plus directed motors
- push attached chromosomes away form the pole
Microtubles’ collar attach to kinetochore using anchoring proteins such as
Ndc80
At first kinetochore binds ____ to a microtubule
laterally
Formation of a stable Bipolar attachement to the kinetochore is detected by
tension
First visible change in cell during cytokinesis is the
cleavage furrow (underlying the cleavage furrow is the contractile ring)
The contractile ring is composed of
actin and myosin filaments
Extracellular signaling molecules that regulate cell size and number are of what 3 classes
- Class 1
- Mitogens
- Stimulate cell division by triggering G1/S-Cdk activity
- Mitogens
- Class 2
- Growth factors
- Stimulate cell growth
- Growth factors
- Class 3
- Survival factors
- suppress form of programmed cell death known as apoptosis
- Survival factors
Mitogens activate the Ras-MAPK pathwaywhich leads to increased gen regualtory proteins including ___, which promotes cell cycle entry by increasing expression of G1 cyclin
Myc
G1-Cdk activates group of gene regulatory factors called ____ proteins, which binds to promoters of G1/S cyclin and S cyclin and DNA synthesis protien genes
E2F
E2F is inhibited by interaction with ______ protein
Rb protein (retinoblastoma)
____ activates ATM and ATR protein kinases
DNA damage
What is the function of ATM and ATR protein kinases
Associate with site of DNA damage and phosphorylate Chk1 and Chk2 proteins (checkpoint kinase 1 and 2) This is a majore target of p53, which stimulates to the transcription of p21. p21 CKI binds to G1/S-Cdk and S-Cdk to inhibit activity
What is Ataxia Telangiectasia caused by
error in ATM protein kinase
What is the most important growth signaling pathway
PI-3 kinase pathway
What are three mechanisms to coordinate cell growth with division
- Rate of cell division determined by extracellular factor leading to cell growth
- Cell growth and division controlled separately by growth factors and mitogens
- Cell growth and division both stimulated by extracellular factor
What is myostatin
It is a mucle cell growth inhibitor
___ inhibits Cdk activity by phoshorylating the roof “site” and ___ dephorylates it to reactivate Cdk
- Wee1, Cdc25
- (note this is important in M-Cdk)
____ CKI binds to both Cdk and Cyclin ot inactivat and is primarily used for control of G1/S-Cdks + S-Cdks early in cell cycle. How is it reactivated
- p27
- SCF-ubiquitin ligase targets the CKI for destruction
