ch 17, 18, 22 (cell cycle, death, and stem cells) Flashcards
What is the cell cycle control system?
a timer that triggers events of the cell cycle in a specific sequence
made up from cyclins and cdks
controls 3 major transitions:
1. start (transition from G1 to S)
2. Transition from G2 to M
3. Transition from metaphase to anaphase (separation into 2 cells)
How was cell cycle control system discovered?
1960s: cell cycle was defined as succession of 4 phases (G1, S, G2, M) and an extra G0
1971: Masui discovered cytosolic unknown factor that can drive cells to enter mitosis
1980s: protein components of cell cycle control system identifified by Maller, Lohka, Hunt, etc
how were cyclins identified and purified
- by observing protein levels in sea urchin eggs during different stages of the cell cycle
- protein was degraded periodically in the cell cycle.
how does CDK activity turn on abruptly?
rapid increase in cyclin availability
how does CDK activity turn off abruptly?
due to the rapid degradation of its cyclin by the APC (which marks the cyclin for destruction by the proteasome)
What are the different cyclin-cdk complexes
G1/S-cyclin: Start–> S ; phase resulting in a commitment to cell-cycle entry.
S-cyclins: after Start –> mitosis; stimulate chromosome duplication
M-cyclins: G2/M transition –> mid-mitosis; entry into mitosis
G1 cyclins – regulate G1/S cyclin activity, respond to extracellular signals
Wee1, p27, PP2A, cdc25
wee1: phosphorylates and inhibits complex
cdc25: dephosphorylates and activates
p27: wraps around complex and inactivates
PP2A: dephosphorylates cdk substrates (does NOT act on complex)
cdk-cyclin complex
CAK
along with cyclin, fully activate complex
when is cdk activity highest?
M-phase
APC/C in regulating cell cycle
Cdc20 activates APC/C when anaphase starts –> APC/C ubiquinates and destructs cyclins → cdk inactivation→ PP2A dephosphorylates targets (negative feedback)
Cdh1 activates APC/C –> Cdk inactivation and cyclin degradation during G1
APC/C ubiquitylates securin –> separase is activated –> cohesin rings are cleaved –> chromatid are separated by being pulled to spindle poles
trigger metaphase to anaphase transition
activates separase for chromatid separation
Cdh1 can only be activated during mid M-phase when M-cyclins are destroyed
What do M-Cdks do?
- inhibit Cdh1 and activates Cdc20
- induce assembly of mitotic spindle
- ensure each sister chromatid is attached to oppsite spindle pole
- trigger chromosome condensaiton
- promote breakdown of nuclear envelope
Cohesins vs condensins
cohesins: form a ring around each of the 2 sister chromatids (glues replicated sister chromatids together until they split at anaphase)
condensin: form ring that encircles DNA, promoting compaction of chromatids
both protein complexes
What does separase do?
cleaves and dissociates cohesin rings
why is chromosome condensation critical?
to avoid DNA breaks during sister chromatid separation
p53/ p21 checkpoint activity
p53 activates the transcription of p21, which arrests cell cycle at the G1 phase (cant go to S phase)
p53 promotes expression of pro-apoptotic proteins like Puma
p21 inhibits cdk
Describe the role of Rb
binds and inhibits E2F
phosphorylation of Rb by G1-cdk reduces binding to E2F –> activates E2F
three examples of events that would trigger checkpoint arrest
DNA damage, incomplete DNA replication, improper chromosome attachment to spindle fibers during mitosis, extreme cellular stress, lack of essential nutrients, and significant changes in cellular environmen
what could happen in cell cycle if cell did not have centrosomes?
cell will still divide and become 2 cells, with chromosome abnormalities
dont need centrosome for microtubules, just y-tubulin ring
When does nuclear envelope break down and chromosomes attach to spindle?
prometaphase
Explain the fundamental difference between meiosis and mitosis
meiosis: duplicated homolog chromosomes pair during prophase
what do kinetichores do?
attach sister chromatids to opposite spindle poles
anaphase A vs B
A: kinetichore microtubules shorten & chromosomes pulled towards poles
B: movement of spindles themselves; poles are pushed and pulled apart
shugoshin
- kinetichore-associated protein that protects cohesins from degradation
- keeps 2 sister chromatids attached
- without it, all chromatids separate prematurely in anaphase I
When does nuclear envelope reform?
telophase
When is nuclear envelope broken down?
prophase
midbody
a tether between 2 daughter cells remaining after contractile ring has been disassembled
(aka cytoplasmic bridge)
What is the final step in cytokinesis?
abscission- membrane on both sides of midbody constricts and is severed by ESCRT-III protein
Define the mechanisms that regulate tissue growth
Compare mitogens and growth factors
mitogens: stimulate cell division by triggering G1/S-cdk activity
growth factors: stimulate cell growth by promoting protein and other molecule synthesis and inhibiting their degradation
how do mitogens control cell division?
promote cell division by activating Ras-MAPK pathway to induce transcription of Myc
what divides cytoplasm into 2?
contractile ring made of actin and myosin
what marks the start of cytokinesis?
cleavage furrow
what is the force for cytokinesis that forms contractile ring?
actin and myosin
bivalent pairing
a 4-chromatid structure with duplicated homolog chromosomes
chiasma
connection between homolog chromosomes during recombination
how does APC/C promote anaphase 1 and 2?
separase activation and cohesin cleavage
what does TGFB do?
mitogen that blocks growth by inhibiting cell cycle progression in G1
Explain how Myc regulates growth
expression of G1-cyclin genes –> G1-cdk activated –> phosphorylates and inactivates Rb –> active E2F transcribes cyclines that activate S-cdk –> DNA synthesis
promotes cell division
E2F
- promote gene expression of multiple protiens needed for S-phase
- inhibited by Rb
Differences between apoptosis and necrosis
apoptosis
* form of cell death by suicide occurring only in animal cells
* cell undergoes characteristic morphological and biochemical changes
* cell debris are engulfed by phagocytic cells
* IRREVERSIBLE
Examples of apoptosis in healthy organisms
- quality control during development- eliminating damaged/misplaced cells
- digit formation during development
- tail disappearance during frog metamorphosis
intrinsic and extrinsic pathway apoptosis induction
extrinsic : initated following activaiton of cell surface death receptors (belong to TNF)
intrinsic : depends on proteins released from the mitochondria: Bcl2 and MOMP proteins (apoptosis is induced when Cytochrome c is released in the cytosol)
Bcl2 proteins
- control permeabilization of mitochondria outer membrane, thus regulating release of cytochrome c to the cytosol
- can be pro or anti-poptotic
What happens when anti-poptotic proteins are inhibited?
allows Bak to oligomerize and induce MOMP
MOMP
- mitochondrial outer membrane permeabilization (when apoptosis starts)
- can be reversed
- only way to stop apoptosis is to stop MOMP from happening
Caspase cascade
Caspase activation cascade:
Activated caspase-9 triggers activation of caspase-3, which ultimately leads to cell death
Inititator vs executioner caspase
Initiatior: begin apoptotic program –> activate executioner caspases (8 & 9 in mammals)
Executioner: orchestrate apoptotic program in vertebrates
Caspase-6: cleaves nuclear lamin –> breakdown of nuclear lamina
Caspase-3: cleaves iCAD (inhibitor protein of DNA-degrading nucelase enzyme)
Caspase-7
Procaspase
inactive caspase precursor in cytosol
FLIP
- dimerizes with caspase-8 and inhibits it from activating executioner caspases to initiate apoptosis
- inhibits the extrinsic pathway of apoptosis
role of mitochondria in the apoptosis process
When MOMP occurs, cytochrome c is released, binds to Apaf-1 and procaspase-9 to form the apoptosome, leading to caspase-9 activation.
Caspase activation cascade:
Activated caspase-9 triggers a cascade of caspase activation, including caspase-3, which ultimately leads to cell death.
Where is cytochrome c released to?
from the mitochondrial intermembrane space into the cytosol
What does Fas ligand promote?
apoptosis through extrinsic pathway
In a tissue with high cell turnover, why dont we see large numbers of dead cells present within that tissue?
phagocytosis: find and clear out apoptosised cells
What happens when cleave of cytoskeleton-regulating proteins?
Causes actin polymerization to form membrane blebbing and detachment from other cells
(How apoptosis is activated)
Which signals activate the initiator caspases?
extrinsic and intrisic pathway (mitochondria)
Apoptosis extrinsic pathway
- death receptors have intracellular death domain that activates the apoptotic program
- TNF: includes death receptor and ligands
Apoptosome
complex containing Apaf 1 and caspase-9
what happens when recruit caspase-9 to apoptosome?
dimerizes –> starts apoptosis
Would WT or injected MOMP-defective cells to undergo apoptosis?
Both- extrinsic pathway links intrinsic pathway
dont care if MOMP is working if already have cytochome C in cytosol
IAP and anti IAP
IAP:
XIAP: inactivates-9,3, 7
anti-IAP
Omi and Smac: released by MOMP, promoting apoptosis; bind XIAP
What are IAP?
- inhibitor of apoptosis
- in cytosol, protect cell from unwatnted apoptosis induction
survival factors
- extracellular signaling molecules that inhbit apoptosis, thus promoting cell survival; cells that do not receive signals wil undergo apoptosis
- increase production of anti-apoptotic Bcl2
- inactivate BH3
flippase and scramblase
flippase: enzyme that ensures correct membrane localization of PS; active in healthy cell membranes
scramblase: enzyme that flips phospholipids from one membrane leaflet to another indiscrimnately; inactive in healthy cells’ membrane
what inactivates flippase?
caspase
what does collective function of caspases to inactive flippase and activate scamblase do?
increased presence of PS on the outer membrane leaflet–> signaling for phagocytic cells to engulf the cell
which way does PS face in healthy cells?
cytosol
PS
phospholipid that serves as “eat me” signal for phagocytosis
What are stem cells?
- undifferentiated self-replicating cells that have the potential to differentiate into multiple cell types supplying tissues with renewable cells for tissue repair and regneration
- Have 2 fundamental properties: self renewal and ability to produce differentiated cells
Tissues that do not have stem cells and cannot renew
auditory epithelium
retinal photoreceptive epithelium
Tissues that regenerate without presence of stem cells
- pancreatic beta cells
- hepatocytes
Difference between multipotent stem cell, transit-amplifying progenitor, and terminally differentiated cell
multipotent progenitor: can form different types of cells
unipotent progenitor: can form one cell type
terminally differentiated: fully differentiated cell
progenitor: transit-amplifying; will have limited number of divisions prior to differentiation
Describe the role of stem cells in tissue renewal – epithelium in the gut
Cell types of the gut epithelium and define the areas where they form
- absorptive cell: take up nutrients from gut lumen; digestion; villi
- goblet cell; secrete mucus into gut lumen, which acts as protective coat; villi
- paneth cell: role in innate immune defense by secreting proteins that kill bacteria; secrete wnt needed to maintain stem cell population; crypts
- enteroendocrine cell: secrete serotonin and peptide hormones; act on neurons to regulate growth, poliferation, and digestion; both
Villus: structure in gut lumen coated by a layer of epithelial cells
crypt: area within connective tissue; location of replicating stem cells that produce differntiated cell types
where do differentiated cells go in the gut?
migrate out of the crypt to populate the villi
Structure of the epidermis and the location of the stem cells
- transient amplifying progenitors in basal cell layer: give rise to epidermis cell types
- squames: dead cells marking the outermost layer of the skin; shed from the surface
- granular cell layer: waterproof barrier of the skin formed by cells
How is the epidermis renewed by new
differentiated cells?
continously renewed by stem cells proliferation in basal cell layer
hematopoietic cells
- stem cells that give rise to RBC and WBC
- located in adult bone marrow
Describe lineage tracing technique using cre/loxp system. What do we learn about the characteristics of the cells by conducting this experiment?
Cre/loxP: edit genome and express a reporter under desired promoter
activity of cre is dependent on the presence of tamoxifen allowing temporal control of genome editing
Why are satellite cells in muscles difficult to observe through traditional lineage tracing experiments?
Satellite cells are quiescent until activated and therefore are not detectable by most lineage tracing
Difference between totipotent and pluripotent stem cells. what type of cells are ES cells?
totipotent- can create a complete organism
pluripotent- can only create the different cell types within the body
ES are pluripotent
How can hematopoietic cells be used in transplantation studies?
- multipotent cells that give rise to transit-amplifying progenitors
- ** cell transplantation**- to replace damaged or dysfunctional bone marrow in patients by infusing healthy stem cells from a donor, allowing the patient to regenerate a healthy blood cell population in their body
- transplantation of bone marrow fro healthy mouse can repopulate the hematopoietic stem cells destroyed by X-irradiation
Describe the stem cell niche
- specialized microenvironment in the tissue containing necessary signaling molecules to promote proliferation and inhibit differentiation
- supporting cells, ECM, excreted factors
Cell fate determinant
molecules required to maintain stem cell identity
what will uneven distribution of cell fate determinant molecules produce?
one stem cell and one differentiatied cell
mechanisms that control the numbers of stem cells within tissues
- limiting size of niche
- asymmetric cell division
- stochastic cell fate determination (randomly decides)
Describe how some animals can regenerate their limbs and organs, give examples
blastema- small bud forming at the site of amputation includes activated stem cells and progenitor cells that regenerate the limb
ex: axolotol
Neoblast- restore the regenerative ability of planarian
compare ES and iPS cells
overexpression of ES genes can induce iPS
How is transdifferentiation different from reprogramming cells to iPS cells?
reprogramming: converts cells into pluripotent stem cells
transdifferentiation: converts cells directly into another cell type
At which two major time points during the cell cycle does the cell-cycle control system normally arrests the cycle if DNA damage is detected?
G1 and G2
What is mainly responsible for maintaining low levels of M-cyclin protein during G1?
Cdh1–APC/C
Teratoma
tumor with a mixture of cell types that did not receive appropriate differentiation cues
organoid
3D organ-like tissues generated by reprogramming ES cells
