Flashcards in Exam 4 Lesson 34 Deck (58):
How many cells in the human body?
At least 37 trillion
How many mitosis/cytokinesis events occur in humans each day?
What is cytokinesis?
Division of the cytoplasm
When does cytokinesis begin?
How does cytokinesis cellular machinery differ from plants to animals?
Animal cells undergo abscission. Plant cells do not separate because of cell wall. This is more of a secretory process for plant cells.
What indicates that contractile ring is forming?
a. F-actin assembly, antiparallel orientation, links to plasma membrane. b. Nonmuscle myosin II – formation of bipolar filaments c. The RhoA GTPAse pathway – the switch for assembly of the contractile ring d. Factors for trafficking and fusion of membrane vesicles e. factors for abscission
What is abscission?
Pinching off the membrane
How does myosin II influence the contractile ring?
Myosin II causes sliding of antiparallel actin filaments, causing the ring to decrease in diameter. Remember, myosin II travels towards the plus end, so actin filaments are traveling toward the minus end.
What happens to actin filaments as contractile ring contracts?
Actin filaments are removed by severing/depolymerization. The membrane is pulled with the ring.
What is the role of RhoA GTPAse in contractile ring assembly?
RhoA GTPAse activates both myosin and actin filament assembly.
How does RhoA GTPAse activate myosin and actin filament assembly?
a. Inactive RhoA GDP is turned on by RhoGEF and becomes RhoA GTP, which is active. b. RhoA GTP stimulates formin, which stimulates actin filament formation. c. RhoA GTP stimulates Rho-activated kinases (Rock). d. Kinases directly stimulate myosin light-chain phosphorylation and indirectly stimulate it by inhibiting myosin phosphatase. This leads to myosin II activation. When actin and myosin II are activated, there is assembly and contraction of actin-myosin ring.
How does contractile ring contract?
Myosin II is two headed, and walks towards plus ends of actin filaments. They are parallel, and close in as the plus ends on each side reach myosin II.
What is the central spindle in cytokinesis?
The central spindle contains the interpolar microtubules of the spindle.
What assembles at the central spindle?
Components for cytokinesis assemble here at anaphase when mitotic cyclin-dependent kinase is inactivated.
In animal cells, what does positioning of cleavage furrow depend on?
The positioning of the mitotic spindle.
What may be source of signals for cleavage furrow positioning?
Central spindle or asters.
What force positions the spindle and contractile ring in some cells?
Force generated at the cell cortex.
How does cell cortex anchor spindle to itself?
Cytoplasmic dynein anchored at cell cortex interacts with dynamic astral microtubules to position spindle.
What three models exist for cleavage furrow positioning?
Astral stimulation model, central spindle stimulation model, and astral relaxation model.
Astral stimulation model
The astral microtubules carry furrow-inducing signals, which are somehow focused in a ring on the cell cortex, halfway between the spindle poles.
Central spindle stimulation model
The spindle midzone, or central spindle, generates a furrow-inducing signal that specifies the site of furrow formation at the cell cortex.
Astral relaxation model
Astral microtubules promote the local relaxation of actin-myosin bundles at the cell cortex. The cortical relaxation is minimal at the spindle equator, thus promoting cortical contraction at that site.
How does experiment with frog egg and bead show that aster microtubules play a role in formation of cleavage furrow?
Cleavage occurs between two asters that are simply adjacent.
How is cytokinesis completed?
Microtubules must be disassembled/cleaved and the membrane must be pinched off on either side of the midbody.
Do daughter cells remained attached after cytokinesis?
They remain attached by a thin strand of cytoplasm containing the remains of the central spindle.
What is abscission?
The final cut on both sides of the midbody.
Stages of abscission.
Furrow ingression, MT bundle compaction, 1st MT bundle disassembly, 2nd MT bundle disassembly
What cleaves the microtubules during abscission? What constricts the membrane tube to pinch off membrane?
Spasmin. ESCRT III filaments.
ESCRT stands for
Endosomal sorting complex required for transport
Besides cytokinesis, what are ESCRT filaments involved in?
Assembling multivesicular bodies.
What is the most ancient and conserved complex?
What does ESCRT-III mediate?
Abscission of narrow membrane stalks filled with cytosol.
In what three types of membrane abscissions does ESCRT machinery play a role?
MVE biogenesis, Cytokinesis, Virus budding
What recruits ESCRT-III in MVE biogenesis?
ESCRT-0, -I, and –II.
What recruits ESCRT-III in cytokinesis?
Centrosome/midbody protein CEP55 and Alix, and ESCRT-I
What recruits ESCRT-III in HIV budding?
Viral GAG protein and ESCRT-I, and Alix.
What is the process of cytokinesis in the plant cell?
a. G2, preprophase band of microtubules sets plane of cell division before mitosis. b. spindle separates c. anti-parallel bundle of microtubule and actin filaments called phragmoplast assembles at midpoint d. telophase, motor proteins carry golgi-derived vesicles that fuse to assemble the cell plate e. cell plate expands to meet the mother cell PM and fuses in a curtain like manner to fully divide two cells
delivery of golgi-derived vesicles during plant cytokinesis is what kind of secretion?
Phragmoplast microtubules in plant cytokinesis
Anti-parallel bundle of microtubule and actin filaments that assembles at midpoint
What must be added to each daughter cell during plant cytokinesis?
By how much does surface area of cell increase during plant cytokinesis?
By about 20%
What supplies the increased surface area of cell during plant cytokinesis?
Secretion from the endomembrane system.
How does membrane insertion happen during cytokinesis?
Kinesin motors carry membrane vesicles along microtubules of the central spindle. Vesicles are inserted to the cleavage furrow.
What is the source of vesicles used to insert membrane during cytokinesis?
Recycled endosomes and vesicles budded off from the trans Golgi network
How do membrane vesicles travel back and forth between golgi and cleavage furrow?
Dynein motors carry vesicles to golgi. Kinesin motors carry vesicles to cleavage furrow.
How do both daughter cells receive a complete set of organelles during cytokinesis?
a. enough copies spread throughout the cell that a contractile ring cannot possibly divide without assuring that each daughter gets at least one copy of the organelle b. coordinate organelle division with the cell cycle. Organelles can be docked onto spindle fibers and be segregated like chromosomes. Centrioles and centrosomes are examples in animal cells.
How do mammalian cells divide up the Golgi, which are clustered near the centrosome?
At M phase, most Golgi membranes recycle back to ER. The remaining Golgi matrix and vesicles split and move with centrosomes into daughter cells. At anaphase-telophase, the Golgi reassemble from the ER. Membrane vesicles from the Golgi are transported to the plasma membrane.
What is FtsZ-mediated fission of organelles?
Fission used by plastids and some mitochondria.
What is FtsZ?
A GTP binding protein related to tubulin.
What types of mitochondria do not require FtsZ for fission?
Green plant, animal and fungal mito
To what genes are mitochondrial FtsZ genes most related to?
Those of alpha-proteobacteria, which come from endosymbiont.
Where are genes encoding plastid FtsZ located now?
Encoded in the nucleus
What are plastid FstZ genes most related to?
Genes of cyanobacteria
What are dynamins?
A protein that drives fission used by mitochondria and peroxisomes. Different type of dynamin also used by plastids along with FtsZ.
Where do dynamins get the energy to twist their membranes?
They hydrolyze GTP
How do plastids use dynamin during cytokinesis?
They retain FtsZ0ring on stromal side, but use a dynamin ring on the cytosolic side. All fission-related proteins are nuclear encoded.
How do mitochondria use dynamin in cytokinesis?
In green plants, animals and fungi, FtsZ is gone. Instead, they use a nuclear-encoded dynamin ring on the cytosolic side.