week 7 the cytoskeleton Flashcards
what are the componets of the cell cytoskeleton
actin filaments, microtubules, intermediate filaments
what are actin filaments
provides mechanical support, determines cell shape, and allows movement of the cell surface, thereby enabling cells to migrate, engulf particles, and divide.
what are microtubules
function both to determine cell shape and in a variety of cell movements, including some forms of cell locomotion, the intracellular transport of organelles, and the separation of chromosomes during mitosis.
What do the daughter cells do after the cell divides?
They reorganize their microtubule and actin cytoskeletons into smaller versions of those present in the mother cell, enabling them to crawl their separate ways.
What is bound in the deep cleft at the center of an actin monomer?
Either ATP or ADP.
What is the orientation of the subunits within an actin filament?
All the subunits within the filament have the same orientation.
What imaging technique is used to visualize an actin filament?
Electron micrograph of a negatively stained actin filament
what does the cytoskeleton network do
This network is important for cell shape, movement, division, trafficking
what is an actin filament made up of
made up an actin monomer which is wither ATP or ADP bound in a deep cleft
what is nucleation
the formation of a helical momomer of actin where one actins binds to another which binds to another forming a ‘neclus’ of action
what are the 2 ends that actin is polymerised
the minus end which is the slow growing end and the plus end which is the fast growing end
what is treadmilling
occurs when one end of a filament grows in length while the other end shrinks, resulting in a section of filament seemingly “moving” across a stratum or the cytosol.
what is profilin
binds momoers, concentrates them at sites of filament assembly
what is thymosin
binds subunits and prevents assembly
What is the cytoskeleton in a cell?
A network of filaments that supports the plasma membrane, gives the cell shape, aids in organelle positioning, provides tracks for vesicle transport, and allows cell movement.
What are the three types of protein fibers in the cytoskeleton of eukaryotes?
Microfilaments, intermediate filaments, and microtubules.
What is another name for microfilaments and why?
Actin filaments, because they are made of actin monomers
Actin filaments, because they are made of actin monomers
Serve as tracks for myosin, involved in cell division, muscle contraction, cargo transport, cell motility, and maintaining cell shape.
How do actin filaments contribute to muscle contraction?
Actin and myosin filaments in sarcomeres slide past each other to contract muscles
What is the primary function of intermediate filaments?
Maintain cell shape and anchor the nucleus and other organelles in place.
What are microtubules made of?
α-tubulin and β-tubulin subunits.
How do microtubules behave dynamically?
They can grow and shrink quickly by adding or removing tubulin proteins.
What specialized roles do microtubules play?
Provide tracks for motor proteins (kinesins and dyneins) to transport vesicles and cargoes, and form the spindle during cell division to pull chromosomes apart.
how does actin filament grow
arp2/3 complexes
-arp2/3 complexes are held together accessory proteins
-when nucleation promoting factor bind they cuse arp2/3 complexes to assemble actin filaments and cause branching
what does proflin do
stimulates proflin elongation
What role does profilin play in actin filament elongation?
Profilin stimulates actin filament elongation.
What do many nucleation-promoting factors (NPFs) contain?
Binding sites for profilin.
What is profilin bound to in the cell?
Actin monomers.
What does the activation of NPFs lead to?
Nucleation of branched actin filaments by Arp2/3 and rapid elongation of new filaments.
what is the pathway for proflin
- inactive arp2/3 comlexes bind to nucleation promoting factors (NPFs)
this causes actin momers to bind and nucleate actin filament
What is the composition of a myosin II molecule?
Two heavy chains and four light chains.
How many types of light chains are present in a myosin II molecule?
Two distinct types, with one copy of each type present on each myosin head.
What causes the dimerization of myosin II?
The two α helices of the heavy chains wrap around each other to form a coiled-coil, driven by the association of regularly spaced hydrophobic amino acids.
What structure does the coiled-coil arrangement of myosin II heavy chains form?
An extended rod in solution, which forms the tail of the myosin II molecule.
Describe the structure of a microtubule.
A microtubule is a stiff hollow tube formed from 13 protofilaments aligned in parallel.
How is the GTP molecule associated with the β-tubulin monomer?
It is less tightly bound and plays an important role in filament dynamics
What is the function of γ-TuRC?
Nucleates microtubule assembly and remains associated with the minus end, forming a centrosome near the nucleus.
Where is γ-TuRC located and what does it form?
Located next to the nucleus in the cytosol, forming a centrosome.
What is the role of γ-tubulin in cells?
Promotes the formation of new microtubules
What is the primary function of katanin?
Severs microtubules.
How does katanin contribute to microtubule dynamics
Promotes complex reorganization of the microtubule array by severing microtubules.
How does microtubule organization differ in other cell types
More complex distributions of microtubule plus and minus ends are observed.
What direction do kinesin and dynein transport cargo along microtubules?
Kinesin transports towards the plus-ends, while dynein transports towards the minus-ends of microtubules.
Describe the structure of kinesins.
Kinesins consist of two heavy chains (heads) that interact with microtubules, joined by a middle domain to two light chains (tails) where the cargo binds
What is the function of kinesins?
Kinesins function in vesicular transport
How do kinesins move?
Kinesins use ATP to fuel their movement, with the ATP-lagging head detaching, passing over the ADP-leading head, and attaching at the next binding site.
How do dyneins bind to cargo
Dyneins need an adaptor protein called dynactin to bind to cargo.
What are the similarities between kinesins and dyneins
Both kinesin and dynein move along microtubules, can engage with cargo, and use ATP to fuel their movements
What are the differences between kinesins and dyneins?
Kinesin moves towards the plus-end and binds directly to cargo via light chains, while dynein moves towards the minus-end and binds to cargo via dynactin. Dynein typically moves faster than kinesin.
Describe the structure of intermediate filaments.
Monomers pair to form a coiled dimer, two dimers form an antiparallel tetramer, and tetramers pack together in a rope-like array to provide mechanical strength to cells.
Why is cell polarity important during early development
ensures that the inside is on the inside and the outside is on the outside, particularly important during the formation of the epithelial sheet and gastrulation.
What is the effect of activating the small GTPase Rac on actin organization?
It promotes the formation of protrusive actin networks in lamellipodia and pseudopodia through alterations in actin accessory proteins.
How does Rac-GTP activate actin nucleation?
Rac-GTP activates members of the WAVE protein family, which then activate actin nucleation and branched network formation by the Arp2/3 complex.
Describe the parallel pathway involving Rac-GTP and PAK.
Rac-GTP activates PAK, which inhibits myosin light-chain kinase (MLCK) through phosphorylation, leading to myosin II filament disassembly and decreased contractile activity.
: What is the overall result of Rac activation on actin networks and contractility
Rac activation leads to branched actin networking in cellular protrusions and less contractility and stress fibers.
What does activation of the GTPase Rho lead to in terms of actin organization?
increases contraction by myosin II
What are the effects of Rho activation on cellular structures
Rho activation increases contractility, stress fibers, and focal adhesion formation.
How do neutrophils achieve polarization and chemotaxis?
Binding of bacterial molecules to G-protein-coupled receptors stimulates directed motility, with receptors more likely bound to bacterial ligands at the front.
What are the two signaling pathways involved in neutrophil polarization?
1) The Rac pathway
The Rho pathway
what is the key objective of cell division
key objective of cell division is to provide each daughter cell with exactly the same genetic information as the parent cell
What are the four stages of the cell cycle?
Gap phase 1 (G1), Synthesis phase (S), Gap phase 2 (G2), and Mitosis (M).
What is G0?
A resting state where a cell may temporarily, indefinitely, or permanently exit the cell cycle.
What are the three checkpoints in the cell cycle?
G1, S, and G2.
What happens during Gap phase 1 (G1)?
Cells grow in size, producing organelles, cytoskeleton, and some DNA and proteins needed for DNA synthesis.
What is interphase?
The collective term for G1, S, and G2 phases.
What occurs during cytokinesis?
Cytoplasmic division.
What is M phase?
The combination of mitosis and cytokinesis.
: What occurs during Gap phase 2 (G2)?
Cells grow in size and produce required proteins, and there is a checkpoint to confirm readiness to enter M phase.
What happens during the Synthesis phase (S)?
DNA synthesis to duplicate chromosomes.
How is the cell cycle controlled in eukaryotic cells?
egulatory proteins specified by cdc genes
What happens at the cell cycle checkpoints?
halts progression, initiates corrective measures, or aborts the cycle if deficiencies are detected.
What is the consequence of the cell cycle continuing when it shouldn’t?
Abnormal and unrestrained proliferation, which is a critical feature of cancer cells.
How can certain drugs affect the cell cycle?
They can cause the cell cycle to arrest in G1 or G2/M by affecting proteins involved in checkpoint mechanism
What is checked during the G2 phase?
The accuracy of DNA replication is thoroughly checked.
What are Cyclin and CDK complexes?
Key components of the cell cycle control system that regulate the cycle by forming protein kinase complexes.
How do Cyclin-CDK complexes function?
When cyclin forms a complex with Cdk, the protein kinase is activated to trigger specific cell-cycle events.
What happens if Cyclin is absent?
Cdk is inactive without Cyclin.
What are the different Cyclins active at different times?
G1/S CDK, S/CDK, and M-CDKs
What blocks the active site of CDK in its inactive state?
The T-loop region of the protein blocks the active site.
What effect does cyclin binding have on CDK?
Cyclin binding causes the T-loop to move out of the active site, partially activating CDK2.
How is CDK2 further activated after cyclin binding?
Phosphorylation of CDK2 at a threonine residue in the T-loop by CAK changes the shape of the T-loop, improving the enzyme’s ability to bind its protein substrates.
What are the two mechanisms of CDK inhibition?
Phosphate addition by kinase and binding of CDK inhibitory protein (CKI)
What happens to polyubiquitylated target proteins?
They are recognized and degraded in a proteasome.
How does DNA polymerase move along the template strand?
DNA polymerase proceeds in the 5’-to-3’ direction along the template strand.
What is the difference between leading strand and lagging strand synthesis?
Leading strand synthesis is continuous, while lagging strand synthesis is discontinuous, producing Okazaki fragments that are later joined together
What key events occur during Prophase in mitosis?
Chromatin condenses into visible chromosomes.
Each chromosome consists of two sister chromatids joined at the centromere.
The mitotic spindle begins to form, originating from the centrosomes.
The nucleolus disappears.
What happens during Prometaphase in mitosis?
The nuclear envelope breaks down.
Microtubules from the mitotic spindle attach to kinetochores on the centromeres of chromosomes.
Chromosomes begin to move toward the cell’s equator
Describe the events of Metaphase in mitosis.
Chromosomes align at the metaphase plate (the cell’s equatorial plane).
The kinetochores of sister chromatids attach to microtubules from opposite spindle poles.
What occurs during Anaphase in mitosis?
Sister chromatids are pulled apart as the microtubules shorten.
The separated chromatids, now individual chromosomes, are pulled to opposite poles of the cell.
The cell elongates as non-kinetochore microtubules lengthen
What key processes happen during Telophase in mitosis
Chromosomes reach the opposite poles and begin to de-condense back into chromatin.
The nuclear envelope re-forms around each set of chromosomes.
The nucleolus reappears.
The mitotic spindle disassembles.
what do Kinetochores
- Kinetochores control the movements of chromosomes during cell division. Sister chromatids are positioned by the kinetochores, which face in opposing directions and attach to the mitotic spindle.
what is the metaphase to anaphase checkpoint
Ensures proper alignment of all chromosomes before anaphase.
what is the mechanism for metaphase tp anaphase checkpoint
Uses APC/C (Anaphase-Promoting Complex) for marking proteins for degradation.
APC/C Function: Targets M-cyclins and triggers breakdown of the cohesin complex holding sister chromatids together.
