unit 4 Flashcards
What are the three major filament systems of the cytoskeleton?
Actin filaments, microtubules, and intermediate filaments
What are the two ends of actin filaments, and what are some differences between them?
Barbed vs pointed end. Barbed: fast growth, plus end. Pointed: slow growth, minus end
What are some general cell processes that involve actin filaments?
Cell migration, signaling, muscle contraction, cell architecture, cytokinesis
Wha are some general cell processes that involve microtubules?
Intracellular transport, signaling cell division, cell migration, cellular architecture?
What are some general cell processes that involve intermediate filaments?
Mechanical stability, neuronal function, nuclear organization
What are actin filaments made of?
Actin monomers, G actin
What is actin generally bound to inside the filament?
ATP or ADP (usually ADP because of ATP hydrolysis)
What does actin polymerization look like kinetically?
Very slow growth during nucleation (lag phase), fast growth during elongation, slower growth + plateau during steady state (aka equilibrium phase). If preformed filament seeds added, nucleation phase skipped
Can actin polymerize spontaneously?
Yes
What is a major barrier to forming actin filaments?
Nucleation
How do actin filaments get formed despite the barrier of nucleation
Different proteins can recruit actin monomers and put them together to form filaments
What is the Arp2/3 complex and what does it do?
Arp2/3 is a branched actin nucleator. Alongside NPF (nucleation promoting factor), the complex recruits actin filaments.
What are formins and what do they do?
Formins are non-branched/straight actin nucleators. Each formin dimer binds to an actin monomer, and puts the monomers together. Moves along filament and adds monomers to the barbed end.
What is profilin and what does it do?
Profilin is a protein that stimulates actin filament formation. It binds to actin monomers (opposite the ATP binding sites) and this profilin bound actin gets added to the barbed end. The profilin then “falls off”.
How does bacteria hijack the cytoskeleton?
Bacteria (listeria in particular) hijacks the Arp2/3 complex and rapidly polymerizes actin as a branch (polymerizes at the barbed end).
Why is it important that actin filaments have polarity?
Polarity allows for directionality
What are myosins?
Myosins are directional motors that move along actin filaments
How does myosin-2 work mechanically?
ADP bound myosin is bound to the actin monomer. Once ATP is introduced and hydrolyzed, it lets goes of the actin monomer. It then binds to the next actin monomer and a conformational change happens (power stroke), returning the myosin back to its original shape.
How does myosin-2 work during muscle contractions?
In muscles, myosins exist in head-to-head configurations. As myosin-2 slides across actin filaments, muscle contractions occur.
What are microtubules made of?
Microtubules are made of tubulins. Microtubules are heterodimers (dimer of 2 different alpha and beta tubulin molecules ).
What do tubulins bind to / what do they hydrolize?
GTP
Are microtubules polar or nonpolar?
Microtubules are polar because of the alpha vs. beta tubulins.
What is the difference between GTP hydrolysis in alpha and beta tubulin?
GTP is more accessible to hydrolysis in beta tubulin compared to alpha tubulin.
Do microtubules grow faster at the plus or minus ends?
Plus end
What are MAPs? What is one of the most important MAPs
Microtubule associated proteins. +TIPs are one of the most important MAPs
What is catastrophe?
When a microtubule rapidly grows then shrinks (from the plus end).
What is rescue?
When a microtubule rapidly shrinks then grows
Where is there more GTP in a microtubule? Why?
Towards the plus end. Because of the GTP cap.
What assists microtubules in nucleation?
y-TuRC, or the y-tubulin ring complex.
How many gamma tubulins per 1 gamma TuSC molecule
2
How many protofilaments in a microtubule?
13
What does the centrosome do?
The centrosome is the major microtubule organizing organelle in most animal cells
What are centrosomes made of?
y-tubulin ring complex and 2 centrioles
What are centrioles?
A cylindrical organelle involved in microtubule development and organization (during mitosis).
What is augmin?
A microtubule nucleator (from the plus end)
In what types of cells will we find (different) microtubule organizations?
Fibroblasts, plant cell, intestinal epithelial cell, neuronal cell, etc.
What do +TIPS do?
They are MAPs that bind at the tips of the plus end of microtubules. They are helpful in visualizing the behavior of the plus (growing) end of microtubules.
What are microtubule motors? What is the major superfamily of microtubule motor proteins.
Microtubule motors transport proteins, vesicles, etc. intracellularly. Kinesins are the major superfamily of microtubule motor proteins.
What are the three families of kinesins?
Plus end directed, Microtubule depolymerases (does not walk), and Minus end directed. Usually more plus end directed than minus end directed.
Describe Kinesin 1’s structure
On of the most conventional kinesin motors. Homodimeric (Two molecules that are the same). Two homodimers. Each contains a head domain (binds to ATP and hydrolyzes), the neck domain, and the tail domain (eventually associate with other motor proteins).
How does kinesin-1 work?
In its initial stage, motor is bound to microtubule, and lagging head/end is tighly bound to ATP. Front/leading head bound to ADP (having hydrolyzed previously bound ATP). As the kinesin walks, ATP bound to lagging end gets hydrolyzed, ADP in leading head is replaced by ATP. When it associates with ATP, tightly binds to microtubule, while neck region swings forward.
What is a processive motor?
They stay bound to the microtubule or actin filament for long distances before they fall off of the microtubule/filament. Allows for transport across long distances.
How many cells are in the human body?
42 trillion
What are the phases of the cell cycle?
G1 phase (growth), S phase (synthesis), G2 phase (growth), and M (mitosis/division) phase
How long is the cell cycle? How long is each phase of the cell cycle?
22 hours. G1: 10 hours, S: 7.5 hours, G2: 3.5 hours, M: 1 hour
What is the restriction point of G1 phase?
The point at which the cell cycle can only continue if conditions are favorable
What could stop the cell cycle at the restriction point of G1?
Lack of nutrients or growth stimuli
How does the restriction point of G1 work molecularly?
If there are no growth stimuli, the cell is held through a protein called Rb. Rb associates with a protein called E2F, a transcriptional gene that drives cell cycle progression. Rb also bound to histone deacetylase (enzyme that deacetylates chromatin or DNA). In deacetylated form, histone is compact, making it less accessible to transcription. Without nutrients/growth stimuli, Rb remains bound to E2F and histone deacetylase, turning off transcriptional genes necessary for cell cycle progression.
In the presence of nutrients and growth stimuli, how does the cell move past G1 phase at the restriction point?
In the presence of nutrients and growth stimuli, a signal transduction cascade is triggered that results in the synthesis of cyclin-D. Cyclin-D activates the Cdk4/6 protein, phosphorylating the Rb, unbinding it from histone deacetylase and the E2F transcription factor. E2F can now recruit RNA polymerase for transcription, and the chromatin are now in an open position (favorable for transcription).
What needs to happen after the cell passes the G1 restriction point but before it can move on to S phase. What proteins carry this out?
The “quick check” for DNA damage, by ATM kinase and p53.
What do ATM and ATR kinases do when a cell is undamaged?
They are kinases that sense DNA damage. At the end of G1 phase, if a cell is undamaged, they are inactive, leading to inhibition of E2F and low amounts of p53
What do ATM and ATR kinases do when a cell is damaged?
If a ds DNA strand is damaged and ATM is active, E2F activates leading to apoptosis, or p53 activates, leading to cell cycle arrest. For ss DNA strands, active ATR localizes to the damage site, eventually leading to cell cycle arrest.
What are the two ways of fixing ds DNA breaks?
Homology based Repairs and non-homologous end joining repairs
What are homology based repairs?
Slower but more precise way of repairing ds DNA breaks
What are non homologous end joining repairs?
Filling gaps in ds DNA strands without surveilling the correct sequence on a homolog. Faster but less precise
What is cohesion establishment, and at what point in the cell cycle does it occur?
It occurs during S phase. It is the establishment of chromosome cohesion (keeping sister chromatids together).
What is cohesion structurally?
Ring structure around the sister chromatids. “Molecular glue”
What happens during S phase (generally)?
Centrosome duplication starts, chromosome duplication,Check for damaged DNA or stalled replication forks
What happens during G2 phase (generally)?
Second growth phase. Check for damaged or unduplicated DNA
What is the main purpose of mitosis?
Cell division
What is cytokinesis?
When actin and myosin pinch one cell into two daughter cells?
What are the stages of mitosis in order?
Interphase (G2), Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Why must centrosome separation happen? When does it happen?
Occurs during prophase. Occurs because during interphase, the two centrosomes appear as one, and during prophase they have to go to the opposite sides of the nucleus in order to form bipolar spindle
How do cancerous cells “cheat” cell death when they have an abnormal # of centrosomes?
They cluster the amplified centrosomes into two to evade cell death
How do motor proteins drive centrosome separation?
Motor proteins and ATP drive apart the centrosomes.
What does kinesin-5 do during centrosome separation
Kinesin-5 has antiparallel sliding activity. 4 motor domains: two mind to one microtubule, two bind to the other. Walk towards opposite sides. (microtubules connected to centrosomes).
What does dynein do during centrosome separation?
Minus end directed motor. At the nuclear envelope, it always walks towards the minus ends of microtubules (towards the centorosomes). Essentially pulls the centrosome to one side while kinesin pulls them apart
Is kinesin-5 plus or minus end directed
Plus end directed
How many poles does the mitotic spindle have? What does each pole contain?
2 poles. Each pole contains a centrosome
What are kinetochores?
Protein complexes that connect chromosomes and microtubules (at the plus end)
What are the 3 ways that kinetochores can attach microtubules and chromosomes? Which is the correct way
Amphitelic (correct), syntelic, merotelic
What is amphitelic microtubule kinetochore attachment?
Each of the kinetochores are attached to microtubules coming from opposite sides of the spindle.
What are syntelic and merotelic microtubule kinetochore attachments?
Syntelic: Only one pole contributes to chromosome attachment // both kinetochores are attached to microtubules coming from one pole
Merotelic: One kinetochore attached to microtubules coming from both poles
What happens during interphase (generally)?
DNA replicate, duplicated centrosomes
What happens during prophase (Generally)?
Centrosome separation, chromosome condensation
What happens during prometaphase (generally)?
Nuclear envelope breakdown, chromosome capture by microtubules, spindle assembly
What happens during metaphase (generally)?
Spindle bipolarization, chromosome bi-orientation at spindle equator
What happens during anaphase (general)?
Completion of chromosome microtubule attachment, cleavage of chromosome cohesion, separation of sister chromatids
What happens during telophase (general)?
Completion of cytokinesis, chromosome decondensation, nuclear envelope reassembly
How is cohesin removed from the sister chromatids during anaphase?
Ubiquitinization. Degradation mechanisms. As cohesin is cleaved, microtubules also pulling sister chromatids apart.
What is the spindle assembly checkpoint?
The spindle assembly checkpoint (SAC) senses the tension of the microtubules. The MCC (mitochic checkpoint complex) is part of the SAC and it has the CDC20 protein. When kinetochores are not attached to microtubules, the MCC complex is present on the kinetochore, and CDC20 is inactive. Inactive CDC20 leads to inactive APC/C (anaphase promoting complex/cyclosome), preventing the progression into anaphase.
What happens when there is tension during metaphase?
The MCC complex dissociates from the kinetochores, CDC20 is released, APC/C is activated, mitosis progresses.
How does APC/C work during metaphase?
Cyclin B and SEC (securin) are APC/C substrates. APC/C activation leads to Cyclin B-CDK1 binding, and SEC-separase binding, activating CDK1 and inactivating separase
What is separase’s job during metaphase?
To cleave cohesin (which initiates chrosome segregation)
What are some main differences between meiosis and mitosis?
Two rounds of division, No DNA replication, 4 cells produced instead of 2
How many cells does meiosis produce in men vs women?
4 vs. 1
What are the steps of meiosis?
prophase, metaphase i, anaphase i, metaphase ii, anaphase ii
What are some unique features of meiosis?
Chromosome segregation is driven by actin and microtubule cytoskeletons, asymmetric division is driven by actin and myosin motors (microtubule dependent), long range vesicle transport is microtubule independent (driven by actin and myosin motors). centrioles are eliminated in early stages and restored during embryonic development
