What is the cytoskeleton?
A network of structural proteins found in all cell types
What defines the shape of a cell?
The cytoskeleton
What are the three classes of structural proteins in the cytoskeleton?
Intermediate filaments, microtubules, actin
What is the primary function of intermediate filaments?
Adds mechanical strength to cells
What is the primary function of microtubules?
Supports trafficking within cells
What is the primary function of actin?
Supports large-scale cell movements like motility and contractility
Like other proteins, cytoskeletal proteins bind to a target to form what?
Polymers
How are cytoskeletal proteins similar to other proteins?
Binding, conformation, and function
Out of these two, which can withstand the most stress before rupturing?
Actin
Which of these are the most common intermediate filaments in humans?
Class I and II keratins
Class I and II keratins are distrbuted where?
Epithelial cells
What is the class III protein of intermediate filaments?
Desmin, GFAP, vimentin, periphevin
What is the class V protein of intermediate filaments?
Lamins
Which keratin is the class I protein of intermediate filaments?
Acidic keratins
Which keratin is the class II protein of intermediate filaments?
Basic keratins
Class V lamin proteins are distributed where?
Nucleus
Where are class IV and VI proteins distributed?
Neurons
Which class of intermediate filament proteins are distributed in muscle, glial cells, mesenchymal cells, and perphevin neurons?
Class III: Desmin, GFAP, vimentin, periphevin
What is the function of class I and II keratins?
Tissue strength and integrity
What is the function of class IV proteins?
Axon organization
What is the function of class III proteins?
Sarcomere organization, integrity
What is the function of class V proteins?
Nuclear structure and organization
Which class of intermediate filament proteins control tissue strength and integrity?
Class I and II keratins
What is the function of class VI proteins?
Axon growth
Which class of intermediate filament proteins control nuclear structure and organization?
Class V: Lamins
Which class of intermediate filament proteins control axon growth?
Class VI: Nestin
Which of these intermediate filament proteins control axon organization?
Class IV neurofilaments
What is the primary structure of intermediate filaments?
A polymer of amino acids linked by peptide bonds
Secondary structure: Intermediate filaments are rich in...
α-helics
What is responsible for the coiled structure and hydrogen bonds of intermediate filaments?
α-helics
What is the tertiary structure of intermediate filaments?
A coiled monomer
Two coiled monomers form what?
A dimer
What is it called when two coiled monomers wrap around each other? (PS. it's very stupid)
A coiled coil
What is the building block of intermediate filaments?
A tetramer
Which compounds are on opposite ends of the antiparallel alignment of the tetramer?
NH2 and COOH
A unit-length filament is formed by what?
8 tetramers
What happens in stage 1 of the assembly of an intermediate filament?
The formation of a unit-length filament
What do unit-length filaments form?
An immature filament
What do immature filaments compact to form?
A mature filament
What controls the assembly and disassembly of intermediate filaments?
Post-translational modifications
What types of post-translational modifications affect intermediate filaments? (select all that apply)
Phosphorylation
Glycosylation
Phosphorylation leads to the dissolution of intermediate filaments into what?
Unit-length filaments
What are the specialized intermediate filaments? (select all that apply)
Lamins
Desmins
Keratins
Which intermediate filament is responsible for muscle structural integrity?
Desmins
What does keratin bind to?
Desmosomes
Where does microtubule assembly occur?
Microtubule-organizing centres (MTOCs)
True or False: A centrosome is an MTOC
True
What are microtubules made of?
Tubulin
What are the 2 types of tubulins?
α-tubulin and β-tubulin
What do α and β-tubulins bind to form?
A dimer
Which tubulin can cleave its GTP to GDP? (and undergo a shape change)
β-tubulin
In microtubule assembly, when does a polymer become more stable?
Once it has 6 dimer subunits
How many protofilaments will form a tube?
13
What is the nucleation site for microtubule elongation?
The protofilament tube
What causes a microtubule to grow?
If the rate of assembly is greater than the rate of disassembly
What causes a microtubule to shorten?
If disassembly occurs faster than assembly
Does dimer polymerization happen when GTP or GDP is bound to β-tubulin?
GTP
Does dimer polymerization cause assembly or disassembly?
Assembly
What happens when β-tubulin's GTP is hydrolysed to GDP?
The dimer undergoes a change that promotes depolymerization and disassembly
Do microtubules extend faster from the plus or minus end?
Plus
What is dynamic instability?
The ability to rapidly grow or shrink
In dynamic instability, a growing microtubule has what kind of cap at its tip?
GTP subunit cap
In dynamic instability, what does GTP hydrolysis expose at the tip?
GDP-bound subunits
What occurs after hydrolysis in dynamic instability?
Catastrophic depolymerization
In dynamic instability, how does depolymerization stop?
By the rapid binding of GTP subunits
What is catastrophe in microtubule dynamic instability?
The rapid depolymerization of tubulin dimers at the plus end which shortens the microtubule
What does catastrophe cause in microtubule dynamic instability?
The shortening of the microtubule
What is the result of GTP rapidly falling off one end of the tubulin dimers when being converted to GDP?
Catastrophe
Can the microtubule grow without GTP-bound tubulin dimers?
No
How can microtubule catastrophe be reversed?
Rescue
What can keep microtubules polymerized if their dimers are in their GDP-bound form?
Capping from capping proteins
To reverse microtubule catastrophe, rescue can occur if what is present?
GTP-bound dimers
Do microtubule capping proteins bind to the plus or minus end of microtubules?
Plus end
What are the advantages of microtubule dynamic instability? (select all that apply)
It allows cells to exert force for molecule transport
It allows cells to create trafficking pathways
What actually controls trafficking in the microtubules?
Motor proteins
What are the 2 most common motor proteins in microtubules?
Dynein and kinesin
Which motor protein moves to the plus end of microtubules?
Kinesins
Which motor protein moves to the minus end of microtubules?
Dyneins
Which area of kinesin and dynein bind to cargo that needs to be trafficked?
Tail
What occurs in step 1 of the walking of motor proteins?
Head 1 binds to the microtubule, head 2 binds to ADP
During the walking of motor proteins, which head does ATP bind to and what does it initiate?
Head 1. It causes conformational changes that swings head 2
During the walking of motor proteins, what is released after head 2 binds to a microtubule?
ADP
During which step of the walking of motor proteins does ATP undergo hydrolysis on head 1 to become ADP?
Step 4
During which step of the walking of motor proteins is ADP released from head 1?
Step 4
What happens during step 5 of the walking of motor proteins?
ATP binds to head 2 making head 1 swing around
What happens when ATP binds to head 2 during the walking of motor proteins?
Head 1 swings around
How is actin similar to microtubules?
Composition, network formation, & movement
What is the building block of the actin cytoskeleton?
Actin monomer
What creates the tensile strength of actin filaments?
Longitudinal and lateral bonding
The 'plus end' in actin is called...?
Barbed end
The 'minus end' in actin is called...?
Pointed end
Microtubule tubulin binds to GTP/GDP. What do actin monomers bind to?
ATP/ADP
Actin assembly/polymerization is prompted when actin monomers bind to what?
ATP
Which of these discourages actin polymerization?
ADP
Order the stages of actin polymerization.
1. Nucleation 2. Elongation 3. Steady State
Stage 1 of actin polymerization (nucleation) occurs when a third actin binds to a dimer to form what?
A nucleus trimer
Which structure is the core of actin filament formation?
Nucleus trimer
When does net actin filament elongation cease?
When the rate of assembly equals the rate of disassembly
What is actin treadmilling?
The addition of monomers to one end with the same rate of removal on the other end
Actin treadmilling gives the cell the ability to do what?
Rapidly adjust the actin cytoskeleton
What is the critical concentration of ATP-actin to polymerize?
Lower at the plus end than the minus
What happens if the ATP-actin concentration rises above the critical concentration?
Actin monomers can be added to the minus end
Monomers are added to which end during actin treadmilling?
Plus end
Proteins that bind to actin monomers and influence polymerization.
Monomer-binding proteins
Proteins that bind to the plus or minus end and can stabilize the polymer to prevent disassembly & assembly.
Capping proteins
Proteins that bind to actin polymers for stability and growth of a new branch.
Nucleating proteins
Proteins that bind to actin polymers and sever or induce disassembly.
Severing and depolymerization proteins
Proteins that bind to the actin filament for movement.
Actin-binding motor proteins
Proteins that allow side-to-side links of actin polymers to form bundles of actin filaments.
Cross-linking proteins
Proteins that link actin filaments to non-actin proteins.
Membrane anchors
What is the most common actin-binding motor protein?
Myosins
What are the subunits of myosins called?
Light chains & heavy chains
What are the 3 domains of myosins?
Motor, regulatory, tail
Which domain of myosins is formed by heavy chains and binds to actin filament and ATP?
Motor
Which domain of myosins is formed by a heavy chain and 2 light chains?
Regulatory
Which domain of myosins binds to other proteins or myosins?
Tail
Which end of the actin filament does myosin move towards?
Barbed end
What commences the movement of actin-binding motor proteins causing a conformational shift in the regulatory domain?
Hydrolysis of ATP to ADP and inorganic phosphate
What happens when the motor domain binds to the actin filament? (select all that apply)
ADP is released and myosin unbinds from the actin filament
Inorganic phosphate is released
A conformational change that pulls the myosin along the actin filament
What causes myosin to detach from the actin filament?
Binding of ATP after ADP is released
What is bound to the motor domain when the motor domain of myosin is bound to actin?
ADP
Where does cell migration occur?
Inside the cell
What are the 3 types of actin filaments in cell migration? (select all that apply)
Filopodia
Lamellipodia
Stress fibres
Thin bundles of filaments that extend in the direction of a cell's intended movement.
Filopodia
Sheet-like bundles of actin filaments that extend the cell membrane in the same direction as the filopedia.
Lamellipodia
Actin filaments that anchor to integrins with the reverse polarity of other actin filaments to allow the cell to move forward.
Stress fibres
What are the 2 crucial phases of the cell cycle?
Interphase & mitosis
Which phase of interphase does most of a cell's life occur during?
G1 phase
What are the 3 phases of interphase? (select all that apply)
G1
G2
S
What are cells doing in interphase?
Actively living or preparing to divide
Does the cell cycle occur clockwise or counterclockwise?
Clockwise
What do CDKs do once activated?
Phosphorylate other proteins to trigger the next stage of the cell cycle
Why would a cell divide in a multicellular organism?
To replace damaged cell tissue and maintain continuously dividing tissues
What are the checkpoints of the cell cycle? (select all that apply)
G1/S checkpoint
S/G2 checkpoint
G2/M checkpoint
Mitotic spindle checkpoint
How are CDKs activated?
When they bind to a cyclin
What are cells doing in the G1 phase?
Being active and growing
Which types of cells are often found in G0 phase?
Nerve and muscle cells
Which of these is the resting phase?
G0
What happens at the G1/S checkpoint of the cell cycle?
Proteins check for DNA damage
What happens during the S phase of the cell cycle?
The cell genome and the centrosome is replicated
What happens at the S/G2 checkpoint of the cell cycle?
DNA integrity is checked
What happens during the G2 phase of the cell cycle?
Cytoplasm and endomembrane system increases
What happens at the G2/M checkpoint of the cell cycle?
Large-scale rearrangement to cell structure
What happens during the M phase of the cell cycle?
Mitosis
What induces cell cycle arrest and/or apoptosis in a cell?
p53 protein
How do cells become able to evade apoptosis and replicate uncontrollably?
If the p53 protein is dysfunctional
What is Li-Fraumeni syndrome?
A hereditary cancer predisposition
At which checkpoint in the cell cycle does p53 perform its functions?
G1/S
Does p53 act before or after DNA replication in the cell cycle?
Before
What is the first phase of mitosis?
Prophase
Chromosomes are condensed and packaged into...?
Chromatids
What are sister chromatids joined by?
A centromere
Other than the sister chromatids, what else attaches at the centromere? (select all that apply)
Kinetochore
Mitotic spindle
During which phase of mitosis is gene transcription shut down?
Prophase
Which organelle remains intact after gene transcription is shut down during prophase?
Mitochondria
After chromosome condensation in prophase, the nuclear envelope dissolves allowing what to happen?
Chromosomes to be released into the cytosol
After condensation during prophase, the microtubule network reorganizes to form what?
The mitotic spindle
What does the mitotic spindle form around in prophase?
Two centrosomes
Which of these is a microtubule organizing centre (MTOC)?
Centrosomes
What are centrosomes formed from?
Centrioles and proteins
Which of these is responsible for microtubule organization during mitosis?
Centrosomes
What moves centrosomes to opposite sides of the cell in prophase?
Tubulin and motor proteins
During what phase of the cell cycle does the kinetochore form?
Prometaphase
What is the kinetochore?
A protein complex that binds to chromatids on each side of the centromere
How do the kinetochores use ATP to move chromosomes in the cell?
ATP will polymerize and depolymerize the microtubule spindles
Where do chromosomes align in metaphase?
The spindle equator
What is the function of the mitotic spindle checkpoint at metaphase?
To ensure that chromosomes are properly aligned at the spindle equator before anaphase
What happens during anaphase?
Sister chromatids separate to opposite end of the cell
Which phase of the cell cycle involves the reformation of the nucleus, cytoskeleton, and endomembrane system?
Telophase
Which phase of the cell cycle involves microtubules gathering at the spindle equator for the development of the contractile ring in cytokinesis?
Anaphase
What happens during prometaphase?
The chromosomes condense and the kinetochores form
In mammalian cells, what forms where the spindle equator was during cytokinesis?
A contractile ring
After the contractile ring tightens during cytokinesis and the plasma membrane snaps, what keeps the plasma membrane from being permanently ruptured?
Its hydrophobicity
How can microtubule catastrophe be averted?
Capping
What is the functional unit of the cell cycle checkpoints?
Cyclins and CDKs