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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