Cell Bio Exam 4

Cytoskeleton

A network of protein fibers that extends throughout the cytoplasm

Which of the following is false about the cytoskeleton?

A. Provides mechanical strength and helps maintain the cell’s shape

B. Enables cell movement and motility

C. Acts as a network for transporting organelles, vesicles, and molecules within the cell

D. Organizes and separates chromosomes and helps divide the cytoplasm

E. Facilitates cell adhesion and signal transduction

All of the above

What are the three main cytoskeletal filaments

A. Microtubules, myofibrils, intermediate filaments

B. Myofibrils, intermediate filaments, microfilaments

C. Microfilaments, sarcomeres, myosin

D. Microtubules, Intermediate filaments, microfilaments

D. Microtubules, Intermediate filaments, microfilaments

T/F: The subunit of a microtubule is made up of an alpha and beta tubulin

True

T/F: The subunit for Intermediate filaments is a heterogeneous family of fibrous proteins

True

T/F: The subunit for Microfilaments is actin

True

Intermediate filament structure

Ropelike

~10 nm diameter

Very flexible

Good tensile strength

Deform under stress, do not rupture

Intermediate filament examples

Nuclear lamina meshwork

Keratins

Microtubule structure

Hollow tubulin cylinders

~25 nm outer diameter

More ridgid

Rupture under stress

Minus (-) end attached to the microtubule organizing center (centrosome)

T/F: Microtubules serve as tracks for kinesin and dynein in vesicular transport.

True

Microfilament (Actin filament) structure

Helical polymers of actin

Flexible

~7 nm diameter

Most concentrated just inside plasma membrane

Organization: linear bundles, 2D networks, 3D gels

If you make a cross-section across an intermediate filament, how many monomers would you typically see in that section?

32 (actual range 16-48)

When the intermediate filament is found in epithelial cells

A. What is the subtype/major class

and

B. is this cytoplasmic or nuclear

A. keratin filaments

B. Cytoplasmic

When the intermediate filament is found in connective tissue, muscle, and glial cells

A. What is the subtype/major class

and

B. is this cytoplasmic or nuclear

A. vimentin and vimnetin-related filaments

B. Cytoplasmic

When the intermediate filament is found in nerve cells

A. What is the subtype/major class

and

B. is this cytoplasmic or nuclear

A. Neurofilaments

B. Cytoplasmic

When the intermediate filament is found in all animals cells

A. What is the subtype/major class

and

B. is this cytoplasmic or nuclear

A. Nuclear lamins

B. Nuclear

Keratin filaments (Cytoplasmic IFs)

Most diverse IF

Present in all epithelial cells (tissue surfaces and linings)

Provide high tensile strength through lace-like networks

Make up hair, nails, feathers, hooves, horns

How do Keratin filaments connect to adjacent cells?

A. Directly through desmosomes

B. Indirectly through desmosomes

C. Directly with no intermediate

B. Indirectly with no intermediate

B. Indirectly through desmosomes

What happens at desmosomes?

Cadherin molecules connect to keratin filaments

Cadherins then bind to each other in extracellular space

T/F: Cytoplasmic IFs do not provide mechanical strength

False

T/F: A mutant form of keratin makes skin less prone to blisters

False

Nuclear Lamina

Made of special IFs called Lamins

Support the nuclear envelope

Provides attachment points for chromosomes

T/F: During interphase, lamins play an important role in gene expression regulation by tethering euchromatin to the nuclear periphery.

False

T/F: Plectins stabilize IFs and link them to other IFs, microtubules, and microfilaments

True

Where do protein complexes bridge the nucleus and cytoplasm

The nuclear envelope

Which of the following statements is FALSE regarding the structure and function of intermediate filaments?

A. Intermediate filaments can connect cells at cell–cell junctions called desmosomes.

B. Intermediate filaments protect cells from mechanical stress because they have high tensile strength and resist stretching.

C. Intermediate filaments are constructed of identical subunits found in all eukaryotic cells.

D. Each filament is made of eight strands, and each strand is made from staggered tetramers linked end to end.

B. Intermediate filaments protect cells from mechanical stress because they have high tensile strength and resist stretching.

Microtubule (MT) structure

Stiff hollow tube made of 13 protofilaments made of tubulin dimers

Tubulin dimers: alpha tubulin, Beta tubulin. GTP bound

Beta tubulin = plus end

Alpha tubulin = minus end

T/F: Microtubules usually do not grow out from an organizing center

False

What is a microtubule organizing center?

centrosome or basal body

T/F: Microtubules typically have their plus ends extending out toward cell periphery

True

T/F: Microtubules are nucleated from gamma tubulin complexes in centrosomes, growing out the + ends

True

T/F: Microtubules display dynamic instability (rapid growth and shrinkage)

True

What is dynamic instabiliy driven by?

GTP hydrolysis

How are Microtubules grown/disassembled?

Beta tubulin is a GTPase

Multiple GTP-tubulin dimers bind together

GTP is hydrolyzed, causing weakness in the protofilament

Section of the protofilament breaks off

GDP is exchanged for GTP

Cycle repeats

GTP ‘Cap’

When microtubules assemble too fast, the most recent Beta-tubulin may still have GTP instead of GDP

‘Cap’ favors growth

T/F: If GTP hydrolysis proceeds faster than tubulin addition, GTP cap is lost and microtubule disassembles

True

T/F: Tubulin dimers carrying GDP bind more tightly to one another than do tubulin dimers carrying GTP.

False

T/F: Colchicine arrests cells in metaphase by inhibiting MT polymerization. This property makes colchicine useful in karyotyping.

True

What does the drug, Taxol do?

Binds to filaments in microtubules and prevents depolymerization

What does the drug Colchicine/colcemid do?

Forms a complex with tubulin dimers, preventing further polymerization

What does the drug Nocodazole do?

Binds to tubulin dimers, preventing their polymerization

What are the roles of microtubules in a cell?

Intracellular transport

Cell organization

Structural support of eukaryotic cilia and flagella

Spindle assembly and function

T/F: Microtubules guide the transport of organelles, vesicles, and macromolecules (e.g. transport along the nerve axon)

True

What is intracellular transport mediated by?

Transport proteins called motor proteins

T/F: Motor proteins use kinesin and dynein to move along microtubules

True

Kinesin moves the motor protein toward the ____ end and Dynein moves the motor protein toward the ____ end

A. minus; plus

B. plus; minus

C. plus or minus; plus or minus

B. plus; minus

T/F: Kinesin and dynein movement is due to conformational changes powered by ATP hydrolysis

True

Which motor protein is used to move vesicles from Golgi apparatus to ER?

Kinesin

What do kinesin and dyein transport within a cell

organelles and vesicles

T/F: Microtubules help position organelles in the cytoplasm

True

T/F: Motor proteins arrange the ER and Golgi apparatus

True

T/F: Stable microtubules nucleated from basal bodies support cilia and flagella

True

What is the characteristic arrangement in cilia and flagella called?

9+2

_____ coordinates cilia and flagella movement

A. Flagellar dynein

B. Flagellar kinesin

C. Ciliary dynein

D. Ciliary kinesin

C. Ciliary dynein

What movement does dynein produce in an isolated doublet?

A. Microtubule Sliding

B. Microtubule Bending

A. Microtubule Sliding

What movement does dynein produce in a normal flagellum?

A. Microtubule Sliding

B. Microtubule Bending

B. Microtubule Bending

Identify the cytoskeletal structures (black lines) depicted in the epithelial cells shown

A. microfilaments

B. microtubules

C. intermediate filaments

D. nanotubes

B. microtubules

What type of cytoskeletal filaments lines the inner side of the nuclear envelope?

A. intermediate filaments

B. microtubules

C. actin filaments

A. intermediate filaments

Microfilaments (MFs/Actin filaments) structure

Monomers of globular actin proteins in a twisted two-stranded helix

Distinct polarity; more likely to be added to the + end

Actin monomer structure

Folds into two lobes

Cleft between lobes contains 1 ATP or ADP

ATP bound monomers tend to polymerize into a stable actin filament

How are Microfilaments grown/dissasembled?

Actin-ATP added to MF + end

ATP hydrolyzed to ADP, “trapping” the Actin-ADP

Actin-ADP dissociates at - end

What does the drug Phalloidin do?

Binds and stabilizes Actin Filaments

Also used for staining actin filaments

What does the drug Cytochalasin do?

Caps the + end of Actin Filaments, preventing polymerization there

What does the drug Latrunculin do?

Binds actin monomers, preventing their polymerization

What is microfilament behavior modified by?

Actin-binding proteins

What does the Nucleating protein do?

Promotes polymerization

What does the monomer sequestering-protein do?

Bind monomers, reducing change of polymerization

What % of the protein in animal cells is Actin?

A. 10%

B. 38%

C. 8%

D. 5%

D. 5%

T/F: Actin-binding proteins regulate when and where actin filaments will form and grow.

True

How many protofilaments are found in a typical actin filament?

A. 4

B. 32

C. 2

D. 9

E. 13

C. 2

What are the two roles of actin filaments?

Stabilizing and Temporary

Stable examples: Microvilli and contractile bundles

Temporary examples: Cytokinetic contractile ring and protrusions needed for cell movement

What does Cell Crawling depend on?

The cortical (cortex) actin

Actin Cortex

A thin layer of actin mesh-work that uniformly underlies the plasma membrane of the entire cell

Lamellipodium

Thin (0.1-0.3 um) and usually long (1-5 um) projection that adheres to the underlying substrate

Branched actin network

T/F: the primary energy source for cell crawling is ATP

True

Filopodia

Unbranched actin network

T/F: Actin polymerization at the leading edge of the cell results in the protrusion of lamellipodia and filopodia, motile structures that form and retract at great speed

True

A web of _____ pushes the leading edge of a lamellipodium forward

A. Actin

B. Microtubules

C. Polymerizing microtubules

D. Polymerizing actin

D. Polymerizing actin

What are Lamellipodia nucleated by?

Actin-Related Protein (ARP) complexes

What is the main structural protein in the extracellular matrix?

Collagen

What do integrins bind to?

Extracellular proteins and internal microfilaments

T/F: Integrins are transmembrane proteins that interact with the crawling cell’s environment

True

______ is an important ECM protein that mediates cell-ECM interaction

Fibronectin

How does a cell know what type of microfilament network to form?

Extracellular signals that alter the arrangement of actin filaments

What do Rho proteins do?

promote contractile bundles

What do Rac proteins do

Promote lamellipodia

What does Cdc42 do?

Promotes filopodia

T/F: Myosin I is the most complex myosin

False

Myosin I

Involved in vesicle movement

Can bind to an actin filament in the cortex

Generally moves toward the + end of actin filaments

Uses ATP hydrolysis for energy

T/F: Actin associates with myosin to form contractile structures

True

The head of Myosin I attaches to _____ and the tail attaches to ____

A. a molecule or organelle; an actin filament

B. an actin filament; a molecule or organelle

B. an actin filament; a molecule or organelle

What do muscle contractions depend on?

Interacting filaments of actin and myosin

Bipolar myosin filaments

When Myosin II molecules associate with one another

What creates a contractile force?

Movement of Myosin II along oppositely orientated action filaments

Where does the Myosin II head group walk toward?

A. + end

B. - end

A. + end

T/F: Myosin II is responsible for muscle contraction by forming cross-bridges with actin filaments and moving toward the plus end of the actin filaments.

True

Sarcomere

Contractile unit of muscle

Mofibril

repeating sarcomeres

T/F: The minus end of actin filaments attach to the Z disc in a sarcomere

True

Muscles contract by a _____ mechanism

sliding-filament