Chapter 17: Cytoskeleton

The basal body is the organizing center.

Ciliary microtubule

The graph in Figure 17-18 shows the time course of the polymerization of pure tubulin in vitro. Assume that the starting concentration of free tubulin is higher than it is in cells. Three parts of the curve are labeled above it as A, B, and C. You conduct a similar in vitro tubulin- polymerization experiment, only this time you include purified centrosomes in your preparation. When you plot your data, which part of your graph should be most dissimilar to the curve shown in Figure 17-18? a. A b. B c. C d. None. The shape of my graph should be identical to the graph produced when tubulin is polymerized in the absence of purified centrosomes.

a. A

Compared to the normal situation, in which actin monomers carry ATP, what do you predict would happen if actin monomers that bind a nonhydrolyzable form of ATP were incorporated into actin filaments? a. Actin filaments would grow longer. b. Actin filaments would grow shorter because depolymerization would be enhanced. c. Actin filaments would grow shorter because new monomers could not be added to the filaments. d. No change, as the addition of monomers binding nonhydrolyzable ATP would not affect actin filament length.

a. Actin filaments would grow longer

Which of the following statements about the function of the centrosome is FALSE? a. Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome. b. Centrosomes contain hundreds of copies of the - tubulin ring complex important for microtubule nucleation. c. Centrosomes typically contain a pair of centrioles, which is made up of a cylindrical array of short microtubules. d. Centrosomes are the major microtubule-organizing center in animal cells.

a. Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome.

Which of the following statements about microtubules is TRUE? a. Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament. b. The centromere nucleates the microtubules of the mitotic spindle. c. Because microtubules are subject to dynamic instability, they are used only for transient structures in a cell. d. ATP hydrolysis by a tubulin heterodimer is important for controlling the growth of a microtubule.

a. Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament.

Which of the situations below will enhance microtubule shrinkage? a. addition of a drug that inhibits GTP exchange on free tubulin dimers b. addition of a drug that inhibits hydrolysis of the GTP carried by tubulin dimers c. addition of a drug that increases the affinity of tubulin molecules carry ing GDP for other tubulin molecules d. addition of a drug that blocks the ability of a tubulin dimer to bind to - tubulin

a. addition of a drug that inhibits GTP exchange on free tubulin dimers

Which of the following items is not impor tant for !agellar movement? a. sarcoplasmic reticulum b. ATP c. dynein d. microtubules

a. sarcoplasmic reticulum

All intermediate filaments are of similar diameter because a. the central rod domains are similar in size and amino acid sequence. b. the globular domains are similar in size and amino acid sequence. c. covalent bonds among tetramers allow them to pack together in a similar fashion. d. there is only a single type of intermediate filament in every organism.

a. the central rod domains are similar in size and amino acid sequence

abundant in filopodia

actin

directly involved in muscle contraction

actin

important for formation of the contractile ring during cytokinesis

actin

monomer that binds ATP

actin

Which of the following statements is FALSE? a. Formins promote the formation of unbranched actin filaments. b. Actin filaments are usually excluded from the cell cortex. c. Integrins are transmembrane proteins that can bind to the extracellular matrix. d. ARPs can promote the formation of branched actin filaments.

b. Actin filaments are usually excluded from the cell cortex

Consider the mechanism by which actin and tubulin polymerize. Which of the items below does not describe something similar about the polymerization mechanisms of actin and microtubules? a. Although both filaments can grow from both ends, the growth rate is faster at the plus ends. b. Depolymerization initiates at the plus ends of filaments. c. Nucleotide hydrolysis promotes depolymerization of filaments. d. Free subunits (actin and tubulin) carry nucleoside triphosphates

b. Depolymerization initiates at the plus ends of filaments

Intermediate filaments are made from elongated fibrous proteins that are assembled into a ropelike structure. Figure 17-6 shows the structure of an intermediate filament subunit. You are interested in how intermediate filaments are formed, and you create an intermediate filament subunit whose α- helical region is twice as long as that of a normal intermediate filament by duplicating the normal α- helical region while keeping a globular head at the N- terminus and a globular tail at the C- terminus; you call this subunit IFαd. If you were to assemble intermediate filaments using IFαd as the subunit, which of the following predictions describes the most likely outcome? a. Filaments assembled using IFαd will interact with different cytoskeletal components. b. Filaments assembled using IFαd will form dimers that are twice as long as dimers assembled from normal intermediate filaments. c. Sixteen tetramers assembled from IFαd will be needed for a ropelike structure to form. d. Dimers of IFαd will form by interactions with the N- terminal globular head and the C- terminal globular tail.

b. Filaments assembled using IFαd will form dimers that are twice as long as dimers assembled from normal intermediate filaments.

Which of the following statements about organellar movement in the cell is FALSE? a. Organelles undergo saltatory movement in the cell. b. Only the microtubule cytoskeleton is involved in organellar movement. c. Motor proteins involved in organellar movement use ATP hydrolysis for energy. d. Organelles are attached to the tail domain of motor proteins.

b. Only the microtubule cytoskeleton is involved in organellar movement

Figure 17-38 shows an electron micrograph of a skeletal muscle fiber, where various points along a fiber and various regions have been labeled. Which of the following statements is TRUE about muscle contraction? a. Point A will move closer to point B. b. Point B will move closer to point C. c. Region D will become smaller. d. Region E will shrink in size.

b. Point B will move closer to point C.

Which of the following statements about skeletal muscle contraction is FALSE? a. When a muscle cell receives a signal from the nervous system, voltage- gated channels open in the T- tubule membrane. b. The changes in voltage across the plasma membrane that occur when a muscle cell receives a signal from the nervous system cause an influx of Ca2+ into the sarcoplasmic reticulum, triggering a muscle contraction. c. A change in the conformation of troponin leads to changes in tropomyosin such that it no longer blocks the binding of myosin heads to the actin filament. d. During muscle contraction, the Z discs move closer together as the myosin heads walk toward the plus ends of the actin filaments.

b. The changes in voltage across the plasma membrane that occur when a muscle cell receives a signal from the nervous system cause an influx of Ca2+ into the sarcoplasmic reticulum, triggering a muscle contraction.

Which of the following conditions is likely to decrease the likelihood of skeletal muscle contraction? a. partial depolarization of the T- tubule membrane, such that the resting potential is closer to zero b. addition of a drug that blocks Ca2+ binding to troponin c. an increase in the amount of ATP in the cell d. a mutation in tropomyosin that decreases its affinity for the actin filament

b. addition of a drug that blocks Ca2+ binding to troponin

You discover a protein, MtA, and find that it binds to the plus ends of microtubules in cells. The hypothesis that best explains this localization is that MtA a. is involved in stabilizing microtubules. b. binds to GTP- bound tubulin on microtubules. c. is important for the interaction of microtubules with the centrosome. d. will not bind to purified microtubules in a test tube.

b. binds to GTP- bound tubulin on microtubules.

Intermediate filaments help protect animal cells from mechanical stress because filaments a. directly extend from the interior of the cell to the extracellular space and into the next cell, linking one cell to the next, helping to distribute locally applied forces. b. in each cell are indirectly connected to the filaments of a neighboring cell through the desmosome, creating a continuous mechanical link between cells. c. remain in de pen dent of other cytoskeletal elements and keep the mechanical stress away from other cellular components. d. make up the desmosome junctions that connect cells; these junctions are more important than the internal network of filaments for protecting cells against mechanical stress.

b. in each cell are indirectly connected to the filaments of a neighboring cell through the desmosome, creating a continuous mechanical link between cells.

it uses dynemin motors

both types of microtubules

Figure 17-24A shows how the movement of dynein causes the flagellum to bend. If instead of the normal situation, the polarity of the adjacent doublet of microtubules were to be reversed (see Figure 17-24B), what do you predict would happen? a. No bending would occur. b. Bending would occur exactly as diagrammed in Figure 17-24A. c. Bending would occur, except that the right microtubule doublet would move down relative to the left one. d. The two microtubule doublets would slide away from each other.

c. Bending would occur, except that the right microtubule doublet would move down relative to the left one

Figure 17-31 shows the leading edge of a lamellipodium. Which of the following statements is FALSE? a. Nucleation of new filaments near the leading edge pushes the plasma membrane forward. b. ARP proteins nucleate the branched actin filaments in the lamellipodium. c. Capping proteins bind to the minus end of actin filaments. d. There is more ATP- bound actin at the leading edge than in the actin filaments away from the leading edge.

c. Capping proteins bind to the minus end of actin filaments.

You are examining a cell line in which activation of the Rho family member Rac promotes lamellipodia formation. Which of the following statements is most likely to be TRUE? a. Cells carry ing a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize more unbranched actin filaments than normal cells. b. Cells carry ing a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more unbranched actin filaments than normal cells. c. Cells carry ing a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize more branched actin filaments than normal cells. d. Cells carry ing a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more branched actin filaments than normal cells.

c. Cells carry ing a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize more branched actin filaments than normal cells.

Which of the following statements about the movement of materials in a nerve axon is TRUE? a. Movement along microtubules in the axon is slower than free diffusion, but necessary due to the importance of directional transport. b. The small jerky steps seen when vesicles move along microtubules are due to the shrinkage of microtubules that occurs when axonal microtubules undergo dynamic instability. c. Microtubules within an axon are arranged such that all microtubules point in the same direction with their minus ends toward the nerve cell body. d. Microtubules within the axon support the unidirectional motion of materials from the nerve cell body to the axon terminal, while materials transported back from the axon terminal to the cell body move along intermediate filaments.

c. Microtubules within an axon are arranged such that all microtubules point in the same direction with their minus ends toward the nerve cell body.

You are studying nuclear lamins and use recombinant DNA technology to alter the coding sequence of a nuclear lamin gene. The alteration you make creates a situation such that the gene now codes for a nuclear lamin protein that can no longer be phosphorylated when the nuclear envelope is broken down during mitosis. What do you predict would happen if the cell only had the altered nuclear lamin gene (and not the unaltered version)? a. Mitosis should proceed as usual because the dephosphorylation of the lamin is what is important for nuclear lamina assembly during mitosis, so phosphorylation will not be necessary. b. Disassembly of the nuclear lamins will occur prematurely because the lamins cannot be phosphorylated. c. Nuclear lamins will no longer disassemble properly during mitosis. d. Nuclear lamins will be unable to produce dimers, as the coiled-coil formation will be disrupted.

c. Nuclear lamins will no longer disassemble properly during mitosis.

Which of the following statements about the cytoskeleton is FALSE? a. The cytoskeleton is made up of three types of protein filaments. b. The cytoskeleton controls the location of organelles in eukaryotic cells. c. Covalent bonds between protein monomers hold together cytoskeletal filaments. d. The cytoskeleton of a cell can change in response to the environment.

c. covalent bonds between protein monomers hold together cytoskeleton filaments

Kinesins and dyneins a. have tails that bind to the filaments. b. move along both microtubules and actin filaments. c. often move in opposite directions to each other. d. derive their energy from GTP hydrolysis.

c. often move in opposite directions to each other

Which of the following structures shorten during muscle contraction? a. myosin filaments b. flagella c. sarcomeres d. actin filaments

c. sacromeres

The microtubules in a cell form a structural framework that can have all the following functions except which one? a. holding internal organelles such as the Golgi apparatus in particular positions in the cell b. creating long, thin cytoplasmic extensions that protrude from one side of the cell c. strengthening the plasma membrane d. moving materials from one place to another inside a cell

c. strengthening the plasma membrane

it is arranged in a "9 + 2" array

ciliary microtubule

it is involved in moving fluid over the surface of cells

ciliary microtubule

myosin

contracting the rear of the cell

Consider the in vitro motility assay using purified kinesin and purified polymerized microtubules shown in Figure#17-37. The three panels are images taken at 1- second intervals. In this figure, three microtubules have been numbered to make it easy to identify them. Which of the following statements about this assay is FALSE? Figure 17-37 a. Kinesin molecules are attached by their tails to a glass slide. b. The microtubules used in this assay must be polymerized using conditions that stabilize tubule formation or else they would undergo dynamic instability. c. ATP must be added for this assay to work. d. Addition of the nonhydrolyzable ATP analog (AMP- PNP) would cause the microtubules to move faster.

d. Addition of the nonhydrolyzable ATP analog (AMP- PNP) would cause the microtubules to move faster.

Keratins, neurofilaments, and vimentins are all categories of intermediate filaments. Which of the following properties is not true of these types of intermediate filaments? a. They strengthen cells against mechanical stress. b. Dimers associate by non-covalent bonding to form a tetramer. c. They are found in the cytoplasm. d. Phosphorylation causes disassembly during every mitotic cycle.

d. Phosphorylation causes disassembly during every mitotic cycle.

Microtubules are important for transporting cargo in nerve cell axons, as diagrammed in Figure 17-20. Notice that the two types of cargo are traveling in opposite directions. Which of the following statements is likely to be FALSE? Figure 17-20 a. The gray cargo is attached to dynein. b. The black cargo and the gray cargo require ATP hydrolysis for their motion. c. The black cargo moving toward the axon terminal contains a domain that specifically interacts with the tail domain of a particular kind of motor. d. The black cargo and the gray cargo are moving along microtubules of opposite polarity.

d. The black cargo and the gray cargo are moving along microtubules of opposite polarity.

Which of the following statements about actin is FALSE? a. ATP hydrolysis decreases actin filament stability. b. Actin at the cell cortex helps govern the shape of the plasma membrane. c. Actin filaments are nucleated at the side of existing actin filaments in lamellipodia. d. The dynamic instability of actin filaments is important for cell movement.

d. The dynamic instability of actin filaments is important for cell movement.

Which of the following statements regarding dynamic instability is FALSE? a. Each microtubule filament grows and shrinks independently of its neighbors. b. The GTP cap helps protect a growing microtubule from depolymerization. c. GTP hydrolysis by the tubulin dimer promotes microtubule shrinking. d. The newly freed tubulin dimers from a shrinking microtubule can be immediately captured by growing microtubules and added to their plus end.

d. The newly freed tubulin dimers from a shrinking microtubule can be immediately captured by growing microtubules and added to their plus end.

Which of the following statements about the cytoskeleton is TRUE? a. All eukaryotic cells have actin, microtubules, and intermediate filaments in their cytoplasm. b. The cytoskeleton provides a rigid and unchangeable structure important for the shape of the cell. c. The three cytoskeletal filaments perform distinct tasks in the cell and act completely independently of one another. d. Actin filaments and microtubules have an inherent polarity, with a plus end that grows more quickly than the minus end.

d. actin filaments and microtubules have an inherent polarity, with a plus end that grows more quickly than the minus end

You are interested in understanding the regulation of nuclear lamina assembly. To create an in vitro system for studying this pro cess you start with partly purified nuclear lamina subunits to which you will add back purified cellular components to drive nuclear lamina assembly. Before you start doing experiments, your instructor suggests that you consider what type of conditions would be most amenable to the assembly of the nuclear lamina from its individual subunits in vitro. Which of the following additions do you predict would be most likely to enhance the assembly of the nuclear lamina? a. addition of phosphatase inhibitors b. addition of ATP c. addition of a concentrated salt solution that is 10 times the concentration normally found in the nucleoplasm d. addition of protein kinase inhibitors

d. addition of protein kinase inhibitors

The hydrolysis of GTP to GDP carried out by tubulin molecules a. provides the energy needed for tubulin to polymerize. b. occurs because the pool of free GDP has run out. c. tips the balance in favor of microtubule assembly. d. allows the behavior of microtubules called dynamic instability.

d. allows the behavior of microtubules called dynamic instability.

For both actin and microtubule polymerization, nucleotide hydrolysis is important for a. stabilizing the filaments once they are formed. b. increasing the rate at which subunits are added to the filaments. c. promoting nucleation of filaments. d. decreasing the binding strength between subunits on filaments

d. decreasing the binding strength between subunits on filaments

Which of the statements below about intermediate filaments is FALSE? a. They can stay intact in cells treated with concentrated salt solutions. b. They can be found in the cytoplasm and the nucleus. c. They can be anchored to the plasma membrane at a cell-cell junction. d. Each filament is about 10 μm in diameter.

d. each filament is about 10μm in diameter.

Your friend works in a biotech company that has just discovered a drug that seems to promote lamellipodia formation in cells. Which of the following molecules is unlikely to be directly involved in the pathway that this drug affects? a. Rac b. ARP c. actin d. myosin

d. myosin

Cell movement involves the coordination of many events in the cell. Which of the following phenomena is not required for cell motility? a. myosin- mediated contraction at the rear of the moving cell b. integrin association with the extracellular environment c. nucleation of new actin filaments d. release of Ca2+ from the sarcoplasmic reticulum

d. release of Ca2+ from the sarcoplasmic reticulum

Which of the following statements about the structure of microtubules is FALSE? a. Microtubules are built from protofilaments that come together to make a hollow structure. b. The two ends of a protofilament are chemically distinct, with α- tubulin exposed at one end and β- tubulin exposed at the other end. c. Within a microtubule, all protofilaments are arranged in the same orientation, giving the microtubule structural polarity. d. α- Tubulin and β- tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments.

d. α- Tubulin and β- tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments.

formins

important for the growth of straight, unbranched actin filaments

can be connected through desmosomes

intermediate filaments

includes keratin and neurofilaments

intermediate filaments

supports and strengthens the nuclear envelope

intermediate filaments

integrins

involvement in focal contacts

it is nucleated at the centrosome

microtubule of the mitotic spindle

Their stability involves a GTP cap.

microtubules

a component of the mitotic spindle

microtubules

used in the eukaryotic flagellum

microtubules

The monomer is sequestered by profilin.

neither type of microtubule

it is involved in sperm mobility

neither type of microtubule

ARP proteins

nucleation of new actin filaments at the side of an existing filament

Profilin

regulation of the availability of actin monomers

Place the following in order of size, from the smallest to the largest. A. protofilament B. microtubule C. α- tubulin D. tubulin dimer E. mitotic spindle

α- tubulin, tubulin dimer, protofilament, microtubule, and lastly mitotic spindle