Deck 17: Cytoskeleton

Question 1

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.

Answer 1

C

Question 2

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. Figure 17-18
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.

Answer 2

A
A constitutively active phospholipase C will lead to the constitutive production of IP₃ and diacylglycerol, leading to activation of PKC in a signal-independent manner; thus, Rafty activation and the lipid modification will be signal-independent.A mutation in the GPCR that bind the signal more tightly and a Ca²⁺ channel with an increased affinity for IP will increase activity of the signal transduction pathway in a signal-dependent manner.A mutation that renders Rafty such that it can no longer by phosphorylated by PKC will prevent PKC from activating Rafty and will thus prevent the lipid modifications.

Question 3

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.

Answer 3

B

Question 4

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.

Question 5

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.

Question 6

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 noncovalent bonding to form a tetramer.
C)They are found in the cytoplasm.
D)Phosphorylation causes disassembly during every mitotic cycle.

Question 7

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.

Question 8

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.

Question 9

You are interested in understanding the regulation of nuclear lamina assembly.To create an in vitro system for studying this process 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

Question 10

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 γ-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.

Question 11

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? Figure 17-6

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.

Question 12

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.

Question 13

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.

Question 14

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.

Question 15

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.

Question 16

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.

Question 17

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

Question 18

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 carrying GDP for other tubulin molecules
D)addition of a drug that blocks the ability of a tubulin dimer to bind to γ-tubulin

Question 19

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.

Question 20

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 independent 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.

Question 21

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

Question 22

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)


(B)

Figure 17-24

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.

Question 23

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.

Question 24

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 carrying 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 carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more unbranched actin filaments than normal cells.
C)Cells carrying 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 carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more branched actin filaments than normal cells.

Question 25

Which of the following items is not important for flagellar movement?

A)sarcoplasmic reticulum
B)ATP
C)dynein
D)microtubules

Question 26

Identify the cytoskeletal structures (black lines) depicted in the epithelial cells shown in Figure 17-1.
Figure 17-1

Question 27

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.

Question 28

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.

Question 29

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

Question 30

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.

Question 31

Rank the following cytoskeletal filaments from smallest to largest in diameter (1 = smallest in diameter, 4 = largest)
_____ intermediate filaments
_____ microtubules
_____ actin filament
_____ myofibril

Question 32

Figure 17-31 shows the leading edge of a lamellipodium.Which of the following statements is FALSE? Figure 17-31

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.

Question 33

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

Question 34

Figure 17-38 shows an electron micrograph of a skeletal muscle fiber, where various points along a fiber and various regions have been labeled. Figure 17-38
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.

Question 35

Which of the following structures shorten during muscle contraction?

A)myosin filaments
B)flagella
C)sarcomeres
D)actin filaments

Question 36

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.

Question 37

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.

Question 38

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.

Question 39

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.

Question 40

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.

Question 41

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; use each word or phrase only once.
antiparallel four tail
β barrel globular ten
coiled-coil head trimeric
covalent rod twenty-five
eight seven two
Intermediate filaments are elongated fibrous proteins with an N-terminal globular __________ region and a C-terminal globular __________ region; these regions flank the elongated rod domain.The α-helical region of the rod interacts with the α-helical region of another monomer in a __________ configuration to form a dimer.__________ dimers will line up to form a staggered tetramer.__________ strands of tetramers come together and twist together to form the __________ nm filament.The __________ domains are exposed on the surface of the intermediate filament, allowing for interaction with cytoplasmic components.

Question 42

The graph in Figure 17-10 shows the time course of the polymerization of pure tubulin in vitro.You can assume that the starting concentration of free tubulin is much higher than it is in cells.
Figure 17-10
A.Explain the reason for the initial lag in the rate of microtubule formation.
B.Why does the curve level out after point C?

Question 43

Do you agree or disagree with this statement? Explain your answer.
Minus-end directed microtubule motors (like dyneins) deliver their cargo to the periphery of the cell, whereas plus-end directed microtubule motors (like kinesins) deliver their cargo to the interior of the cell.

Question 44

In the three cell outlines in Figure 17-62, indicate the arrangement of the microtubules, showing clearly their free and attached ends.On each figure, indicate the plus end for one of the microtubules.
Figure 17-19

Question 45

You isolate some muscle fibers to examine what regulates muscle contraction.When you bathe the muscle fibers in a solution containing ATP and Ca²⁺, you see muscle contraction (experiment 3 in Table 17-22).Ca²⁺ is necessary, as solutions containing ATP alone or nothing do not stimulate contraction and thus the muscle remains in a relaxed state (experiments 1 and 2 in Table 17-22).From what you know about the mechanism of muscle contraction, fill in your predictions of whether the muscle will be contracted or relaxed for experiments 4, 5, and 6.Explain your answers.
Table 17-22
Extra credit: In what state would the muscle be if you added Ca²⁺ but no ATP?

Question 46

Figure 17-21 shows two isolated outer-doublet microtubules from a eukaryotic flagellum with their associated dynein molecules.
Figure 17-21
A.Sketch what will happen to this structure if it is supplied with ATP.
B.Sketch what will happen to this structure if the linking proteins are removed and it is supplied with ATP.
C.In a complete flagellum, what would happen if all the dynein molecules were active at the same time?

Question 47

Your friend discovers a protein that she names EBP.EBP binds to microtubule plus ends, and she hypothesizes a role for EBP in increasing dynamic instability.To determine the function of EBP, she examines its effect on microtubules.She polymerizes microtubules from purified centrosomes in a Petri plate and determines the number of shrinking microtubules over a three-minute time interval for different concentrations of EBP.The data she obtained are shown in Figure 17-8.
Figure 17-8
Is this result consistent with her hypothesis? Explain.

Question 48

Do you agree or disagree with the following statement? Explain your answer.
When skeletal muscle receives a signal from the nervous system to contract, the signal from the motor neuron triggers the opening of a voltage-sensitive Ca²⁺ channel in the muscle cells' plasma membrane, allowing Ca²⁺ to flow into the cell.

Question 49

Phosphorylation of nuclear lamins regulates their assembly and disassembly during mitosis.You add a drug to cells undergoing mitosis that inhibits the activity of an enzyme that dephosphorylates nuclear lamins.What do you predict will happen to these cells? Why?

Question 50

You are curious about the dynamic instability of microtubules and decide to join a lab that works on microtubule polymerization.The people in the lab help you grow some microtubules in culture using conditions that allow you to watch individual microtubules under a microscope.You can see the microtubules growing and shrinking, as you expect.The professor who runs the lab gets in a new piece of equipment, a very fine laser beam that can be used to sever microtubules.She is very excited and wants to sever growing microtubules at their middle, using the laser beam.
A.Do you predict that the newly exposed microtubule plus ends will grow or shrink? Explain your answer.
B.What do you expect would happen to the newly exposed plus ends if you were to grow the microtubules in the presence of an analog of GTP that cannot be hydrolyzed, and you then severed the microtubules in the middle with a laser beam?

Question 51

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

Question 52

In the budding yeast, activation of the GTP-binding protein Cdc42 occurs on binding of an external signal (pheromone) to a G-protein-coupled receptor.Activation of Cdc42 promotes actin polymerization.Predict what would happen to actin polymerization, in comparison with pheromone-treated cells, in the following cases.
A.You add pheromone to an inhibitor of G-protein-coupled receptors.
B.You add pheromone to a nonhydrolyzable analog of GTP.

Question 53

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; use each word or phrase only once.
α-tubulin dynein nine
ATP four thirteen
basal body γ-tubulin twenty-one
β-tubulin GTP UTP
centrosome kinesin two vimentin
δ-tubulin myosin
Microtubules are formed from the tubulin heterodimer, which is composed of the nucleotide-binding __________ protein and the __________ protein.Tubulin dimers are stacked together into protofilaments; __________ parallel protofilaments form the tube-like structure of a microtubule.__________ rings are important for microtubule nucleation and are found in the __________ , which is usually found near the cell's nucleus in cells that are not undergoing mitosis.A microtubule that is quickly growing will have a __________ cap that helps prevent the loss of subunits from its growing end.Stable microtubules are used in cilia and flagella; these microtubules are nucleated from a ___________ and involve a "__________ plus two" array of microtubules.The motor protein __________ generates the bending motion in cilia; the lack of this protein can cause Kartagener's syndrome in humans.

Question 54

You are interested in studying kinesin movements.You therefore prepare silica beads and coat them with kinesin molecules so that each bead, on average, has only one kinesin molecule attached to it.You add these kinesin-coated beads to a preparation of microtubules you have polymerized.Using video microscopy, you watch the kinesin [labeled with green fluorescent protein (GFP)] move down the microtubules.
A.Kinesin-GFP has been measured to move along microtubules at a rate of 0.3 μm/sec, and single-molecule studies have revealed that kinesin moves along microtubules progressively, with each step being 8 nm.How many steps can the kinesin molecule take in 4 seconds, assuming that the kinesin stays attached to the microtubule for the entire 4 seconds?
B.Because each kinesin molecule is thought to take approximately 100 steps before falling off the microtubule, will you see your silica beads detach from the microtubule during your 4 seconds of observation?
C.What would you predict would happen to the kinesin-coated silica beads if you were to add AMP-PNP (a nonhydrolyzable ATP analog)?

Question 55

Cytochalasin is a drug that caps actin filament plus ends, thus preventing actin polymerization.Phalloidin is a drug that binds to and stabilizes actin filaments, preventing actin depolymerization.Even though these drugs have opposite effects on actin polymerization, the addition of either of these drugs instantaneously freezes the cell movements that depends on actin filaments.Explain why drugs that have opposite effects on actin filaments can have a similar effect on cell movements.

Question 56

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; use each word or phrase only once.
desmosome lamin synapse
keratin neurofilament vimentin
kinase plectin
Intermediate filaments are found mainly in cells that are subject to mechanical stress.Gene mutations that disrupt intermediate filaments cause some rare human genetic diseases.For example, the skin of people with epidermolysis bullosa simplex is very susceptible to mechanical injury; people with this disorder have mutations in their __________ genes, which code for the intermediate filament found in epithelial cells.These filaments are usually connected from cell to cell through junctions called __________s.The main filaments found in muscle cells belong to the __________ family; people with disruptions in these intermediate filaments can have muscular dystrophy.In the nervous system, __________s help strengthen the extremely long extensions often present in nerve cell axons; disruptions in these intermediate filaments can lead to neurodegeneration.People who carry mutations in the gene for __________, an important protein for cross-linking intermediate filaments, have a disease that combines symptoms of epidermolysis bullosa simplex, muscular dystrophy, and neurodegeneration.Humans with progeria, a disease that causes premature aging, carry mutations in a nuclear __________.

Question 57

Indicate whether each of the following statements refers to a ciliary microtubule, a microtubule of the mitotic spindle, both types of microtubule, or neither type of microtubule.
A.The basal body is the organizing center.
B.The monomer is sequestered by profilin.
C.It is arranged in a "9 + 2" array.
D.It is nucleated at the centrosome.
E.It uses dynein motors.
F.It is involved in sperm motility.
G.It is involved in moving fluid over the surface of cells.

Question 58

Kinesins were purified by adding the nonhydrolyzable analog AMP-PNP to cytoplasmic extracts containing microtubules, purifying the microtubules, and then releasing the kinesin proteins, which were still attached to the microtubules, by adding ATP.Would this trick have worked to purify myosin motors attached to actin filaments? Explain.

Question 59

Match the following labels to the numbered lines on Figure 17-12.
Figure 17-12
A.minus end of microtubule
B.tail of motor protein
C.cargo of motor protein
D.head of motor protein
Which of the two motors in Figure 17-12 is most probably a kinesin? Explain your answer.