17

Front 1

17-2 Which of the following statements about the cytoskeleton is false?

Back 1

(c) Covalent bonds between protein monomers hold together cytoskeletal filaments

Hint 1

(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.

Front 2

17-4 Which of the following statements about the cytoskeleton is true?

Back 2

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

Hint 2

(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.

Front 3

17-6 Which of the statements below about intermediate filaments is false?

Back 3

(d) Each filament is about 10 μm in diameter

Hint 3

(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.

Front 4

17-8 All intermediate filaments are of similar diameter because ____________.

Back 4

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

Hint 4

(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.

Front 5

17-9 Intermediate filaments help protect animal cells from mechanical stress because ____________.

Back 5

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

Hint 5

(a) filaments 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) filaments in each cell are indirectly connected to the filaments of a neighboring cell through the desmosome, creating a continuous mechanical link between cells.
(c) filaments remain independent of other cytoskeletal elements and keep the mechanical stress away from other cellular components.
(d) filaments 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.

Front 6

17-10 Intermediate filaments are made from elongated fibrous proteins that are assembled into a ropelike structure. Figure Q17-10 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?

Back 6

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

Hint 6

Figure Q17-10

(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.

Front 7

17-13 Keratins, neurofilaments, and vimentins are all categories of intermediate filaments.

Back 7

(b) Dimers associate by noncovalent bonding to form a tetramer.

Hint 7

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.

Front 8

17-15 You are studying nuclear lamins in yeast. Using recombinant DNA technology, you alter the coding sequence of a nuclear lamin gene 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 yeast cell only had the altered nuclear lamin gene (and not the unaltered version)?

Back 8

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

Hint 8

(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 coiled-coil formation will be disrupted.

Front 9

17-16 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 conditions do you predict would be most likely to enhance the assembly of the nuclear lamina?

Back 9

(d) addition of protein kinase inhibitors

Hint 9

(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

Front 10

17-19 Which of the following statements about the structure of microtubules is false?

Back 10

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

Hint 10

(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.

Front 11

17-20 Which of the following statements about the function of the centrosome is false?

Back 11

(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.

Hint 11

(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.

Front 12

17-21 Which of the following statements about microtubules is true?

Back 12

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

Hint 12

(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.

Front 13

17-23 The hydrolysis of GTP to GDP carried out by tubulin molecules ________________.

Back 13

(d) allows the behavior of microtubules called dynamic instability.

Hint 13

(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.

Front 14

17-24 The microtubules in a cell form a structural framework that can have all the following functions except which one?

Back 14

(c) strengthening the plasma membrane

Hint 14

(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

Front 15

17-26 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 ________________.

Back 15

(b) MtA binds to GTP-bound tubulin on microtubules.

Hint 15

(a) MtA is involved in stabilizing microtubules.
(b) MtA binds to GTP-bound tubulin on microtubules.
(c) MtA is important for the interaction of microtubules with the centrosome.
(d) MtA will not bind to purified microtubules in a test tube.

Front 16

17-28 Which of the following statements regarding dynamic instability is false?

Back 16

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

Hint 16

(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.

Front 17

17-29 Which of the situations below will enhance microtubule shrinkage?

Back 17

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

Hint 17

(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

Front 18

17-31 The graph in Figure Q17-31 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 Q17-31

Three parts of the curve are labeled above it as A, B, and C. You conduct a similar in vitro tubulin-polymerization experiment, only 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 Q17-31?

Back 18

(a) A

Hint 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.

Front 19

17-32 Which of the following statements about organellar movement in the cell is false?

Back 19

(b) Only the microtubule cytoskeleton is involved in organellar movement.

Hint 19

(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.

Front 20

17-33 Microtubules are important for transporting cargo in nerve cell axons, as diagrammed in Figure Q17-33. Notice that the two types of cargo are traveling in opposite directions. Which of the following statements is likely to be false?

Back 20

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

Hint 20

Figure Q17-33

(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.

Front 21

17-35 Which of the following statements is correct? Kinesins and dyneins ____________________.

Back 21

(c) often move in opposite directions to each other.

Hint 21

(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.

Front 22

17-39 Which of the following items is not important for flagellar movement?

Back 22

(a) sarcoplasmic reticulum

Hint 22

(a) sarcoplasmic reticulum
(b) ATP
(c) dynein
(d) microtubules

Front 23

17-40 Figure Q17-40A 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 Q17-40B), what do you predict would happen?

Back 23

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

Hint 23

Figure Q17-40

(a) No bending would occur.
(b) Bending would occur exactly as diagrammed in Figure Q17-40A.
(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.

Front 24

17-42 Which of the following statements about actin is false?

Back 24

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

Hint 24

(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.

Front 25

17-44 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?

Back 25

(b) Depolymerization initiates at the plus ends of filaments.

Hint 25

(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.

Front 26

17-45 For both actin and microtubule polymerization, nucleotide hydrolysis is important for ______.

Back 26

(d) decreasing the binding strength between subunits on filaments.

Hint 26

(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.

Front 27

17-46 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?

Back 27

(a) Actin filaments would grow longer.

Hint 27

(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 addition of monomers binding nonhydrolyzable ATP would not affect actin filament length.

Front 28

17-49 Which of the following statements is false?

Back 28

(b) Actin filaments are usually excluded from the cell cortex.

Hint 28

(a) Cytochalasins prevent actin polymerization.
(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.

Front 29

17-50 Cell movement involves the coordination of many events in the cell. Which of the following phenomena is not required for cell motility?

Back 29

(d) Release of Ca2+ from the sarcoplasmic reticulum.

Hint 29

(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.

Front 30

17-52 Figure Q17-52 shows the leading edge of a lamellipodium. Which of the following statements is false?

Back 30

(c) Capping proteins bind to the minus end of actin filaments

Hint 30

Figure Q17-52

(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.

Front 31

17-53 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?

Back 31

(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.

Hint 31

(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.

Front 32

17-55 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 involved in the pathway that this drug affects?

Back 32

(d) myosin

Hint 32

(a) Rac
(b) ARP
(c) actin
(d) myosin

Front 33

17-56 Which of the following structures shorten during muscle contraction?

Back 33

c) sarcomeres

Hint 33

(a) myosin filaments
(b) flagella
(c) sarcomeres
(d) actin filaments

Front 34

17-57 Figure Q17-57shows an electron micrograph of a skeletal muscle fiber, where various points along a fiber and various regions have been labeled.

Back 34

(b) Point B will move closer to point C.

Hint 34

Figure Q17-57

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.

Front 35

17-58 Which of the following conditions is likely to decrease the likelihood of skeletal muscle contraction?

Back 35

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

Hint 35

(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

Front 36

17-59 Which of the following statements about skeletal muscle contraction is false?

Back 36

(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.

Hint 36

(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.

Front 37

17-63 Consider the in vitro motility assay using purified kinesin and purified polymerized microtubules shown in Figure Q17-63. 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?

Back 37

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

Hint 37

Figure Q17-63

(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.