Molecular Cell Bio Chapter 17

Which of the statements below about intermediate filaments is FALSE?

 

1. They can stay intact in cells treated with concentrated salt solutions.

2. They can be found in the cytoplasm and the nucleus.

3. They can be anchored to the plasma membrane at a cell–cell junction.

4. Each filament is about 25 nm in diameter.

4

The graph in Figure 17-2 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 the cell cytosol. Three parts of the curve are labeled above it as A, B, and C.

Figure 17-2

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 17-2?

 

1. A

2. B

3. C

4. None. The shape of my graph should be identical to the graph produced when tubulin is polymerized in the absence of purified centrosomes.

1

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?

 

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.

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

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.

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

3

The hydrolysis of GTP to GDP carried out by tubulin molecules

 

1. provides the energy needed for tubulin to polymerize.

2. occurs because the pool of free GDP has run out.

3. tips the balance in favor of microtubule assembly.

4. allows the behavior of microtubules called dynamic instability.

4

Which of the following statements about the structure of microtubules is FALSE?

 

1. Microtubules are built from protofilaments that come together to make a hollow structure.

2. The two ends of a protofilament are chemically distinct, with α-tubulin exposed at one end and β-tubulin exposed at the other end.

3. Within a microtubule, all protofilaments are arranged in the same orientation, giving the microtubule structural polarity.

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

4

Which of the following structures shortens during muscle contraction?

 

1. myosin filaments

2. overlap regions

3. sarcomeres

4. actin filaments

3

For both actin and microtubule polymerization, nucleotide hydrolysis is important for

 

1. stabilizing the filaments once they are formed.

2. increasing the rate at which subunits are added to the filaments.

3. promoting nucleation of filaments.

4. decreasing the binding strength between subunits on filaments.

4

Figure 17-5 shows the leading edge of a lamellipodium. Which of the following statements is FALSE?

Figure 17-5

 

1. Nucleation of new filaments near the leading edge pushes the plasma membrane forward.

2. ARP proteins nucleate the branched actin filaments in the lamellipodium.

3. Capping proteins bind to the minus ends of actin filaments.

4. There is more ATP-bound actin at the leading edge than in the actin filaments away from the leading edge.

3

Consider the in vitro motility assay shown in Figure 17-6. It uses purified kinesin and polymerized microtubules to assess the function of microtubule motor proteins. The three panels are microscopic images taken at one-second intervals. Three microtubules have been numbered to make it easy to identify them. Which of the following statements about this assay is FALSE?

Figure 17-6

 

1. Kinesin molecules are attached by their tails to a glass slide.

2. The microtubules used in this assay must be polymerized using conditions that stabilize tubule formation or else they would undergo dynamic instability.

3. ATP must be added for this assay to work.

4. Addition of a nonhydrolyzable ATP analog (e.g., AMP-PNP) would cause the microtubules to move faster.

4

Which of the following statements about microtubules is true?

 

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

2. Centromeres on chromosomes nucleate the microtubules of the mitotic spindle.

3. Because microtubules are subject to dynamic instability, they are used only for transient structures in a cell.

4. ATP hydrolysis by a tubulin heterodimer is important for controlling the growth of a microtubule.

1

Which of the following statements about the movement of materials in a nerve axon is true?

 

1. Movement along microtubules in the axon is slower than free diffusion, but it is necessary due to the importance of directional transport.

2. 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.

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

4. 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.

3

Which of the following conditions would decrease the likelihood of skeletal muscle contraction?

 

1. partial depolarization of the T-tubule membrane, such that the resting potential is closer to zero

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

3. an increase in the amount of ATP in the cell

4. a mutation in tropomyosin that decreases its affinity for the actin filament

2

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?

 

1. Rac

2. ARP

3. actin

4. myosin

4

Which of the following components is NOT important for ciliary movement?

 

1. sarcoplasmic reticulum

2. ATP

3. dynein

4. microtubules

1

Which of the following statements regarding dynamic instability is FALSE?

 

1. Each microtubule grows and shrinks independently of its neighbors.

2. The GTP cap helps protect a growing microtubule from depolymerization.

3. GTP hydrolysis by the tubulin dimer promotes microtubule shrinking.

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

4

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?

 

1. Actin filaments would grow longer.

2. Actin filaments would grow shorter because depolymerization would be enhanced.

3. Actin filaments would grow shorter because new monomers could not be added to the filaments.

4. No change, as the addition of monomers binding nonhydrolyzable ATP would not affect actin filament length.

1

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 components would be most likely to enhance the assembly of the nuclear lamina?

 

1. phosphatase inhibitors

2. ATP

3. a concentrated salt solution that is 10 times the concentration normally found in the nucleoplasm

4. protein kinase inhibitors

4

Which of the following statements about skeletal muscle contraction is FALSE?

 

1. When a muscle cell receives a signal from the nervous system, voltage-gated channels open in the T-tubule membrane.

2. 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.

3. 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.

4. During muscle contraction, the Z discs move closer together as the myosin heads walk toward the plus ends of the actin filaments.

2

Actin-mediated movement involves the coordination of many events in the cell. Which of the following is NOT required for cell crawling?

 

1. myosin-mediated contraction at the rear of the moving cell

2. integrin association with the extracellular environment

3. nucleation of new actin filaments

4. release of Ca²⁺ from the sarcoplasmic reticulum

4

Which of the following statements about the function of the centrosome is FALSE?

 

1. 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.

2. Centrosomes contain hundreds of copies of the γ-tubulin ring complex important for microtubule nucleation.

3. Centrosomes typically contain a pair of centrioles comprised of cylindrical arrays of short microtubules.

4. Centrosomes are the major microtubule-organizing center in animal cells.

1

Figure 17-4A shows how the movement of dynein causes a sperm cell’s flagellum to bend. If the polarity of the adjacent doublets of microtubules is reversed (see Figure 17-4B), what do you predict would happen?

Figure 17-4

 

1. No bending would occur.

2. Bending would occur exactly as diagrammed in Figure 17-4A.

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

4. The two microtubule doublets would slide away from each other.

3

Kinesins and dyneins

 

1. have tail domains that bind to microtubules.

2. move along both microtubules and actin filaments.

3. often move in opposite directions.

4. derive their energy from GTP hydrolysis.

3

Microtubules are important for transporting cargo in nerve cell axons, as diagrammed in Figure 17-3. Notice that the two types of cargo are traveling in opposite directions. Which of the following statements is likely to be FALSE?

Figure 17-3

 

1. The gray cargo is attached to dynein.

2. The black cargo and the gray cargo require ATP hydrolysis for their motion.

3. The black cargo moving toward the axon terminal contains a domain that specifically interacts with the tail domain of a particular kind of motor.

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

4

Three of the options below represent similarities in the polymerization mechanisms of actin filaments and microtubules. Which one applies to actin filaments or microtubules, but not both?

 

1. Although growth can occur at both ends, the growth rate is faster at the plus end.

2. Depolymerization initiates at the plus ends.

3. Nucleotide hydrolysis promotes depolymerization.

4. Free subunits (actin and tubulin) carry nucleoside triphosphates.

2

You are studying nuclear lamins and use recombinant DNA technology to alter the coding sequence of a nuclear lamin gene. The altered gene codes for a nuclear lamin protein that can no longer be phosphorylated during prophase of mitosis. What do you predict would happen if the cell only had the altered nuclear lamin gene (and not the unaltered version)?

 

1. Mitosis should proceed as usual because dephosphorylation of the lamin is what is important for nuclear lamina disassembly during mitosis.

2. Disassembly of the nuclear lamins will occur prematurely because the lamins cannot be phosphorylated.

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

4. Nuclear lamins will be unable to produce dimers, as the coiled-coil formation will be disrupted.

3