Essential Cell Biology

Bruce Alberts, Dennis Bray, Karen Hopkin

Chapter 17

Cytoskeleton - all with Video Answers

Educators

Chapter Questions

Problem 1

Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm? Explain your answers.
A. Amoeba proteus (a free-living amoeba)
B. Skin epithelial cell
C. Smooth muscle cell in the digestive tract
D. Escherichia coli
E. Nerve cell in the spinal cord
F. Sperm cell
G. Plant cell

Mariana Roldan

Mariana Roldan

Numerade Educator

Problem 2

Why do you suppose it is much easier to add tubulin to existing microtubules than to start a new microtubule from scratch? Explain how $\gamma$ -tubulin in the centrosome helps to overcome this hurdle.

Mariana Roldan

Numerade Educator

Problem 3

Dynamic instability causes microtubules either to grow or to shrink rapidly. Consider an individual microtubule that is in its shrinking phase.
A. What must happen at the end of the microtubule in order for it to stop shrinking and to start growing again?
B. How would a change in the tubulin concentration affect this switch?
C. What would happen if only GDP, but no GTP, were present in the solution?
D. What would happen if the solution contained an analog of GTP that cannot be hydrolyzed?

Mariana Roldan

Numerade Educator

Problem 4

Dynein arms in a cilium are arranged so that, when activated, the heads push their neighboring outer doublet outward toward the tip of the cilium. Consider a cross section of a cilium (see Figure $17-26$ ). Why would no bending motion of the cilium result if all dynein molecules were active at the same time? What pattern of dynein activity can account for the bending of a cilium in one direction?

Mariana Roldan

Numerade Educator

Problem 5

The formation of actin filaments in the cytosol is controlled by actinbinding proteins. Some actin-binding proteins significantly increase the rate at which the formation of an actin filament is initiated. Suggest a mechanism by which they might do this.

Mariana Roldan

Numerade Educator

Problem 6

Suppose that the actin molecules in a cultured skin cell have been randomly labeled in such a way that 1 in 10,000 molecules carries a fluorescent marker. What would you expect to see if you examined the lamellipodium (leading edge) of this cell through a fluorescence microscope? Assume that your microscope is sensitive enough to detect single fluorescent molecules.

Mariana Roldan

Numerade Educator

Problem 7

At the leading edge of a crawling cell, the plus ends of actin filaments are located close to the plasma membrane, and actin monomers are added at these ends, pushing the membrane outward to form lamellipodia or filopodia. What do you suppose holds the filaments at their other ends to prevent them from just being pushed into the cell's interior?

Mariana Roldan

Numerade Educator

Problem 8

If both the actin and myosin filaments of muscle are made up of subunits held together by weak noncovalent bonds, how is it possible for a human being to lift heavy objects?

Mariana Roldan

Numerade Educator

Problem 9

Compare the structure of intermediate filaments with that of the myosin-II filaments in skeletal muscle cells. What are the major similarities? What are the major differences? How do the differences in structure relate to their function?

Mariana Roldan

Numerade Educator

Problem 10

A. Note that in Figure $17-46$, troponin molecules are evenly spaced along an actin filament, with one troponin found every seventh actin molecule. How do you suppose troponin molecules can be positioned this regularly? What does this tell you about the binding of troponin to actin filaments?
B. What do you suppose would happen if you mixed actin filaments with (i) troponin alone,
(ii) tropomyosin alone, or
(iii) troponin plus tropomyosin, and then added myosin? Would the effects be dependent on $\mathrm{Ca}^{2+}$ ?

Mariana Roldan

Numerade Educator

Problem 11

Which of the following statements are correct? Explain your answers.
A. Kinesin moves endoplasmic reticulum membranes along microtubules so that the network of ER tubules becomes stretched throughout the cell.
B. Without actin, cells can form a functional mitotic spindle and pull their chromosomes apart but cannot divide.
C. Lamellipodia and filopodia are "feelers" that a cell extends to find anchor points on the substratum that it will then crawl over.
D. GTP is hydrolyzed by tubulin to cause the bending of flagella.
E. Cells having an intermediate-filament network that cannot be depolymerized would die.
F. The plus ends of microtubules grow faster because they have a larger GTP cap.
G. The transverse tubules in muscle cells are an extension of the plasma membrane, with which they are continuous; similarly, the sarcoplasmic reticulum is an extension of the endoplasmic reticulum.
H. Activation of myosin movement on actin filaments is triggered by the phosphorylation of troponin in some situations and by $\mathrm{Ca}^{2+}$ binding to troponin in others.

Mariana Roldan

Numerade Educator

Problem 12

The average time taken for a molecule or an organelle to diffuse a distance of $x \mathrm{cm}$ is given by the formul $t=x^{2} / 2 D$ where $t$ is the time in seconds and $D$ is a constant called the diffusion coefficient for the molecule or particle. Using the above formula, calculate the time it would take for a small molecule, a protein, and a membrane vesicle to diffuse from one side to another of a cell $10 \mu \mathrm{m}$ across. Typical diffusion coefficients in units of $\mathrm{cm}^{2} / \mathrm{sec}$ are: small molecule, $5 \times$ $10^{-6} ;$ protein molecule, $5 \times 10^{-7} ;$ vesicle, $5 \times 10^{-8}$. How long would a membrane vesicle take to reach the end of an axon $10 \mathrm{cm}$ long by free diffusion? How long would it take if it was transported along microtubules at $1 \mu \mathrm{m} / \mathrm{sec} ?$

Sana Riaz

Numerade Educator

Problem 13

Why do eukaryotic cells, and especially animal cells, have such large and complex cytoskeletons? List the differences between animal cells and bacteria that depend on the eukaryotic cytoskeleton.

Mariana Roldan

Numerade Educator

Problem 14

Examine the structure of an intermediate filament shown in Figure $17-4 .$ Does the filament have a unique polarity-that is, could you distinguish one end from the other by chemical or other means? Explain your answer.

Mariana Roldan

Numerade Educator

Problem 15

There are no known motor proteins that move on intermediate filaments. Suggest an explanation for this.

Mariana Roldan

Numerade Educator

Problem 16

When cells enter mitosis, their existing array of cytoplasmic microtubules has to be rapidly broken down and replaced with the mitotic spindle that forms to pull the chromosomes into the daughter cells. The enzyme katanin, named after Japanese samurai swords, is activated during the onset of mitosis, and chops microtubules into short pieces. What do you suppose is the fate of the microtubule fragments created by katanin? Explain your answer.

Mariana Roldan

Numerade Educator

Problem 17

The drug Taxol, extracted from the bark of yew trees, has an opposite effect to the drug colchicine, an alkaloid from autumn crocus. Taxol binds tightly to microtubules and stabilizes them; when added to cells, it causes much of the free tubulin to assemble into microtubules. In contrast, colchicine prevents microtubule formation. Taxol is just as pernicious to dividing cells as colchicine, and both are used as anticancer drugs. Based on your knowledge of microtubule dynamics, suggest why both drugs are toxic to dividing cells despite their opposite actions.

Mariana Roldan

Numerade Educator

Problem 18

A useful technique for studying microtubule motors is to attach them by their tails to a glass coverslip (which can be accomplished quite easily because the tails stick avidly to a clean glass surface) and then allow them to settle. The microtubules may then be viewed in a light microscope as they are propelled over the surface of the coverslip by the heads of the motor proteins. Because the motor proteins attach at random orientations to the coverslip, however, how can they generate coordinated movement of individual microtubules rather than engaging in a tug-of-war? In which direction will microtubules crawl on a 'bed' of kinesin molecules (i.e., will they move plus-end first, or minus-end first)?

Mariana Roldan

Numerade Educator

Problem 19

A typical time course of polymerization of purified tubulin to form microtubules is shown in Figure $017-19$.
A. Explain the different parts of the curve (labeled $A, B$, and $C$ ). Draw a diagram that shows the behavior of tubulin molecules in each of the three phases.
B. How would the curve in the figure change if centrosomes were added at the outset?

Sana Riaz

Numerade Educator

Problem 20

The electron micrographs shown in Figure $Q 17-20 A$ were obtained from a population of microtubules that were growing rapidly. Figure $Q 17-20 B$ was obtained from microtubules undergoing "catastrophic" shrinking. Comment on any differences between $A$ and $B$, and suggest likely explanations for the differences that you observe.

Mariana Roldan

Numerade Educator

Problem 21

The locomotion of fibroblasts in culture is immediately halted by the drug cytochalasin, whereas colchicine causes fibroblasts to cease to move directionally and to begin extending lamellipodia in seemingly random directions. Injection of fibroblasts with antibodies to vimentin has no discernible effect on their migration. What do these observations suggest to you about the involvement of the three different cytoskeletal filaments in fibroblast locomotion?

Mariana Roldan

Numerade Educator

Problem 22

Complete the following sentence accurately, explaining your reason for accepting or rejecting each of the four phrases (more than one can be correct). The role of calcium in muscle contraction is:
A. To detach myosin heads from actin.
B. To spread the action potential from the plasma membrane to the contractile machinery.
C. To bind to troponin, cause it to move tropomyosin, and thereby expose actin filaments to myosin heads.
D. To maintain the structure of the myosin filament.

Mariana Roldan

Numerade Educator

Problem 23

Which of the following changes takes place when a skeletal muscle contracts?
A. $Z$ discs move farther apart.
B. Actin filaments contract.
C. Myosin filaments contract.
D. Sarcomeres become shorter.

Mariana Roldan

Numerade Educator