Cell and Molecular Biology: Concepts and Experiments

Gerald Karp

Chapter 9

The Cytoskeleton and Cell Motility - all with Video Answers

Educators

Chapter Questions

Problem 1

If a myofibril were pulled so that the sarcomeres increased in length by approximately 50 percent, what effect would you expect this to have on the contractile ability of the myofibril? Why? What effects would this have on the $\mathrm{H}, \mathrm{A},$ and I bands?

Rachel Peterson

Rachel Peterson

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Problem 2

What are three different radioactively or fluorescently labeled substances that you might inject into a cell that would label the microtubules of a cell without labeling the other cytoskeletal elements?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 3

What are two types of nonmuscle motility that might be unaffected by antibodies against both myosin I and myosin II? Why?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 4

A centriole contains _________ complete microtubules, and a cilium contains ________ complete microtubules.

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 5

Microtubules can be formed in vitro from tubulin that is bound to GTP analogues that (unlike GTP) cannot be hydrolyzed. What properties would you expect these microtubules to possess?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 6

List two things you could do that would shift the dynamic equilibrium of an in vitro preparation of tubulin and microtubules toward the formation of microtubules. List two treatments that would shift the equilibrium in the opposite direction.

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 7

It was noted that a demembranated ciliary or flagellar axoneme is able to beat with a normal frequency and pattern. Can you conclude that the plasma membrane is not important in ciliary or flagellar function?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 8

Because cytoplasmic vesicles are seen to move in both directions within an axon, can you conclude that some microtubules are oriented with their plus end facing the axon terminus and others oriented with the opposite polarity? Why or why not?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 9

Would you agree with the statement that the centrosome plays a key role in determining the rates of lengthening and shortening of the microtubules of an animal cell? Why or why not?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 10

If you were comparing the molecular structure of kinesin and myosin, which are thought to have been derived from a common ancestral protein, which part (heads or tails) would you expect to be most similar between them? Why?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 11

Figure $9.31 a$ shows the appearance of a cross section of a ciliary axoneme cut at an interior portion of the cilium. How might the image of a cross section differ if it had been cut very near the tip of a cilium at the beginning of the recovery stroke?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 12

If an individual kinesin molecule can move at a rate of $800 \mathrm{nm} / \mathrm{sec}$ in an in vitro motility assay, what is the maximal turnover rate (ATP molecules hydrolyzed per second) by one of the molecule's motor domains?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 13

Why do you suppose more can be learned about microtubule dynamics by injecting fluorescent tubulin into a cell than radioactively labeled tubulin? Can you think of a question that would be better answered by radioactively labeled tubulin?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 14

Suppose you discovered that a mouse lacking copies of one of the conventional kinesin genes appeared to show no ill effects and lived to a ripe old age. What could you conclude about the role of this kinesin in intracellular locomotion?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 15

Which type of vertebrate tissue would you expect to be an excellent source of tubulin? of actin? of keratin? Which protein would you expect to be the least soluble and most difficult to extract? What types of protein would you expect as contaminants in a preparation of tubulin? Which in a preparation of actin?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 16

Actin is one of the most evolutionarily conserved proteins. What does this tell you about the structure and function of this protein in eukaryotic cells?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 17

According to data from genome sequences, cytoplasmic dynein is absent in some plants (e.g., Arabidopsis) and present in others (e.g., rice). Does this finding surprise you? What else might you do to confirm or refute such a statement? How is it possible that higher plant cells could operate without cytoplasmic dynein?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 18

Myosin II action (Figure 9.61 ) differs from that of kinesin (Figure 9.15 ) in that one of the kinesin heads is always in contact with a microtubule, whereas both myosin heads become completely detached from the actin filament. How are these differences correlated with the two types of motor activities in which these proteins engage?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 19

The microtubules of an axon are thought to be nucleated at the centrosome, then severed from their nucleation site and moved into the axon. It was noted in the text that cytoplasmic dynein is responsible for the retrograde movement of organelles in axons, yet this same motor is thought to mediate the anterograde movement of microtubules in these same cellular processes. How is it possible that the same minus end-directed motor can be involved in both anterograde and retrograde movements?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 20

Motor proteins are often described as mechanoenzymes. Why? Could this same term be applied to virtually all enzymes? Why or why not?

Danielle Ashley

Danielle Ashley

Numerade Educator