Exam 17: Cytoskeleton

Which of the following statements regarding dynamic instability is FALSE?

Keratins, neurofilaments, and vimentins are all categories of intermediate filaments.Which of the following properties is not true of these types of intermediate filaments?

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

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.

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

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.

The hydrolysis of GTP to GDP carried out by tubulin molecules

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

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. \alpha -tubulin dynein nine ATP four thirteen basal body \gamma -tubulin twenty-one \beta -tubulin GTP UTP centrosome kinesin two vimentin \delta -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.

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

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

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

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)?

Kinesins and dyneins

Which of the situations below will enhance microtubule shrinkage?

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. \begin{array}{llcc} \text { Experiment } & \\ \text { number } & \text { added to muscle fibers}& \text {muscle state}\\\hline 1& \text { nothing } & \text {relaxed}\\\hline 2& \text { ATP } & \text {relaxed}\\\hline3& \text { ATP and \mathrm{Ca}^{2+} } & \text {contracted}\\ \hline4& \text { ATP, \( \mathrm{Ca}^{2+} \), and a drug that inhibits troponin from binding \( \mathrm{Ca}^{2+} \)} &\\\hline 5&\text { ATP and a drug that inhibits binding of tropomyosin to actin } &\\\hline 6&\text { a nonhydrolvzable analog of ATP } &\end{array} Table 17-22 Extra credit: In what state would the muscle be if you added Ca²⁺ but no ATP?

Intermediate filaments help protect animal cells from mechanical stress because filaments

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?

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)?

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