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Capítulo 9
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1. It would inhibit contractility because the thin filaments would be pulled beyond the point where myosin heads could engage them. It would increase the H and I bands without affecting the A band.

2. GTP, tubulin, MAPs

3. Bacterial movement in a phagocyte and the extension of lamellipodia. Both depend on actin polymerization as a force-generating mechanism.

4. 9; 11

5. They would be exceptionally stable because microtubules become labile when they contain GDP-tubulin subunits at their plus end.

6. Warm the preparation, add GTP, add EGTA, add more tubulin, add taxol. Chill the preparation, add colchicine or another depolymerizing agent, add Ca 2+ , place the mixture under hydrostatic pressure.

7. No, since you are providing the axoneme with substances whose concentration may be controlled by the plasma membrane. In fact, the plasma membrane controls cAMP and Ca 2+ concentrations, both of which influence axonemal activity.

8. No. In fact, all the microtubules of the axon are oriented in the same direction; different motor proteins move vesicles in anterograde and retrograde directions.

9. No. The centrosome is located at the opposite end from that where these events are occurring. Events at the plus end are determined by dynamic instability and plus-end bound proteins.

10. The heads, because they must all have a structure that enables them to move along the same type of microtubular track and all of them require a catalytic site that can hydrolyze ATP and use the energy to drive the conformational changes of the power stroke. In contrast, the structure of the tails would be expected to vary according to the type of cargo being transported.

11. The doublets on one side of the axoneme or the other would be missing, depending on the direction the cilium is bent.

12. 50/sec, which is 100 for a two-headed molecule with a step size of 8nm and rate of movement of 800 nm per sec.

13. Changes in fluorescence can be followed during microscopic observation of a living cell, whereas radioactive localization requires that the cell be killed. Radioactivity is better suited for quantitative measurements, such as rates of tubulin synthesis or of tubulin incorporation into microtubules in a brief period of time.

14. Very little, since other kinesin-like proteins may be able to compensate for the loss of kinesin. This does not mean that kinesin normally does not play a major role in cellular activities.

15. Brain; muscle; and skin; keratin; MAPs, tropomyosin, troponin, or other actin-binding proteins.

16. That virtually every residue in the protein is indispensable for either maintaining the protein's structure or carrying out its function and that the processes of cell contractility/motility operate by a similar molecular mechanism in all eukaryotic organisms.

17. It is surprising that a protein expected to be involved in such basic cellular activities would show such a dramatic difference in related organisms. You would want to determine if the species that appears to encode the protein actually expresses the gene in its cells and whether the protein is involved in essential activities. They might be able to manage without cytoplasmic dynein by relying on a minus end-directed kinesin to move organelles in this direction along microtubules. In addition, plant cells utilize actin filaments for long range intracellular movements to a much greater extent than do animal cells.

18. This difference is explained by the fact that myosin molecules are part of a stable thick filament, which holds the motor proteins in place. In contrast, kinesin molecules are not part of an organized structure and they would diffuse away from the microtubule if both heads were to detach at the same time.

19. Microtubules are oriented with their plus ends toward the nerve terminal so that movement of these structures is accomplished by a motor that would move toward the minus end of the transported microtubule. If the motor is stationary, the microtubule attached to the head of the dynein would move toward the end of the axon.







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