Ch. 13: The Cytoskeleton & Cell Movement Online Quiz (Essay) Flashcards
A human hereditary disease has two seemingly unrelated symptoms: an inability to clear mucus from the respiratory system and male sterility. What could be the cause of this disease?
The failure of cilia to beat would disable the clearing of the respiratory tract, and the failure of flagella to beat would render sperm immobile—one source of male sterility. This disease is caused by a failure to produce ___, the motor protein that drives ciliary and flagellar motion.
dynein
The positioning of various organelles—for example, the Golgi apparatus in cells—is the outcome of a balance between dynein and kinesin. What is the expected distribution of the Golgi apparatus in cells in which the dynein function is inhibited?
Disruption of activity would mean there is no minus-end-directed motor tugging at the Golgi apparatus. Kinesin, a plus-end-directed motor, continues to be active, and the Golgi apparatus (or fragments thereof) will be pulled toward the ___ end of microtubules (i.e., the cell periphery).
plus
What is one way in which the more rapid growth of actin filaments at one end of the cell (the plus end) compared to the other end (the minus end) is advantageous to the cell?
Since the plus end grows 5 to 10 times more rapidly than the minus end, microfilaments essentially grow in one ___. Thus, correct orientation of the plus end will cause a cell to ___ toward an attractant without any counteracting force in the opposite direction.
direction, move
Describe the genetic defect that causes Duchenne’s and Becker’s muscular dystrophy and the molecular processes that characterize the diseases:
The diseases are caused by a mutation in the gene for ___, a spectrin-related gene that links the actin network beneath the cell surface (the cortex) to transmembrane proteins in the plasma membrane. The transmembrane proteins, in turn, are bound to components of the extracellular matrix, and thus ___ plays a role in linking the cortex to the extracellular matrix. This firm anchoring to the extracellular matrix stabilizes muscle cells; without it, the constant stress of contraction results in their destruction and consequently in the loss of muscle tissue that characterizes the diseases.
dystrophin
The kinetic properties of myosin have been optimized such that it moves very rapidly along actin filaments. Specifically, this movement is accomplished by myosin’s very brief attachments to the microfilament along which it moves. In contrast, kinesin moves along microtubules more slowly, with longer periods of attachment between each “step.” How might these differences be explained in evolutionary terms?
Because myosin works in conjunction with so many other myosin molecules in muscle contraction, it is unlikely that hundreds of myosin molecules will all let go at once. Therefore, myosin can afford to sacrifice attachment for ___. In contrast, because vesicles and organelles are moved along a microtubule by just a few molecules of kinesin, the chances that they will all let go at once are higher. Therefore, kinesin takes shorter “steps” along the microtubule with ___ attachment periods. This increases the chances that the cargo will make it to its final destination without falling off the microtubule.
speed, longer
One approach to testing whether keratin intermediate filaments are dynamic is to inject biotin-labeled keratin into living fibroblasts and then to compare, at different times, the distribution of biotin-labeled keratin with that of endogenous keratin intermediate filaments. Assuming that keratin intermediate filaments turn over dynamically with a half-time of one hour, predict the comparative distribution of biotin-labeled keratin and keratin intermediate filaments 10 minutes and four hours after microinjection.
Any incorporation of the biotin-labeled keratin into keratin intermediate filaments takes time. After ___ minutes, the injected biotin-labeled keratin should be diffusely distributed in the cell. After ___ hours, individual keratin intermediate filaments will have turned over (with a ___ half-time). The biotin-labeled keratin should now be incorporated into the keratin intermediate filaments, and the distribution of the biotin-labeled keratin and of endogenous keratin in the intermediate filaments should be the same.
10 minutes, four hours, one-hour half-time
Actin filaments are a major element of the cytoskeleton. The cytoskeleton provides a framework for the cell and acts as a scaffold that both determines cell shape and positions organelles within cells. For example, the cytoskeleton provides the tracks along which organelles move. At the same time, the cytoskeleton is subject to cleavage by proteins like cofilin. What is the apparent function of cofilin?
Cofilin is an actin filament; it ___ protein that binds to actin freed during filament severing. Actin filaments must be dynamic for the cell to able to move. Hence, the ability of cofilin to sever actin filaments creates a dynamic actin cytoskeleton.
severs
In vitro, at tubulin concentrations intermediate between the critical concentration for assembly at the plus and minus ends, microtubules treadmill. As a consequence of treadmilling, the microtubules move. In what direction do the microtubules move and why?
Tubulin addition is ___ at the plus end than at the minus end of microtubules. There will be an intermediate concentration of free tubulin dimer at which there is net growth at the plus end and net loss of tubulin at the minus end—this is treadmilling. As a result of treadmilling, individual microtubules move in the direction of their plus ends.
faster
How does Ca2+ molecularly regulate the activity of myosin motors?
In striated muscle, actin-myosin contraction is regulated by the binding of Ca2+ to ___. ___ usually binds to actin in the absence of Ca2+ and blocks the binding of myosin to actin. In the presence of Ca2+, ___ changes shape, and myosin can bind to actin. In non-muscle and smooth muscle cells, myosin activity is regulated by phosphorylation. Myosin light-chain kinase (MLCK) is the responsible enzyme. MLCK activity is regulated by the binding of Ca2+ to calmodulin, which in turn binds to MLCK.
troponin
Why might colchicine, a tubulin-binding drug, be used to treat cancer?
Colchicine, an alkaloid derived from plants, binds tightly to tubulin and inhibits its polymerization, thereby preventing the ___of the mitotic spindle. The drug acts very quickly and inhibits cell division within a few minutes, hence its usefulness as an anticancer drug.
assembly
