细胞生物学
适用课程: 细胞生物学(三)(BIOS1002)【访问量:142451】

Chapter 08 Cytoskeleton

Intermediate filament: Cytoskeletal fiber (10nm in diameter) formed by polymerization of related, but tissue-specific, subunit proteins including keratins, lamins, and neurofilaments.

Actin: Abundant structural protein in eukaryotic cells that interacts with many other proteins. The monomeric globular from (G actin) polymerizes to form actin filaments (F actin). In muscle cells, F actin interacts with myosin during contraction. See also microfilaments.

Cytoskeleton: Network of fibrous elements, consisting primarily of microtubules, microfilaments, and intermediate filaments, found in the cytoplasm of eukaryotic cells. The cytoskeleton provides organization and structural support for the cell and permits directed movement of organelles, chromosomes, and the cell itself.

Dyneins: A class of motor proteins that use the energy released by ATP hydrolysis to move toward the () end of microtubdes. Dyneins can transport vesicles and organelles, are responsible for the movement of cilia and flagella, and play a role in chromosome movement during mitosis.

Kinesins: A class of motor proteins that use energy released by ATP hydrolysis to move toward the (+) end of a microtubule. Kinesins can transport vesicles and organelles and play a role in chromosome movement during mitosis.

Microfilament: Cytoskeletal fiber (≈7nm in diameter) that is formed by polymerization of monomeric globular (G) actin, also called actin filament. Microfilaments play an important role in muscle contraction, cytokinesis, cell movement, and other cellular functions and structures.

Microtubule: Cytoskeletal fiber (≈25 nm in diameter)that is formed by polymerization of α, β-tubulin monomers and exhibits structural and functional polarity. Microtubules are important components of cilia, flagella, the mitotic spindle, and other cellular structures.

Microtubule-associated protein(MAP) Any protein that binds to microtubules and regulates their stability.

Motor protein: Any raember of a special class of mechanochemical enzymes that use energy from ATP hydrolysis to generate either linear or rotary motion; also called molecular motor. See also dyneins, kinesins, and myosins.

Myofibril: Long, slender structures within cytoplasm of muscle cells consisting of a regular repeating array of sarcomeres composed of thick(myosin)filaments and thin (actin) filaments.

Myosins: A class of motor proteins that have actin-stimulated ATPase activity. Myosins move along actin microfilaments during muscle contraction and cytokinesis and also mediate vesicle translocation

1.  What word or phrase below characterizes the three cytoskeletal filaments?

     a.       polymer of protein subunits

     b.       protein subunits connected to one another by covalent bonds

     c.       protein subunits connected to one another by weak, noncovalent bonds

     d.       a and c

     e.       a and b

2.  The advantage of the type of structure displayed by cytoskeletal filaments is that _________.

     a.  it lends itself to rapid assembly and disassembly

     b.  it is dependent upon and responsive to complex cellular regulation

     c.  it is tremendously stable

     d.  a and c

     e.  a and b

3.  Which type of cytoskeletal element is characterized as a hollow, rigid cylindrical tube with walls composed of tubulin subunits?

     a.  microfilaments

     b.  microtubules

     c.  intermediate filaments

     d.  all of the above

     e.  minitubules

4.  Which type of cytoskeletal element is described as a solid structure composed of actin subunits?

     a.  microfilaments

     b.  microtubules

     c.  intermediate filaments

     d.  all of the above

     e.  macrofilaments

5.  Which type of cytoskeletal element is described as a tough, ropelike fibers composed of a variety of related proteins like keratin?

     a.  microfilaments

     b.  microtubules

     c.  intermediate filaments

     d.  a and c

     e.  macrofilaments

6.  Which organisms below have been shown to have at least some semblance of a cytoskeleton?

     a.  mouse

     b.  bacteria

     c.  eukaryotes

     d.  all of the above

     e.  a and c

7.  Which of the following activities are identified as functions of the cytoskeleton?

     a.  a dynamic scaffold that provides structural support that helps to determine cell shape

     b.  internal framework that positions organelles in cell interior

     c.  a network of tracks that direct the movement of materials and organelles within the cell

     d.  a force-generating apparatus

     e.  all of the above

8.  What kinds of materials appear to be moved around the cell by the tracks of the cytoskeleton?

     a.  mRNA molecules

     b.  membranous carriers between the ER and Golgi complex

     c.  transport vesicles containing DNA

     d.  a and b

     e.  all of the above

9.  You use a nonionic detergent to extract a cell.  When you do, much of the translation machinery stays behind with the cytoskeleton, which is not solubilized by this treatment.  What do these results mean?

     a.  The cytoskeleton denatures in nonionic detergents.

     b.  The translation machinery and mRNA are solubilized by the nonionic detergents.

     c.  The translation machinery and mRNA is anchored to the cytoskeleton.

     d.  The cytoplasm Is anchored to the translation machinery.

     e.  The cytoskeleton is destabilized by the nonionic detergent.

10.  The splitting of a parent cell into two daughter cells is called ______.

     a.  diakinesis

     b.  cytokinesis

     c.  mitosis

     d.  meiosis

     e.  cytomegaly

11.  Fluorescence microscopy allows investigators to view things _________ of regular light microscopes.

     a.  above the limits of resolution

     b.  above the limits of magnification

     c.  below the limits of resolution

     d.  below the limits of magnification

     e.  at the limits of magnification

12.  One of the advantages of fluorescence microscopy is that investigators can use the technique on _____ cells.

     a.  living

     b.  dead

     c.  round

     d.  very large

     e.  very small

13.  You inject an antibody against the protein actin into a living animal cell.  After the injection, the cell begins mitosis and the process proceeds normally until cytokinesis would normally occur.  Instead, the contractile ring does not form and cytokinesis does not occur.  What do these results mean?

     a.  Cytokinesis is dependent upon microtubules.

     b.  Cytokinesis is dependent upon microfilaments.

     c.  The separation of chromosomes by the spindle is dependent upon microtubules.

     d.  A major component of the contractile ring is microfilaments.

     e.  b and d

14.  You inject an antibody against the protein tubulin into a living animal cell.  After the injection, the cell begins mitosis but the chromosomes do not become attached to the spindle; in fact, there is no spindle.  The contractile ring does form however.  What do these results mean?

     a.  The contractile ring is composed of microtubules.

     b.  Cytokinesis is dependent upon microtubules.

     c.  The separation of chromosomes by the spindle is dependent upon microtubules.

     d.  The spindle is composed, at least in part, by microtubules..

     e.  c and d

15.  You inject a fluorescent antibody against the protein actin into a living animal cell.  After the injection, the cell begins mitosis and fluorescence appears over the contractile ring.  What do these results mean?

     a.  Cytokinesis is dependent upon microtubules.

     b.  Cytokinesis is dependent upon microfilaments.

     c.  The contractile ring contains actin.

     d.  A major component of the contractile ring is likely to be microfilaments.

     e.  c and d

16.  You inject an antibody against the protein tubulin into a living animal cell.  After the injection, the cell begins mitosis and the spindle fluoresces.  What do these results mean?

     a.  The contractile ring is composed of microtubules.

     b.  Cytokinesis is dependent upon microtubules.

     c.  The spindle contains tubulin.

     d.  The spindle is composed, at least in part, by microtubules.

     e.  c and d

17.  Which of the following is an advantage of video microscopy?

     a.  Digital video has exceptional contrast.

     b.  Digital video can be enhanced by computer processing.

         c.  Digital video is used to visualize only things that are at or above the limits of resolution of a light microscope.

     d.  a and b

     e.  a, b and c

18.  When laser beams are shone through the objective lens of a microscope, a weak attractive is generated near the point of focus.  This allows the grasping of microscopic objects.  Such a device is called a(n) ________.

     a.  optical density

     b.  optical tweezers

     c.  confocal laser scanning

     d.  absorbance microscopy

     e.  laser pointer

2.  How can the rate of movement generated by molecular motors be measured?  After attaching MTs to a cover slip, microscopic beads with molecular motors (one or a few) attached can be placed on the MT surface using focused laser beams (optical tweezers).  The distance moved by the bead per unit time can be measured and the rate of movement thus determined.

3.  How can genetic mutations be used to determine cytoskeletal function?  What precaution must be taken in interpreting such data?  If the gene for a particular cytoskeletal element can be altered or "knocked out", its function would also be altered or eliminated.  Such a change could alter the cell's behavior and thus suggest the function of the cytoskeletal element.  Caution in interpretation must be exercised, however, since the absence or malfunction of one gene product may be compensated for by the function of another.  The cytoskeleton exhibits redundancy as do many other systems in living cells.

4.  Why do detergents extract soluble and membranous material from cells?  Detergents, being amphipathic, can break up and solubilize amphipathic membrane structures and thus break cells open.  This releases any soluble materials in the cell as well.  Larger interconnected structures like the cytoskeleton will be left behind.

5.  Why would the addition of an irreversibly binding ATP analog to an in vitro system for monitoring molecular motors stop motor function?  Such an ATP analog would bind to the molecular motor, but it would not be hydrolyzed or released.  This would paralyze further movement of the motor since the cycle of conformational changes could not be completed.

6.  An element of the cytoskeleton is treated with ferritin-labeled antibodies to dynein and visualized in the electron microscope.  Ferritin can be seen attached to these elements.  What are they?  What physical property of the cytoskeletal element would support your conclusion?  They are microtubules since dynein, a molecular motor identified with microtubules, is part of the structure.  If the element were hollow and the right diameter, this conclusion would be supported.

7.  A scientist observes the motion of a certain type of vesicle around the cell.  She notices that vesicle movement ceases when colchicine is added to the culture medium.  What conclusion can she draw? Since colchicine inhibits microtubule function, it is likely that the vesicular movement is orchestrated by microtubules.

8.  In an electron micrograph, an object is visible in cross-section.  It appears to be made of nine sets of three hollow tubes that are fused together.  They are arranged in a pattern that looks much like a pinwheel.  Next to this object is a similar object oriented perpendicularly to it.  What is the object in the micrograph?  It is a centriole.

9.  You are observing a structure in the microscope and have exposed it to fluorescent antibodies against -, - and -tubulin.  It stains positively for all three.  What kind of structure is it?  If the antibody for -tubulin is injected into these cells following disassembly of microtubules, what happens?  Since it is positive for -tubulin, it must be an MTOC.  If the antibody for -tubulinis injected into these cells after microtubule disassembly, the cell will be unable to reassemble its microtubules.

10.       A scientist is observing a dividing cell.  For a few minutes, he steps away from his microscope, which is fitted with a device to maintain the cell being observed at 37°C.  The thermostat on his device malfunctions and the temperature of the cell drops to about 4°C.  He is unable to see the spindle when he returns - why?  The low temperature causes the disassembly of microtubules.  Since the spindle is made of microtubules, it disassembles and therefore cannot be seen.

11.       If complete axonemes are placed in a test tube in the presence of an antibody that recognizes the globular heads of the dynein arms, what would happen when ATP is added to the medium?  It is likely that the antibody will bind to the globular heads of dynein in the axoneme.  If this is the case, the ATPase activity of dynein may be hampered and thus the axoneme might not be able to move since ATP hydrolysis is required.

12.       An investigator has purified tubulin subunits and added everything to a test tube required for microtubule polymerization. Fluorescently-labeled tubulin subunits are added for a brief period of time.  Immediately after the incubation, the microtubules are visualized under conditions making the fluorescent label visible.  What does she see?  If the experiment were repeated with living cells, what would she see?  The microtubules should be labeled at both the "+"-end and "-" end, since in vitro tubulin subunits can be added at both ends.  If labeled subunits are injected into living cells followed by a short incubation, most of the added and labeled subunits should attach to the "+"-end of the microtubules since in vivo, microtubules grow predominantly at this end.

13.       Radiolabeled subunits of an intermediate filament are microinjected into a living cell and their incorporation into existing filaments was noted.  What was seen?  The subunits enter the existing intermediate filaments at scattered sites.  Eventually, the intermediate filament is fully labeled.