Chapter 14 Flashcards
1
Q
Actin filaments are approximately _______ in diameter.
5 Å
7 nm
11 nm
25 nm
A
7nm
2
Q
Actin exists in cells in two major forms called
monomers and dimers.
α-actin and β-actin.
G actin and D actin.
G actin and F actin.
A
G actin and F actin.
3
Q
ATP is hydrolyzed by actin
in the process of assembly into a filament.
after assembly but before disassembly.
in the process of disassociation.
after disassociation from the filament.
A
after assembly but before disassembly
4
Q
Each monomer of actin binds one molecule of the nucleotide triphosphate
ATP.
GTP.
CTP.
UTP.
A
ATP.
5
Q
The phenomenon that illustrates the dynamic behavior of actin filaments and is critical to regulating the structure and function of actin filaments is known as
assembly.
dynamic instability.
treadmilling.
disassembly.
A
treadmilling
6
Q
Actin filaments are stabilized by
cofilin.
gelsolin.
thymosin.
tropomyosin.
A
tropomyosin
7
Q
Branching of actin filaments can be initiated by
Arp2/3.
formin.
ADF/cofilin.
fimbrin.
A
Arp2/3.
8
Q
Actin filaments are bound into bundles of parallel filaments by the proteins
filamin and spectrin.
troponin and tropomyosin.
profilin and thymosin.
α-actinin and fimbrin.
A
α-actinin and fimbrin.
9
Q
Short actin filaments bind to tetramers of which protein to form the cytoskeleton of erythrocytes?
Myosin
α-actinin
Spectrin
Ankyrin
A
Spectrin
10
Q
Which movement is not based on actin–myosin interactions?
Cell migration (crawling) over surfaces
Chromosome movement during anaphase A
Cytokinesis of animal cells
Phagocytosis
A
Chromosome movement during anaphase A
11
Q
In actin filament assembly, the process in which ATP-actin monomers are added to the barbed end of the filament while ADP-actin monomers are concurrently dissociating from the pointed end of the filament is referred to as
equilibrium.
dynamic instability.
treadmilling.
recycling.
A
treadmilling
12
Q
Duchenne’s muscular dystrophy is characterized by
X-chromosomal inheritance.
childhood onset.
abnormal dystrophin function.
All of the above
A
abnormal dystrophin function.
13
Q
Actin filaments are anchored at junctions called
adherens junctions.
tight junctions.
desmosomes.
gap junctions.
A
adherens junctions.
14
Q
During muscle contraction, the A band
stays the same width, and the I bands and H zone shorten.
and H zone stay the same width, and the I bands shorten.
shortens, and the I bands and H zone stay the same.
and H zone shorten, and the I bands stay the same.
A
stays the same width, and the I bands and H zone shorten.
15
Q
The barbed (fast growing) end of actin filaments is located in muscle
at the A/I junction.
near the M line of a contracted sarcomere.
at the inner margin of the A/I zone.
at the Z disc.
A
at the Z disc.
16
Q
Myosin _______ is present in muscle sarcomeres.
I
II
III
IV
A
II
17
Q
Microtubules are typically _______ in diameter.
7 nm
10–12 nm
25 nm
35 nm
A
25 nm
18
Q
Microtubules are assembled from
α-tubulin dimers.
β-tubulin dimers.
alternating α-tubulin dimers and β-tubulin dimers.
dimers of α- and β-tubulin.
A
dimers of α- and β-tubulin
19
Q
Which nucleotide triphosphate is hydrolyzed during a cycle of microtubule assembly and disassembly?
ATP
TTP
CTP
GTP
A
GTP
20
Q
The GTP bound to β-tubulin hydrolyzes to GDP and Pi
during depolymerization of the α–β dimer.
during polymerization of dimers onto microtubules.
during depolymerization of dimers from microtubules.
following polymerization but before depolymerization.
A
following polymerization but before depolymerization
21
Q
The microtubule behavior in which individual microtubules alternate between cycles of growth and shrinkage is called
an equilibrium state.
dynamic instability.
treadmilling.
recycling.
A
dynamic instability.
22
Q
Both colchicine and colcemid
block microtubule organizing centers.
block microtubule disassembly by binding to microtubule ends.
block microtubule assembly by binding to free tubulin.
accelerate microtubule disassembly by binding to tubulin in microtubules, causing those molecules to exit the microtubule more quickly.
A
block microtubule assembly by binding to free tubulin.
23
Q
The anticancer drug taxol
blocks microtubule organizing centers.
stabilizes microtubules and thus inhibits disassembly.
blocks microtubule assembly by binding to free tubulin.
accelerates microtubule disassembly by binding to tubulin in microtubules, causing those molecules to exit the microtubule more quickly.
A
stabilizes microtubules and thus inhibits disassembly.
24
Q
Microtubules are not involved in
movement of chromosomes during mitosis.
transport of membranous vesicles in the cytoplasm.
cytokinesis of animal cells.
movement of cilia and flagella
A
cytokinesis of animal cells.
25
The major microtubule-organizing center in most animal cells is the
kinetochore.
nucleus.
centrosome.
centromere.
centrosome.
26
The role of the centrosome is to
determine the center of the cell.
determine the position of the nucleus.
initiate microtubule growth.
adhere to the plus ends of microtubules.
initiate microtubule growth.
27
Rings of the protein _______ in the pericentriolar material nucleate microtubule assembly.
centrin
pericentrin
α-tubulin
γ-tubulin
γ-tubulin
wc associates w other proteins and forms a ring shape structure = γ-tubulin ring complex.
This complex is thought to bypass the rate-limiting nucleation step, speeding microtubule growth.
28
At the end of interphase, the part of the microtubule farthest from the centrosome is the _______ end.
capped
barbed
minus
plus
plus
29
Kinesin I is a motor protein molecule consisting of
two heavy chains.
one heavy chain and two light chains.
two heavy chains and two light chains.
two heavy chains and four light chains.
two heavy chains and two light chains.
30
The cargo carried by kinesin along microtubules binds to kinesin on which region?
Head
Neck
Coiled-coil
Tail
tail
31
Kinesins are able to transport _______ along microtubules.
membranous vesicles
mitochondria
All of the above
Membranous vesicles and mitochondria
Endoplasmic reticulum
mRNAs
32
Cytoplasmic dynein plays a key role in the positioning of which organelle?
Nucleus
Peroxisome
Mitochondrion
Golgi apparatus
Cytoplasmic dynein has a role in positioning the Golgi apparatus near the centrosome.
33
A male patient at a medical clinic presents with infertility due to nonmotile sperm and an inability to clear mucous from his respiratory tract. Other tissues are normal. You suspect that these symptoms may be caused by mutant
tubulin.
kinesin.
dynein.
tau protein.
dynein
34
In a motile cilium or flagellum, _______ microtubules are arranged _______.
13; in a circle
9 triplet; in a circle
9 doublet; in a circle around a central pair of microtubules
2 microtubules; perpendicular to each other
9 doublet; in a circle around a central pair of microtubules
35
The basal bodies of cilia and flagella are similar in structure to (and can form from)
centromeres.
kinetomeres.
kinetochores.
centrioles
centrioles
36
Adjacent microtubule doublets in cilia and flagella produce a bending movement because
tubulin is contracting on one side of the microtubules.
dynein is contracting on one side of the microtubules.
kinesin is contracting on one side of the microtubules.
nexin links between microtubule doublets convert a sliding movement into a bending movement.
nexin links between microtubule doublets convert a sliding movement into a bending movement.
37
The beating of cilia and flagella occurs by means of _______-based _______.
dynein; microtubule sliding
kinesin; microtubule sliding
myosin; microfilament sliding
tubulin; microtubule contraction
dynein; microtubule sliding
38
The microtubules that overlap in the center of the mitotic spindle are called _______ microtubules.
astral
minus-end
kinetochore
interpolar
interpolar
Overlapping interpolar microtubules elongate and slide against one another
to push the spindle poles apart
39
The drug taxol stabilizes microtubules so they cannot shorten. If taxol were added during anaphase of mitosis, what effect would you expect it to have on anaphase movements?
It would stop all movements.
It would stop anaphase A but not anaphase B.
It would stop anaphase B but not anaphase A.
It would have no effect.
It would stop all movements
40
The intermediate filaments in the nucleus are made of
keratins.
lamins.
desmin.
vimentin.
keratins
41
The desmin filaments in muscle cells connect
actin filaments to the Z line.
actin filaments to the plasma membrane at the ends of myofibrils.
Z lines of adjacent myofibrils.
myosin filaments to the Z line.
Z lines of adjacent myofibrils.
42
Keratin filaments are found in which of the following cell types?
Fibroblasts
Adipocytes
Muscle cells
Epithelial cells
Epithelial cells
43
Vimentin is the major intermediate filament protein of _______ cells.
epithelial
striated muscle
nerve
fibroblast
fibroblast
44
Which of the following is not an intermediate filament protein?
```
Vimentin
nestin
Lamin
lamin A
desmin
Fibronectin
```
Fibronectin
45
Intermediate filaments are typically _______ in diameter.
5–7 nm
10–12 nm
16–22 nm
24–26 nm
10–12 nm
46
Intermediate filaments function in
cell motility.
providing mechanical strength for cells.
nuclear pore structure.
All of the above
providing mechanical strength for cells.
47
Keratin filaments are anchored to junctions called
adherens junctions.
tight junctions.
desmosomes.
gap junctions.
desmosomes
48
Which protein can link intermediate filaments with actin filaments and microtubules?
α-actinin
β-catenin
Integrin
Plectin
Plectin
49
Expression of a shortened skin keratin gene in place of the normal keratin gene in transgenic mice results in a phenotype in which mice have
thick skin.
no hair.
fragile, easily blistered skin.
white hair.
fragile, easily blistered skin.
50
When a striated muscle is stimulated to contract, calcium is released primarily from the __i_____ and interacts with a group of three thin-filament proteins called ___ii____, which in turn cause the long, thin protein ___iii____ to move away from the cross-bridge binding sites on the __iv_____ molecules. This allows the cross-bridge cycle to occur.
i.sarcoplasmic
reticulum
ii. Troponin complex (Tnl,TnC,TnT)
iii. Tropomyosin
iv. myosin on actin
51
The initial role of ATP in the cross-bridge cycle is to bind to _______ and cause it to _______.
myosin heads
dissociates myosin from actin
cause a conformational change resulting in mvt of myosin heads along actin filaments
52
Dynein and kinesin are similar in that they consist of heavy chains with globular heads that bind both _______ and _______.
actin and microtubules
or and
form cross-bridges btw thick and thin filaments
53
The intermediate filament protein ____i___ is the major protein of the human skin, hair, and fingernails. The most prominent secondary structure in this protein is the ___ii____. The monomers of this protein associate first to form dimers and then are assembled into protofilaments, __iii_____ (number) of which associate laterally to form an intermediate filament.
i. Keratin
ii. desmoplakin
iii. tetramers
54
In cells, actin filaments form a network that provides mechanical support, determines cell shape, and allows for movement of the cell surface.
true
55
Actin exists in two forms, a globular G form and a filamentous F form.
true
56
Formation of actin filaments requires energy in the form of ATP.
true
57
Actin may be cross-linked into antiparallel bundles.
f
58
In muscle, every myofibril is organized as a chain of contractile units called myocytes.
f
59
Microtubules are approximately 1/3 the diameter of actin filaments.
f
60
The cycle of alternating growth and shrinkage of microtubules is referred to as dynamic instability, or rescue and catastrophe.
true
61
Kinesins and dyneins are molecular motor proteins.
true
62
Epidermolysis bullosa simplex is a skin disease caused by a mutation in the gene encoding keratin.
true
63
Desmosomes are specialized junctions that help hold cells together, while their counterpart hemidesmosomes prevent cell-to-cell contact.
f
Desmosomes—junctions between adjacent cells.
Hemidesmosomes -junctions between
epithelial cells and underlying connective tissue
64
What is the normal function of dystrophin in muscle cells?
Dystrophin (a calponin) in muscle cells
links actin filaments to transmembrane proteins in the plasma membrane, which link to the extracellular matrix, helping maintain cell stability during muscle contraction.
65
Nebulin filaments attach to actin filaments in muscle for what purpose?
Nebulin filaments are associated with thin actin filaments length, it's activation, and cross bridge recruitment
Titin - thick myosin filament to Z line
66
Describe the myosin II molecule and its component parts
(the type of myosin in muscle that produces contraction by sliding actin filaments) has two heavy chains and two pairs of light chains.
The heavy chains have a globular head region and a long α-helical tail. The tails twist around each other in a coiled-coil.
The globular heads bind actin, forming cross-bridges between thick and thin filaments
67
What is a sarcomere? (Include the structures that bound it in your definition.)
functional contractile unit of myofibril of striated muscle. composed of interacting two main protein filaments—actin thin filament which is made up of troponin (TnT,Tnc,Tnl), tropomyosin, actin) and myosin, —which are the active structures responsible for muscular contraction. It is the length of one z disc to another z disc with a A band in the middle
68
How is contraction regulated in smooth muscle?
Smooth-muscle contraction is regulated by two systems, sliding filament model and the cross-bridge cycle that forms the molecular basis of the sliding filament theory dissociation of actin-myosin complex.
Nerve impulse stimulates muscle fibers that leads to muscle contraction. Calcium ions are released from the sarcoplasmic reticulum which affects the actin-thin filament. In the excess of calcium ions, the TnC is bound to it which in turn causes a change in orientation of the troponin complex and tropomyosin. This leaves myosin binding sites open for the myosin head to bind to it. Binding of ATP dissociates myosin head from actin. ATP hydrolysis on the myosin head induces a conformational change that displaces it. It binds to new position on actin filament and Pi is released. Sliding filament results in Z disc to get closer together, no change in the A band, I band, and H zone almost disappear leading to actin and myosin sliding past one another.
69
How are the plus and minus ends of microtubules arranged in the dendrites of neurons?
the microtubules are oriented in both directions, alternating from plus and negative ends.
70
What is the polarity of microtubule assembly in an axon of a nerve?
In Axons microtubules have plus ends towards tips
71
What MAP is the main component of the lesions found in the brains of Alzheimer's patients?
Tau proteins
72
In which directions do kinesin and dynein transport vesicles in an axon?
kinesins move along microtubules towards plus end
| dyneins move towards the minus end
73
Describe the two separate mechanisms by which the poles of the mitotic spindle move apart in anaphase B.
Overlapping interpolar microtubules elongate and slide against one another and push the spindle poles apart.
The plus-end-directed kinesins criss-cross interpolar microtubules and move them toward the plus end.
74
Intermediate filament proteins vary in size but share structural features. Describe these shared features
They are intermediates between actin filaments and microtubules
Have a central alpha-helical rod domain for filament assembly
The monomers of this protein associate first to form dimers and then are assembled into protofilaments, tetramer of which associate laterally to form an intermediate filament.
head and tail domains for spefic functions
More stable than actin and myosin thus have no dynamic nature
75
Which of the following is not one of the functions of the cytoskeleton?
To provide a structural framework for the cell
Cell locomotion
Protein translocation into the ER
Intracellular movement of organelles and other structures
Protein translocation into the ER
76
The approximate diameter of an actin filament is
7 nm.
10–12 nm.
25 nm.
5 mm.
7
77
Which of the following is not true of the assembly of actin filaments?
It begins with the formation of an aggregate of three actin monomers.
It requires ATP.
Polymerization occurs from both the plus and minus ends.
Polymerization is faster from the plus end than from the minus end.
Polymerization occurs from both the plus and minus ends.
78
Which of the following is not a function of actin-binding proteins?
Filament initiation and polymerization
End capping
Filament severing/depolymerization
Incorporation of microfilaments into the extracellular matrix
Incorporation of microfilaments into the extracellular matrix
79
Cofilin plays a role in the
disassembly of actin filaments.
stimulation of actin filament formation.
nucleation of microfilaments.
disassembly of microtubules.
disassembly of actin filaments.
80
Which cytosolic protein in red blood cells is the link between the plasma membrane and the spectrin/actin network beneath the cell surface?
Band 3
Glycophorin
Dystrophin
Ankyrin
Ankyrin
81
Which of the following does not mediate the association between actin filaments and the plasma membrane?
Catenin–cadherin links
Myosin I–calmodulin links
Talin–integrin links
Direct interaction between actin and the plasma membrane
Direct interaction between actin and the plasma membrane
82
Which statement about myosin I is true?
It is involved in muscle contraction.
It has a long α-helical tail through which it forms homodimers.
It does not act as a molecular motor.
It links the actin bundles to the plasma membrane in the microvilli of intestinal cells.
It links the actin bundles to the plasma membrane in the microvilli of intestinal cells.
83
In cell movement, branched actin filament growth pushes against the cell membrane. Proteins involved in this process include all of the following except
Arp2/3.
profilin.
vinculin.
WASP proteins.
vinculin
| vinculin and talin activate integrins to bind to extracellular matrix (actin to intergrins)
84
The basis for muscle contraction is the
rotation of myosin fibers around actin fibers.
expansion of the sarcomere.
sliding of myosin and actin fibers past each other.
movement of the Z discs away from each other.
sliding of myosin and actin fibers past each other.
85
The major cation responsible for regulating actin-myosin contraction is
Ca2+.
H+.
K+.
Na+.
Ca2+.
86
Myosin II is found in skeletal muscle and the contractile ring. It is a two-headed myosin with tails that can form thick filaments. Other myosins, such as myosin I or myosin V, do not form thick filaments and yet are still capable of producing movement along actin filaments. To what do the tail(s) of myosin I and myosin V bind?
Cargo such as membrane vesicles or intermediate filaments
Centrosomes
Chromosomes
Tubulin
Cargo such as membrane vesicles or intermediate filaments
87
Whether a microtubule shrinks or grows is determined by the
rate of GTP-bound tubulin addition relative to the rate of tubulin GTP hydrolysis.
phosphorylation state of β-tubulin.
rate of ATP hydrolysis relative to the rate of ATP-bound tubulin addition.
presence or absence of γ-tubulin.
rate of GTP-bound tubulin addition relative to the rate of tubulin GTP hydrolysis.
88
Kinesin and dynein are
intermediate filament proteins.
microtubule motor proteins.
proteins within centrosomes that mediate nucleation of microtubules.
microfilament motor proteins.
microtubule motor proteins.
89
Like myosins, kinesins and dyneins are both families of proteins. Which statement is true of all kinesins and dyneins?
They are microtubule-dependent motors.
They are minus-end-directed motors.
The motor activity of the proteins resides in their light chains.
They are plus-end-directed motors.
They are microtubule-dependent motors.
90
Which statement about cilia is false?
They are about 10 micrometers in length, and cells that have them usually have many of them.
They can be used to move fluids over the cell surface or to move the cell through fluids.
They are projections of the plasma membrane supported by microfilaments.
Their movement relies on the motor activity of axonemal dynein.
They are projections of the plasma membrane supported by microfilaments.
91
A centrosome is
a cylindrical structure made up of nine triplets of microtubules.
a chromosomal region that connects sister chromatids during mitosis and attaches them to the spindle.
a protein structure that binds centromeres and mediates the attachment of chromosomes to the spindle.
the major microtubule-organizing center in animal cells.
the major microtubule-organizing center in animal cells.
92
Which of the following microtubules are attached to chromosomes?
Astral microtubules
Interphase microtubules
Kinetochore microtubules
Interpolar microtubules
Kinetochore microtubules
93
The discovery that the intermediate filament protein keratin is essential for mechanical strength of epithelial cell layers was made in
cell cultures.
organ cultures.
small invertebrates.
transgenic mice.
transgenic mice.
94
Which statement about intermediate filaments is true?
Rather than consisting of a single type of protein, they can be made up of a number of different proteins.
They are involved in cell movement.
The basic structure of an intermediate filament protein is a globular head and a long α-helical tail.
Like microfilaments, they exhibit treadmilling
They are involved in cell movement.
95
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 in creating a dynamic cytoskeleton?
Cofilin serve /slice filaments for generstion of new ends for polymerization. Leaving the pointe and barbed ends open for polymerization
96
Describe the genetic defect that causes Duchenne's and Becker's muscular dystrophy and the molecular processes that characterize the diseases.
Absent dystrophin led to Duchenne's muscular dystrophy and abnormal dystrophin muscular dystrophy
muscular dystrophy is a X-linked inherited disease.
Dystrophin links actin filaments to transmembrane proteins in plasma membrane to the extracellular matrix and stability during muscle contraction.
97
How does Ca2+ regulate the activity of myosin motors in striated muscle, nonmuscle, and smooth muscle cells?
increase ca2+ conc
decrease ca2+ conc
Nerve impulse stimulates muscle fibers that leads to muscle contraction. Calcium ions are released from the sarcoplasmic reticulum which affects the actin-thin filament. In the excess of calcium ions, the TnC is bound to it which in turn causes a change in orientation of the troponin complex and tropomyosin. This leaves myosin binding sites open for the myosin head to bind to it. Binding of ATP dissociates myosin head from actin. ATP hydrolysis on the myosin head induces a conformational change that displaces it. It binds to new position on actin filament and Pi is released. Sliding filament results in Z disc to get closer together, no change in the A band, I band, and H zone almost disappear leading to actin and myosin sliding past one another
98
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?
The advantage of such an arrangement is that the cell can maintain a large pool of subunits for explosive growth at the sites and times of its choosing.
99
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?
Kinesin and myosin evolved from common ancestor and are structurally similar.
100
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?
Cytoplasmic dynein has a role in positioning the Golgi apparatus near the centrosome.
101
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?
dynein, the motor protein that drives ciliary and flagellar motion.
102
Why might colchicine, a tubulin-binding drug, be used to treat cancer?
affect microtubule assembly
103
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
104
When is the centrosome duplicated?
During interphase
105
What happens to the centrosomes during prophase ?
Migrate to form the two pole of the mitotic spindle
106
Do plant cells have centrosomes?
No
107
Kinesin I moves along microtubules in one direction—toward the
plus end.
108
What shows that kinesin
and myosin evolved from a common
ancestor and are structurally similar
X-ray crystallography
109
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
110
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
111
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
112
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
113
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
114
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
115
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
116
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
117
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
