1 Indicate if each of the following structures is based on actin filaments (A), microtubules
(M), or intermediate filaments (I). Your answer would be a five-letter string composed of letters
A, M, and I only; e.g. AAAMM.
( ) The cell cortex
( ) The mitotic spindle
( ) The nuclear lamina
( ) Cilia
( ) Filopodia
2 Indicate if each of the following changes occurring during mitosis in a fibroblast is the
result of the reorganization of actin filaments (A), microtubules (M), or intermediate filaments (I).
Your answer would be a five-letter string composed of letters A, M, and I only; e.g. AIAAM.
( ) The cell rounds up.
( ) The endoplasmic reticulum collapses.
( ) The Golgi apparatus fragments.
( ) The primary cilium is resorbed (disappears).
( ) The contractile ring forms and constricts.
3 Which of the following cytoskeletal filaments are abundant in an animal cell nucleus?
A. Microfilaments
B. Microtubules
C. Septins
D. Intermediate filaments
E. Spectrin filaments
4 From left to right, indicate if each of the following schematic drawings represents the
typical overall organization of actin filaments (A), microtubules (M), or intermediate filaments (I)
in an animal cell. Your answer would be a three-letter string composed of letters A, M, and I only;
e.g. AMI.
,5 Indicate true (T) and false (F) statements below regarding the cell cytoskeleton. Your
answer would be a four-letter string composed of letters T and F only; e.g. TTFF.
( ) The three major building blocks of cytoskeletal filaments can bind to and hydrolyze
nucleotides.
( ) The building blocks of microfilaments and microtubules are globular proteins, whereas
those of intermediate filaments are themselves filamentous proteins.
( ) Intermediate filaments are typically thicker than actin filaments but thinner than
microtubules.
( ) Plant cells lack microtubules.
6 Fill in the blank: Each microtubule is typically made of thirteen parallel …. that associate
laterally to form a hollow tube.
7 Indicate if each of the following descriptions matches actin filaments (A), microtubules
(M), or intermediate filaments (I). Your answer would be a four-letter string composed of letters
A, M, and I only; e.g. AAMM.
( ) They form hollow structures with multiple lateral interactions.
( ) They form strong structures that are more resilient than the other two cytoskeletal
filaments.
( ) Their subunits bind GTP and hydrolyze it.
( ) They form coiled-coil interactions between the subunits.
8 Bacteria contain homologs of cytoskeletal filament subunits …
, A. except those of the intermediate filaments.
B. but these homologs are incapable of nucleotide binding and hydrolysis.
C. that are less diverse in their function relative to their eukaryotic counterparts.
D. that can have different functions to those of their eukaryotic counterparts.
E. that are dispensable for cell growth and proliferation.
9 Persistence length for a cytoskeletal filament is the minimum filament length at which
random thermal fluctuations are likely to cause it to bend. Which of the following comparisons are
true, considering the persistence lengths of (a) an actin filament, (b) a bundle of cross-linked actin
filaments, and (c) a microtubule?
A. (a > b) and (a > c)
B. (a > b) and (a < c)
C. (a < b) and (a > c)
D. (a < b) and (a < c)
E. (a > b) and (b > c)
10 You have prepared actin filament seeds in a microfluidic chamber through which you then
pass either unlabeled or fluorescently labeled actin subunits in alternation several times, keeping
the total monomer concentration constant. You then observe the resulting filaments under a
fluorescence microscope. If the filaments appear as drawn schematically in the example below (in
which fluorescence is indicated by black color), which end do you think is the plus end—(a) or
(b)? Was the concentration of free actin subunits below or above the critical concentration (Cc) for
polymerization at the barbed end?
(b)
(a)
A. (a); below
B. (b); below
C. (a); above
D. (b); above
, 11 F-actin is not a straight polymer but can be considered a double-helical assembly. The two
protofilaments twist around each other and cross over approximately every 13 actin subunits, and
the subunits in each protofilament are repeated every 2.8 nm along the helix. F-actin is almost 8
nm wide. Different myosin motors have different “step lengths” on actin depending on their
function. Which of the following step lengths would you expect to belong to a processive myosin
motor that carries large cellular cargoes such as endocytic vesicles?
A. About 2.8 nm
B. About 8 nm
C. About 13 nm
D. About 27 nm
E. About 36 nm
12 In the polymerization in vitro of actin filaments and microtubules from their subunits, what
does the “lag phase” correspond to?
A. Nucleation
B. Reaching steady state
C. Nucleotide exchange
D. ATP or GTP hydrolysis
E. Treadmilling
13 The time courses of seeded actin polymerization under two different conditions are
compared in the following graph. If the overall kon for polymerization is known to be the same
under both of these conditions, which curve—(1) or (2)—corresponds to the condition with a
higher koff rate constant? Which one corresponds to a higher Cc for polymerization?
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