#Question id: 2583
#Unit 2. Cellular Organization
Following statements are regarding to the microtubules.
A. Tubulin is the major structural component of microtubules
B. Microtubule-associated proteins (MAPs) associate with tubulin and help mediate the assembly, dynamics, and function of microtubules.
C. Free tubulin exists as a dimer in which the α-subunit contains a trapped and hydrolyzable GTP and theβ-subunit binds an exchangeable and nonhydrolyzable GTP
D. All microtubules are nucleated from microtubule-organizing centers (MTOCs), and many remain anchored with their (−) ends there. Thus the end away from the MTOC is always the (+) end.
Which of the following statements is incorrect?
#Question id: 2584
#Unit 2. Cellular Organization
The polymerization of pure G-actin in vitro proceeds in three phases.
|
Column I |
Column II |
|
A. nucleation phase |
i. G-actin monomers exchange with subunits at the filament ends, but there is no net change in the total length of filaments |
|
B. elongation phase |
ii. marked by a lag period in which G-actin subunits combine into an oligomer of two or three subunits |
|
C. steady-state phase |
iii. the short oligomer rapidly increases in length by the addition of actin monomers to both of its ends |
Which of the following represent correct match?
#Question id: 2585
#Unit 2. Cellular Organization
Which of the following is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber?
A. Tropomyosin shifts and unblocks the cross-bridge binding sites.
B. Calcium is released and binds to the troponin complex.
C. Transverse tubules depolarize the sarcoplasmic reticulum.
D. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.
E. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane.
#Question id: 2586
#Unit 2. Cellular Organization
Intermediate filaments are the only nonpolar fibrous component of the cytoskeleton and are not associated with motor proteins. Intermediate filaments are built from coiled coil dimers that associate in an antiparallel fashion into tetramers and then into protofilaments, 16 of which make up the filament. There are five major classes of intermediate filament proteins. Match the following proteins (Column I) with their class (Column II).
|
Column I |
Column II |
|
A. Lamins |
a. Class I |
|
B. Keratins |
b. Class II |
|
C. Desmin |
c. Class III |
|
D. Neurofilaments |
d. Class IV |
|
e. Class V |
Which of the following is correct?
#Question id: 2588
#Unit 2. Cellular Organization
Which of the following statement is correct regarding attachment of microtubule to the kinetochore?
#Question id: 2589
#Unit 2. Cellular Organization
Following statements are regarding to the mechanism of thick filament regulation.
A. In vertebrate smooth muscle, phosphorylation of the myosin regulatory light chain (LC) activates contraction.
B. At Ca2+ concentrations of more than 10−6 M, the regulatory light chain is not phosphorylated, and the myosin adopts a folded conformation.
C. When the Ca2+ level rises, Ca2+ binds calmodulin (CaM), which undergoes a conformational change (CaM*). The CaM*-Ca2+ complex binds and activates myosin light chain kinase (MLC kinase), which then phosphorylates the myosin LC. This phosphorylation event unfolds the myosin II, which is now active and can assemble into bipolar filaments to participate in contraction.
D. When the Ca2+ levels drop, the myosin LC is dephosphorylated by myosin light-chain (MLC) phosphatase, which is dependent on Ca2+ for activity, causing muscle relaxation.
Which of the following combination is correct?