You are given an unknown organism to identify. It is unicellular and heterotrophic. It is motile, using many short extensions of the cytoplasm, each featuring the 9+2

filament pattern. It has well-developed organelles and three nuclei, one large and two small. This organism is most likely to be a member of which group?

#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).

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?

#Question id: 2590

#Unit 2. Cellular Organization

Following graph represents dynamic instability of microtubules in vitro. Identify the correct activity at A, B, C and D.

#Question id: 2591

#Unit 2. Cellular Organization

What kind of information can be obtained by injecting cells with fragments of heavy meromyosin (HMM) or S1 subfragments of HMM?