Structure and Function of Microtubules Quiz

What is the result of all the monomers within an actin filament being pointed in the same direction?

The filament becomes a highly branched network
The filament forms tightly anchored bundles
The filament becomes polar with 'barbed' and 'pointed' ends (correct)
The filament transforms into an ordered array

Which molecule does an actin monomer bind before being incorporated into a filament?

ATP (correct)
S1 subunit of myosin
GTP
ADP

What kind of enzyme is actin, as mentioned in the text?

ATPase (correct)
Helicase
GTPase
Polymerase

Which end of an actin filament incorporates monomers at a rate about 10 times faster than the other end?

Barbed end (A) Signup and view all the answers

What occurs much more rapidly in actin filament formation after the initial nucleation event?

Filament elongation (D) Signup and view all the answers

What defines actin as a major protein in every eukaryotic cell?

Its ATPase activity (B) Signup and view all the answers

What is the characteristic of kinesin that allows it to walk along a microtubule for considerable distances without falling off?

Its hand-over-hand movement mechanism (B) Signup and view all the answers

What is the primary function of kinesin-mediated organelle transport?

To drive the movement of cargo and organelles in an outward direction toward the cell's plasma membrane (A) Signup and view all the answers

How does the movement of cytoplasmic dynein along a microtubule compare to that of most kinesins?

Cytoplasmic dynein moves in the opposite direction to most kinesins, toward the minus end of the microtubule. (D) Signup and view all the answers

What is the structure of a cytoplasmic dynein molecule?

Two identical heavy chains, a variety of intermediate and light chains, and a large globular force-generating head (A) Signup and view all the answers

What is the effect on the phenotype of a cell when a member of the kinesin superfamily is absent?

The cell's microtubule network becomes disrupted, leading to changes in the cell's overall morphology. (C) Signup and view all the answers

Which of the following is a key characteristic of the 'hand-over-hand' movement mechanism of kinesin?

It enables kinesin to move processively along a microtubule for considerable distances. (B) Signup and view all the answers

What are the structural characteristics of tubulin subunits within a microtubule?

They are organized in a linear array, asymmetric, and polar with α-tubulin at one end and β-tubulin at the other. (B) Signup and view all the answers

What is the primary function of microtubule-associated proteins (MAPs)?

To increase the stability of microtubules and promote their assembly. (C) Signup and view all the answers

Which of the following statements about the microtubule-associated protein tau is correct?

An abnormally high level of phosphorylation of tau has been implicated in Alzheimer's disease. (A) Signup and view all the answers

What is one of the primary functions of microtubules in cells?

To provide mechanical support and resist forces that might compress or bend the cell. (B) Signup and view all the answers

How does the distribution of microtubules contribute to cell shape?

It determines the round, flattened shape of the cell. (A) Signup and view all the answers

What is the significance of the polar nature of microtubules?

It allows for the transport of molecules in both directions along the microtubule. (C) Signup and view all the answers

What happens to the GDP molecule after it is formed during the hydrolysis of GTP in the assembled microtubule?

The GDP molecule remains bound to the assembled polymer (C) Signup and view all the answers

What is the purpose of the nucleotide exchange from GDP to GTP during the disassembly of tubulin dimers?

It allows the dimer to serve as a building block for polymerization again (B) Signup and view all the answers

What is the structural difference between GTP-tubulin and GDP-tubulin that affects their ability to fit into a protofilament?

GTP-tubulin has a different conformation than GDP-tubulin, making it more able to fit into a protofilament (B) Signup and view all the answers

What is the role of the open sheet of tubulin-GTP subunits at the tip of the growing microtubule?

It provides a stable structure that allows the microtubule to continue growing (A) Signup and view all the answers

What is the mechanism that leads to the catastrophic shrinkage of the microtubule?

The strain from the GDP-tubulin at the plus end causes the microtubule to shrink (A) Signup and view all the answers

Which of the following is NOT a dynamic process that the cell can reorganize its actin cytoskeleton for?

Apoptosis (A) Signup and view all the answers

What is the primary function of conventional (type II) myosins?

Primary motors for muscle contraction (D) Signup and view all the answers

What is the defining characteristic of the head domain of all myosin proteins?

It contains both a site that binds an actin filament and a site that binds and hydrolyzes ATP (D) Signup and view all the answers

How do conventional (type II) myosins differ from unconventional myosins?

Conventional myosins have similar head domains but highly divergent tail domains, while unconventional myosins have similar tail domains but divergent head domains (A) Signup and view all the answers

What is the directionality of movement of conventional (type II) myosins along actin filaments?

They move toward the barbed end of the actin filament (A) Signup and view all the answers

What is the primary function of the tail domains of myosin proteins?

To anchor the myosin motor to specific cellular structures (B) Signup and view all the answers