Typically, mutations that modify the active site of an enzyme are more likely to be harmful than mutations that affect other parts of the enzyme. A hypothetical enzyme consists of four domains (A—D), and the amino acid sequences of these four domains have been determined in five related species. Given the proportion of amino acid homologies among the five species at each of the four domains, which domain probably contains the active site?

#Question id: 2439

#Unit 2. Cellular Organization

α- and β-tubulin

a. are highly conserved.                                 

b. bind GTP.

c. hydrolyze GTP to GDP.                 

d. are found in all eukaryotes.

e. are related to the bacterial protein FtsZ.

#Question id: 2440

#Unit 2. Cellular Organization

Microtubule assembly in vitro requires

a. ATP.                                                           

b. a threshold (critical) concentration of tubulin.

c. warming the solution to 37°C.                   

d. microtubule-associated proteins.         

e. GTP.

#Question id: 2441

#Unit 2. Cellular Organization

Which of the following statement(s) is (are) true of microtubule assembly?

a. GDP-tubulin normally assembles into microtubules.

b. Microtubule (-) ends grow faster than microtubule (+) ends.

c. Tubulin subunits assemble to form a protofilament.

d. Microtubules consist of multiple protofilaments.

e. Microtubules with a GTP cap will shorten.

#Question id: 2442

#Unit 2. Cellular Organization

The drug colchicine acts to

a. promote microtubule assembly.     

b. inhibit microtubule assembly.

c. sever microtubules.                        

d. block cell division.

e. cap microtubule ends.

#Question id: 2443

#Unit 2. Cellular Organization

Microtubule-associated proteins

a. typically co-localize with microtubules inside cells.

b. are divided into two types: those that stabilize microtubules and those that destabilize microtubules.

c. typically co-purify with microtubules.                  

d. are similar to α- and β-tubulin.

#Question id: 2444

#Unit 2. Cellular Organization

Kinesin I moves vesicles