all tissue types possess a unique stem cell niche, and despite many differences among the niche architecture of different tissues, several common principles of stem cell regulation can be applied to all environments. These principles involve extracellular mechanisms leading to intracellular changes that regulate stem cell behavior.

Column A	Column B
I. Extracellular regulatory mechanisms	i. Regulation by endocrine, paracrine, or juxtacrine mechanisms
II. Intracellular regulatory mechanisms	ii. Regulation by a network of transcription factors
	iii. Epigenetic regulation
	iv. Regulation by cell-to-cell and cell-to-matrix adhesions
	v. Regulation by cytoplasmic determinants

Match the following regulatory mechanism (column A) with the factors that regulate stem cell behavior (column B).

#Question id: 1226

#Unit 4. Cell Communication and Cell Signaling

GTPase switch proteins

a. are active when bound to GDP but inactive when bound to GTP.

b. are active when bound to GTP but inactive when bound to GDP.

c. are converted from the inactive to active state by a guanine nucleotide exchange factor (GEF).

d. synthesizes GTP from GDP.

#Question id: 1227

#Unit 4. Cell Communication and Cell Signaling

Which of the following is a mechanism for down regulation of signaling from G protein-coupled receptors?

a. a change in the Kd of the receptor-hormone complex

b. proteolysis of the Gα subunit                     

c. hydrolysis of cAMP to 5´-AMP

d. dephosphorylation of adenylyl cyclase

#Question id: 1228

#Unit 4. Cell Communication and Cell Signaling

Place the events that follow the binding of a ligand to a G protein-coupled receptor that acts on adenylyl cyclase in the correct order:

a. GTP is hydrolyzed to GDP.           

b. GDP is displaced from Gα.           

c. the Gα-GTP complex dissociates from the Gβγcomplex.  

d. GTP is bound to Gα. 

#Question id: 1229

#Unit 4. Cell Communication and Cell Signaling

Which of the following events occur(s) during light activation of the rhodopsin receptor?

a. 11-cis-retinal is converted to all-trans-retinal

b. activation of the trimeric G protein, transducin

c. activation of cGMP phosphodiesterase

d. opening of the cGMP-gated ion channel

#Question id: 1230

#Unit 4. Cell Communication and Cell Signaling

Which of the following events occur(s) following the binding of a ligand to a G protein–coupled receptor that triggers release of Ca2+?

a. activation of protein kinase A by IP3

b. phospholipase cleavage of PIP2 to IP3 and DAG

c. release of DAG into the cytosol

d. IP3-mediated opening of Ca2+ channels in the ER membrane

#Question id: 1231

#Unit 4. Cell Communication and Cell Signaling

Arrange the following events in the proper order in which they occur during transduction of the TGFβ signal:

a. association of Smad3 and Smad4  

b. phosphorylation of TGFβ receptor I by TGFβ receptor II            

c. phosphorylation of Smad3 by TGFβ receptor I     

d. nuclear import of Smad3   

e. binding of TGFβ to TGFβ receptor II