Question 1

Which of the following best describes the role of GEFs (guanine nucleotide exchange factors) in the regulation of monomeric GTPases?

Accepted Answer

They facilitate the exchange of GDP for GTP, activating the GTPase.

Question 2

A mutation in the gene encoding the α subunit of a trimeric G protein prevents GTP hydrolysis. What is the most likely cellular consequence?

Accepted Answer

The α subunit will remain constitutively active, continuously stimulating downstream signaling.

Question 3

Which of the following is a direct result of adenylyl cyclase activation by a G protein?

Accepted Answer

Conversion of ATP to cAMP

Question 4

In the context of GPCR signaling, what is the primary function of cyclic AMP (cAMP)?

Accepted Answer

It activates protein kinase A (PKA) by binding to its regulatory subunits.

Question 5

Which of the following best explains why the cytosolic concentration of Ca$$^{2+} is $$kept very low under resting conditions?

Accepted Answer

Low cytosolic Ca$$^{2+}$$ allows for rapid signaling responses when Ca$$^{2+}$$ channels open.

Question 6

A patient has a mutation that prevents the phosphorylation of CREB by PKA. Which cellular process would be most directly affected?

Accepted Answer

Gene transcription in response to cAMP signaling

Question 7

Which of the following is a correct sequence of events in the activation of the MAP kinase pathway by receptor tyrosine kinases (RTKs)?

Accepted Answer

Ligand binding → RTK dimerization → Ras activation → Raf activation → MAP kinase cascade

Question 8

Which of the following best describes the role of phospholipase C-β (PLCβ) in GPCR signaling?

Accepted Answer

It hydrolyzes PI(4,5)P$$_2 to $$generate IP$$_3$$ and diacylglycerol.

Question 9

Which of the following events occurs immediately after the binding of an odorant to an olfactory GPCR?

Accepted Answer

Activation of Golf and increase in cAMP concentration

Question 10

A researcher observes that a cell line has constitutively active Ras protein. Which of the following is the most likely consequence?

Accepted Answer

Continuous activation of the MAP kinase pathway, leading to increased cell division

Question 11

Which of the following best explains the amplification observed in the vertebrate rod photoreceptor response to light?

Accepted Answer

One activated rhodopsin molecule can activate many G proteins, each of which activates many phosphodiesterase molecules.

Question 12

Which of the following is a direct effect of IP$$_3$$ generated by phospholipase C-β activity?

Accepted Answer

Release of Ca$$^{2+}$$ from the endoplasmic reticulum

Question 13

Which of the following statements about calmodulin is correct?

Accepted Answer

It undergoes a conformational change upon binding Ca$$^{2+}$$, allowing it to activate target proteins.

Question 14

A cell is exposed to a hormone that activates a GPCR coupled to G$$_q. $$Which of the following is the most likely sequence of events?

Accepted Answer

Activation of phospholipase C-β → production of IP$$_3$$ and diacylglycerol → increase in cytosolic Ca$$^{2+}$$

Question 15

Which of the following is a common feature of all receptor tyrosine kinases (RTKs)?

Accepted Answer

They have a single transmembrane domain and dimerize upon ligand binding.

Question 16

Which of the following best describes the function of scaffold proteins in MAP kinase signaling pathways?

Accepted Answer

They organize multiple kinases into signaling modules, increasing specificity and efficiency.

Question 17

Which of the following is a direct effect of the activation of protein kinase C (PKC)?

Accepted Answer

Phosphorylation of target proteins in response to diacylglycerol and Ca$$^{2+}$$

Question 18

Which of the following best explains why mutations in Ras are commonly found in human tumors?

Accepted Answer

Mutant Ras proteins are often constitutively active, leading to uncontrolled cell division.

Question 19

Which of the following is a correct description of the difference between signaling by phosphorylation and signaling by GTP binding?

Accepted Answer

Phosphorylation involves the addition/removal of phosphate groups by kinases and phosphatases, while GTP binding involves exchange and hydrolysis of GTP.

20: Cell Signaling Pathways: G Proteins, GPCRs, and RTKs

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