Question 1

Which of the following best describes the mechanism by which cholera toxin disrupts normal cell signaling in the gut?

Accepted Answer

It inhibits the hydrolysis of GTP by Gs, causing prolonged activation of adenylyl cyclase and excessive cAMP production.

Question 2

A researcher introduces a dominant negative mutation in the fibroblast growth factor receptor (FGFR) in frog embryos. What is the most likely outcome?

Accepted Answer

The embryos will fail to develop correctly in the trunk and tail regions due to disrupted signaling.

Question 3

Which equation correctly represents the dissociation constant ($$K_d) $$for ligand-receptor binding?

Accepted Answer

$$K_d = \frac{[R][L]}{[RL]}$$

Question 4

Which of the following best explains why cells adapt to prolonged exposure to a ligand?

Accepted Answer

Receptor desensitization occurs, which can involve receptor-mediated endocytosis or phosphorylation of the receptor's cytoplasmic tail.

Question 5

Which of the following is a correct sequence of events in the Ras/MAPK pathway following growth factor binding to a receptor tyrosine kinase?

Accepted Answer

Ligand binding → receptor dimerization → autophosphorylation → GRB2 binds SH2 domain → Sos activates Ras → Ras activates Raf → Raf phosphorylates MEK → MEK phosphorylates MAPK

Question 6

A patient with whooping cough is found to have abnormal G protein signaling. Which molecular event is most likely responsible for the symptoms?

Accepted Answer

Pertussis toxin prevents Gi from inhibiting adenylyl cyclase, resulting in excess cAMP and fluid accumulation in the lungs.

Question 7

Which of the following statements about second messengers is correct?

Accepted Answer

Second messengers such as cAMP, IP3, and DAG relay signals from the cell surface to intracellular targets, amplifying the original signal.

Question 8

Which of the following best describes the role of calmodulin in cellular signaling?

Accepted Answer

Calmodulin binds Ca$$^{2+}$$ ions, undergoes a conformational change, and activates target proteins by wrapping around their binding sites.

Question 9

Which of the following is a key difference between G protein-linked receptors and protein kinase-associated receptors?

Accepted Answer

G protein-linked receptors activate G proteins, while protein kinase-associated receptors directly phosphorylate target proteins upon ligand binding.

Question 10

Which of the following best explains the effect of insulin signaling on glucose uptake and glycogen synthesis?

Accepted Answer

Insulin activates PI 3-kinase, leading to PIP3 production, which activates Akt. Akt promotes GLUT4 translocation to the membrane and inhibits GSK3, increasing glycogen synthase activity.

Question 11

Which of the following equations correctly represents the conversion of ATP to cAMP by adenylyl cyclase?

Accepted Answer

$$ATP \rightarrow cAMP + PP_i$$

Question 12

A cell exposed to epinephrine shows increased cAMP levels and rapid glycogen breakdown. Which receptor and G protein are most likely involved?

Accepted Answer

$$\beta-$$adrenergic receptor and G_s$

Question 13

Which of the following best describes the process of signal amplification in cell signaling?

Accepted Answer

Each step in a signaling cascade results in the production of many molecules needed for the next step, allowing a small amount of ligand to elicit a large cellular response.

Question 14

Which of the following is a correct description of the role of IP3 in cellular signaling?

Accepted Answer

IP3 binds to ligand-gated calcium channels in the ER, causing release of Ca$$^{2+}$$ into the cytosol.

Question 15

Which of the following best explains why type II diabetes cannot be effectively treated with insulin?

Accepted Answer

Type II diabetes is characterized by resistance to insulin, so cells do not respond properly to the hormone.

Question 16

Which of the following best describes the function of GTPase activating proteins (GAPs) in cell signaling?

Accepted Answer

GAPs facilitate the hydrolysis of GTP to GDP, inactivating G proteins such as Ras.

Question 17

Which of the following is a correct sequence of events in TGF$$\beta$$ signaling?

Accepted Answer

TGF$$\beta$$ binds to type II receptor → type II receptor phosphorylates type I receptor → type I receptor phosphorylates R-Smads → R-Smads form complex with Smad4 → complex enters nucleus

Question 18

Which of the following best explains the effect of calcium release following fertilization of an animal egg?

Accepted Answer

Calcium release stimulates cortical granules to alter the vitelline envelope and induces egg activation, resuming metabolic processes.

Question 19

Which of the following best describes the action of steroid hormone receptors?

Accepted Answer

Steroid hormone receptors are located primarily in the nucleus, where they regulate transcription of target genes upon hormone binding.

Signal Transduction Mechanisms: Messengers and Receptors

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