Question 1

Which of the following best describes the role of a second messenger in signal transduction pathways?

Accepted Answer

They amplify the signal initiated by the primary messenger and freely diffuse within the cell to activate downstream signaling proteins.

Question 2

A patient is given sildenafil (Viagra), which inhibits phosphodiesterase type 5 (PDE5). What is the primary biochemical effect of this drug in vascular smooth muscle?

Accepted Answer

It prevents the breakdown of cGMP, resulting in prolonged muscle relaxation and increased blood flow.

Question 3

Which of the following is a direct result of receptor tyrosine kinase (RTK) dimerization and autophosphorylation?

Accepted Answer

Formation of binding sites for intracellular signaling proteins, initiating downstream signaling cascades

Question 4

Cholera toxin modifies the Gs protein by ADP-ribosylation. What is the consequence of this modification on the cAMP signaling pathway?

Accepted Answer

It locks Gs in its active GTP-bound state, leading to persistent activation of adenylate cyclase and elevated cAMP.

Question 5

Which of the following is NOT a typical mechanism for terminating a signal transduction pathway?

Accepted Answer

Continuous activation of protein kinases

Question 6

A mutation causes Ras to be constitutively active (always GTP-bound). What is the most likely cellular consequence?

Accepted Answer

Uncontrolled cell proliferation due to persistent activation of downstream kinase cascades

Question 7

Which of the following best explains why steroid hormones do NOT require membrane receptors for signal transduction?

Accepted Answer

They are nonpolar and can diffuse through the lipid bilayer to bind intracellular receptors.

Question 8

In the phosphoinositide pathway, which molecule acts as a substrate for phospholipase C (PLC) and what are the resulting second messengers?

Accepted Answer

Phosphatidylinositol-4,5-bisphosphate (PIP2); diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3)

Question 9

Which of the following statements about calmodulin (CaM) is TRUE?

Accepted Answer

It binds Ca$$^{2+}$$, undergoes a conformational change, and activates kinases by exposing their active sites.

Question 10

Arachidonic acid is a precursor for which of the following classes of signaling molecules?

Accepted Answer

Prostaglandins, thromboxanes, and leukotrienes

Question 11

Which of the following best describes the function of SH2 and SH3 domains in intracellular signaling proteins?

Accepted Answer

SH2 domains bind phosphorylated tyrosines, and SH3 domains bind proline-rich regions of proteins.

Question 12

Which of the following is a direct effect of IP$$_3$$ production in the cell?

Accepted Answer

Opening of Ca$$^{2+}$$ channels in the endoplasmic reticulum, increasing cytosolic Ca$$^{2+}$$ concentration

Question 13

Non-steroidal anti-inflammatory drugs (NSAIDs) can cause gastrointestinal side effects primarily because they:

Accepted Answer

Inhibit COX-1, decreasing prostaglandin synthesis needed for gastric mucosal protection.

Question 14

Which of the following is an example of signal amplification in a typical signal transduction pathway?

Accepted Answer

One ligand binding to a receptor leads to the activation of multiple G proteins, each activating adenylate cyclase to produce many cAMP molecules.

Question 15

Which of the following is a correct sequence of events in G protein-coupled receptor (GPCR) signaling?

Accepted Answer

Ligand binds GPCR → G protein exchanges GDP for GTP → G protein activates effector enzyme → Second messenger produced

Question 16

Which of the following best explains why different cell types can respond differently to the same extracellular signal?

Accepted Answer

Different cells express different subsets of receptors and intracellular signaling proteins, leading to cell-specific responses.

Question 17

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

Accepted Answer

Phosphorylation of target proteins involved in cellular responses

Question 18

A drug that selectively inhibits COX-2 but not COX-1 would be expected to:

Accepted Answer

Reduce inflammation and pain with less risk of gastrointestinal side effects compared to non-selective NSAIDs.

Question 19

Which of the following is a biochemical basis for signal transduction?

Accepted Answer

Protein conformational changes, covalent protein modifications, and altered rates of gene expression

Signal Transduction Pathways in Biochemistry: Mechanisms, Messengers, and Cellular Responses

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