Selected MCQ

Current Question
Which of the following events in protein targeting to the endoplasmic reticulum is directly mediated by the signal recognition particle (SRP)?
  • A. Cleavage of the signal peptide after translocation
  • B. Pausing of translation until the ribosome binds the ER
  • C. Glycosylation of the protein in the ER lumen
  • D. Folding of the protein by lumenal chaperones
Correct Answer: B
Explanation:
The SRP binds the signal peptide and ribosome, pausing translation until the complex docks on the SRP receptor on the ER membrane.
Related Question 1
The smooth endoplasmic reticulum (SER) is primarily involved in which of the following cellular functions?
  • A. Synthesis of secretory and membrane proteins
  • B. Detoxification of xenobiotics and synthesis of lipids
  • C. ATP production via oxidative phosphorylation
  • D. Packaging and modification of proteins in vesicles
Correct Answer: B
Explanation:
The smooth ER specializes in lipid and steroid synthesis, as well as the detoxification of drugs and other harmful compounds. (Protein synthesis occurs on the rough ER; packaging occurs in the Golgi.)
Related Question 2
A mutation prevents dynein from binding microtubules in eukaryotic cells. Which of the following processes would be most directly impaired?
  • A. Skeletal muscle contraction
  • B. Movement of vesicles from the Golgi apparatus to the endoplasmic reticulum
  • C. Movement of chromosomes toward the spindle poles during mitosis
  • D. Sliding of microtubule doublets in eukaryotic cilia and flagella
Correct Answer: D
Explanation:
Dynein motor proteins drive the sliding of microtubule doublets in cilia and flagella. Without functional dynein, ciliary and flagellar motion would be impaired. (Other processes involve myosin or different motor proteins.)
Related Question 3
Phospholipase C (PLC) activation results in the generation of which second messenger that releases Ca²⁺ from the endoplasmic reticulum?
  • A. cAMP
  • B. cGMP
  • C. IP₃
  • D. DAG
Correct Answer: C
Explanation:
PLC cleaves PIP₂ into IP₃ and DAG. IP₃ diffuses to the ER, causing Ca²⁺ release, while DAG activates PKC.
Related Question 4
Binding of extracellular matrix protein fibronectin to a cell’s integrin receptor most directly triggers which intracellular signaling pathway?
  • A. cAMP/PKA pathway
  • B. MAP kinase (ERK) pathway
  • C. JAK/STAT pathway
  • D. Hedgehog signaling pathway
Correct Answer: B
Explanation:
Integrin activation often activates focal adhesion kinase and the MAPK/ERK cascade, promoting cell survival and proliferation.
Related Question 5
During the G1/S transition in the eukaryotic cell cycle, phosphorylation of the retinoblastoma (Rb) protein results in which of the following?
  • A. Release of E2F transcription factors, activating S-phase gene expression
  • B. Activation of cyclin-dependent kinase inhibitors to halt cell cycle progression
  • C. Induction of apoptosis via activation of p53
  • D. Inhibition of cyclin D synthesis
Correct Answer: A
Explanation:
Phosphorylated Rb releases bound E2F transcription factors, which then activate genes required for DNA replication in S phase.