Selected MCQ

Current Question
The signal recognition particle (SRP) functions primarily to:
  • A. Direct ribosomes to the rough ER membrane for secretory proteins
  • B. Unfold misfolded proteins in the cytosol
  • C. Bind and degrade abnormal mRNA
  • D. Stimulate nuclear export of ribosomal subunits
Correct Answer: A
Explanation:
SRP binds the signal peptide of a nascent polypeptide and pauses translation until the ribosome docks on the ER, targeting the protein to the secretory pathway.
Related Question 1
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 2
Histone H1 differs from the core histones (H2A, H2B, H3, H4) because it primarily functions to:
  • A. Form the core nucleosome octamer
  • B. Bind linker DNA between nucleosomes
  • C. Methylate DNA
  • D. Act as a transcription factor
Correct Answer: B
Explanation:
Histone H1 binds to the linker DNA between nucleosomes, helping to compact chromatin into higher-order structures.
Related Question 3
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 4
Which chromatography technique separates proteins primarily based on size using a gel matrix?
  • A. Ion-exchange chromatography
  • B. Gel filtration (size-exclusion) chromatography
  • C. Affinity chromatography
  • D. Reverse-phase chromatography
Correct Answer: B
Explanation:
Gel filtration chromatography (size-exclusion) separates proteins by size: larger molecules elute first because they enter fewer pores in the matrix, while smaller molecules elute later.
Related Question 5
In flow cytometry, cells are labeled with fluorescent antibodies and passed in a stream through a laser. This technique primarily measures:
  • A. Cell size and internal complexity
  • B. DNA sequence variants
  • C. Protein structure by X-ray diffraction
  • D. Glycolytic activity
Correct Answer: A
Explanation:
Flow cytometry measures light scattering (size and granularity) and fluorescence from labeled markers, allowing rapid analysis of cell populations (size, complexity, marker expression).