Selected MCQ

Current Question
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.
Related Question 1
During the eukaryotic cell cycle, which checkpoint ensures that all chromosomes are correctly attached to the mitotic spindle before proceeding to anaphase?
  • A. G1/S checkpoint
  • B. G2/M checkpoint
  • C. Spindle (metaphase) checkpoint
  • D. S-phase checkpoint
Correct Answer: C
Explanation:
The spindle assembly (metaphase) checkpoint monitors attachment of all kinetochores to spindle microtubules. It prevents the onset of anaphase until all chromosomes are properly aligned and attached.
Related Question 2
Why is the G0 phase considered distinct from G1 in the cell cycle?
  • A. G0 cells are rapidly dividing
  • B. G0 is a quiescent/nondividing state outside the normal cycle
  • C. G0 occurs immediately after the M phase in all cells
  • D. Cells in G0 cannot re-enter the cell cycle under any condition
Correct Answer: B
Explanation:
G0 is a resting or terminal differentiation state where cells are metabolically active but not dividing. Cells can re-enter G1 from G0 given appropriate signals.
Related Question 3
Which mechanism allows a single eukaryotic gene to encode multiple protein isoforms?
  • A. Alternative splicing
  • B. Alternative sigma factors
  • C. Gene duplication
  • D. Horizontal gene transfer
Correct Answer: A
Explanation:
Alternative splicing of a pre-mRNA can include or exclude exons, yielding different mRNAs and thus different protein isoforms from one gene.
Related Question 4
Which organelle contains hydrolytic enzymes and is involved in the degradation of cellular components during autophagy?
  • A. Lysosome
  • B. Proteasome
  • C. Golgi apparatus
  • D. Smooth endoplasmic reticulum
Correct Answer: A
Explanation:
Lysosomes contain acid hydrolases that fuse with autophagosomes to degrade cellular organelles and proteins.
Related Question 5
Where in the nucleus are ribosomal RNAs transcribed in eukaryotic cells?
  • A. Nucleolus
  • B. Euchromatin
  • C. Chromatin loops
  • D. Nuclear lamina
Correct Answer: A
Explanation:
The nucleolus is the site of rRNA gene transcription (by RNA Pol I) and ribosome assembly.