🏠 Home
← Back Start Quiz
MCQ Practice

Biology MCQs

Which organelle is primarily involved in the detoxification of hydrogen peroxide (H₂O₂) in eukaryotic cells?

  • A. Lysosome
  • B. Mitochondrion
  • C. Peroxisome
  • D. Golgi apparatus
Explanation:
Peroxisomes contain the enzyme catalase, which converts H₂O₂ to water and oxygen. They specialize in detoxifying reactive oxygen species.

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
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.

In a kinase signaling cascade, signal amplification occurs because:

  • A. Each kinase molecule activates only one target molecule
  • B. Each activated kinase can phosphorylate and activate many target molecules
  • C. Kinases use ATP as a second messenger to spread the signal
  • D. Kinases permanently bind their substrates to propagate the signal
Explanation:
In a cascade, each activated kinase enzyme can phosphorylate multiple downstream targets, amplifying the signal at each step.

Which of the following best describes clathrin function in eukaryotic cells?

  • A. It mediates the transport of proteins from the ER to the Golgi.
  • B. It forms a coat around vesicles during receptor-mediated endocytosis.
  • C. It phosphorylates tyrosine residues in signaling receptors.
  • D. It generates a proton gradient in endosomes.
Explanation:
Clathrin polymerizes into a lattice on the inner surface of the plasma membrane, forming coated pits and vesicles that internalize receptors and ligands.

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
Explanation:
Integrin activation often activates focal adhesion kinase and the MAPK/ERK cascade, promoting cell survival and proliferation.

The checkpoint that verifies DNA is completely replicated and undamaged before a cell enters mitosis is called:

  • A. G1/S checkpoint
  • B. S-phase checkpoint
  • C. G2/M checkpoint
  • D. Spindle assembly checkpoint
Explanation:
The G2/M checkpoint ensures that all DNA has been accurately replicated and that any damage is repaired before mitosis. If errors are found, the cycle is halted for repair.

Which cytoskeletal element is directly involved in separating sister chromatids during anaphase of mitosis?

  • A. Actin filaments (microfilaments)
  • B. Intermediate filaments
  • C. Microtubules of the mitotic spindle
  • D. Myosin thick filaments
Explanation:
Kinetochore microtubules shorten during anaphase, pulling sister chromatids apart. Actin and myosin are involved in cytokinesis, not chromatid separation.

Which of the following is NOT characteristic of the lysosomal degradation pathway?

  • A. Lysosomes contain acid hydrolases active at low pH
  • B. Autophagosomes deliver cytoplasmic components to lysosomes
  • C. Degraded macromolecules are exported to the cytosol for reuse
  • D. Ubiquitinated proteins targeted for degradation are broken down in lysosomes
Explanation:
Lysosomes degrade extracellular and organelle material. The proteasome, not lysosomes, primarily degrades ubiquitinated proteins.

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
Explanation:
The SRP binds the signal peptide and ribosome, pausing translation until the complex docks on the SRP receptor on the ER membrane.

In a resting mammalian cell membrane, which ion has the highest permeability and thus most strongly influences the resting membrane potential?

  • A. Sodium (Na+)
  • B. Potassium (K+)
  • C. Chloride (Cl-)
  • D. Calcium (Ca2+)
Explanation:
At rest, cells have many open potassium leak channels, making K+ the most permeable ion. The resting membrane potential is therefore close to the K+ equilibrium potential.