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MCQ Practice

Biology MCQs

In the lac operon of *E. coli*, if both lactose and glucose are abundant in the medium, what is the expected effect on lac operon transcription?

  • A. High transcription of lac genes
  • B. Low transcription due to catabolite repression
  • C. No transcription at all
  • D. On only if allolactose is present
Explanation:
High glucose lowers cAMP levels, so CAP is inactive. Even with lactose present (inactivating the repressor), transcription is reduced (catabolite repression).

Which eukaryotic RNA polymerase transcribes tRNA genes?

  • A. RNA polymerase I
  • B. RNA polymerase II
  • C. RNA polymerase III
  • D. RNA polymerase IV
Explanation:
RNA Pol III transcribes tRNAs and 5S rRNA. Pol I makes most rRNAs, and Pol II makes mRNA and some snRNAs.

In eukaryotes, removal of RNA primers during DNA replication is primarily carried out by:

  • A. DNA polymerase I
  • B. RNase H
  • C. DNA ligase
  • D. DNA gyrase
Explanation:
RNase H degrades the RNA in RNA-DNA hybrids (RNA primers). DNA Pol I is bacterial; eukaryotes use RNase H (and FEN1) before ligation.

Telomerase maintains chromosome ends by using a built-in RNA template. In which cells is telomerase typically active?

  • A. Most somatic cells
  • B. Germ cells and many cancer cells
  • C. Red blood cells
  • D. Neurons
Explanation:
Telomerase is active in germline and stem cells (and reactivated in many cancers) to elongate telomeres, but is silent in most somatic cells.

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
Explanation:
Lysosomes contain acid hydrolases that fuse with autophagosomes to degrade cellular organelles and proteins.

Which organelle in animal cells contains its own circular DNA and is involved in ATP production?

  • A. Mitochondrion
  • B. Golgi apparatus
  • C. Peroxisome
  • D. Endoplasmic reticulum
Explanation:
Mitochondria have a circular genome and produce ATP through oxidative phosphorylation.

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
Explanation:
Histone H1 binds to the linker DNA between nucleosomes, helping to compact chromatin into higher-order structures.

Where in the nucleus are ribosomal RNAs transcribed in eukaryotic cells?

  • A. Nucleolus
  • B. Euchromatin
  • C. Chromatin loops
  • D. Nuclear lamina
Explanation:
The nucleolus is the site of rRNA gene transcription (by RNA Pol I) and ribosome assembly.

A red blood cell (internal osmolarity ~290 mOsm) is placed in a 100 mM NaCl solution (assume full dissociation). Which best describes the movement of water?

  • A. The cell loses water and shrinks
  • B. The cell gains water and swells
  • C. No net water movement
  • D. The cell bursts immediately
Explanation:
100 mM NaCl yields ~200 mOsm outside (hypotonic to the cell). Water enters the cell, causing it to swell.

Which cell cycle regulator acts as a tumor suppressor that can halt cell cycle progression after DNA damage?

  • A. Cyclin D
  • B. p53
  • C. E2F
  • D. Myc
Explanation:
p53 induces cell cycle arrest or apoptosis in response to DNA damage, preventing propagation of mutations, and is a classic tumor suppressor.