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MCQ Practice

Biology MCQs

What is the sedimentation coefficient (S value) of a complete eukaryotic ribosome?

  • A. 70S
  • B. 80S
  • C. 90S
  • D. 100S
Explanation:
A eukaryotic ribosome is 80S, composed of a 60S large subunit and a 40S small subunit (70S in bacteria).

DNA microarray analysis is used to:

  • A. Sequence whole genomes quickly
  • B. Measure expression levels of thousands of genes simultaneously
  • C. Detect protein-protein interactions
  • D. Separate DNA fragments by size
Explanation:
DNA microarrays contain thousands of gene-specific probes. Labeled cDNA from a sample hybridizes to the array to quantify the expression of many genes in parallel.

Small interfering RNAs (siRNAs) are primarily involved in which cellular process?

  • A. DNA replication
  • B. RNA interference (RNAi) leading to mRNA degradation
  • C. Protein synthesis
  • D. Glycolysis
Explanation:
siRNAs guide the RNA-induced silencing complex (RISC) to complementary mRNA, causing its cleavage and degradation, a form of RNA interference.

Prokaryotic mRNAs often encode multiple proteins on a single transcript. This feature is known as:

  • A. Monocistronic transcription
  • B. Polycistronic transcription
  • C. Post-transcriptional splicing
  • D. Alternative splicing
Explanation:
Polycistronic mRNA contains several open reading frames, each encoding a different protein. This is common in bacteria (e.g., operons).

During PCR, Taq DNA polymerase has an optimal extension temperature of approximately:

  • A. 50°C
  • B. 60°C
  • C. 72°C
  • D. 95°C
Explanation:
Taq polymerase is most active at around 72°C, which is used for the extension step to synthesize new DNA strands.

In the *E. coli* trp operon, attenuation causes premature termination of transcription when tryptophan levels are:

  • A. High, because the ribosome does not stall on the leader peptide
  • B. Low, because the ribosome stalls on the leader peptide
  • C. High, because tryptophan activates the terminator hairpin
  • D. Low, because tryptophan activates the attenuator sequence
Explanation:
High tryptophan allows the ribosome to quickly translate the leader peptide, enabling formation of a terminator hairpin and early transcription termination (attenuation).

Histone deacetylases (HDACs) generally have what effect on gene transcription?

  • A. Increase transcription by opening chromatin
  • B. Repress transcription by condensing chromatin
  • C. Add acetyl groups to histones
  • D. Methylate DNA
Explanation:
HDACs deacetylate histones, reducing positive charge and weakening DNA-histone interactions, thereby condensing chromatin and repressing transcription.

Which DNA repair pathway primarily corrects UV-induced thymine dimers?

  • A. Base-excision repair
  • B. Nucleotide-excision repair
  • C. Mismatch repair
  • D. Non-homologous end joining
Explanation:
Nucleotide-excision repair recognizes bulky lesions like thymine dimers from UV exposure and removes a short single-stranded DNA segment including the damage.

In the CRISPR-Cas9 system, which component confers sequence specificity to target DNA?

  • A. Cas9 enzyme
  • B. Guide RNA (gRNA)
  • C. PAM sequence
  • D. DNA ligase
Explanation:
The guide RNA (gRNA) base-pairs with the target DNA sequence, directing Cas9 to the specific genomic locus, where Cas9 then induces a double-strand break.

Which viral vector carries a high risk of insertional mutagenesis due to genome integration?

  • A. Adenoviral vector
  • B. Retroviral vector
  • C. Adeno-associated viral vector
  • D. Liposome-based vector
Explanation:
Retroviruses integrate their genome into the host DNA. This random insertion can disrupt host genes, increasing the risk of mutagenesis (used with caution in gene therapy).