DNA and the flow of molecular information MCQs With Answer

Understanding DNA and the flow of molecular information is essential for B.Pharm students studying pharmacology, drug design, and molecular medicine. This topic covers DNA structure, replication, transcription, RNA processing, translation, the genetic code, mutations, regulation of gene expression and epigenetic modifications, plus therapeutic relevance such as nucleotide analog drugs, topoisomerase inhibitors and gene therapy. Mastery of enzymes (DNA/RNA polymerases, helicase, ligase, reverse transcriptase), DNA repair, and experimental tools (PCR, cloning, sequencing) equips students to evaluate drug–gene interactions and resistance mechanisms. The questions below reinforce core concepts and clinical applications with focused multiple-choice practice. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which structural feature best describes the native DNA molecule?

• Two antiparallel polynucleotide strands wound into a right-handed double helix
• Two parallel polynucleotide strands wound into a left-handed double helix
• Single-stranded linear polymer with bases exposed
• Triple-stranded helix stabilized by Hoogsteen bonds

Correct Answer: Two antiparallel polynucleotide strands wound into a right-handed double helix

Q2. Chargaff’s rules state which relationship between bases in double-stranded DNA?

• %A = %T and %G = %C
• %A = %G and %C = %T
• %A = %C and %G = %T
• Total purines = total pyrimidines only in single strands

Correct Answer: %A = %T and %G = %C

Q3. DNA polymerases synthesize new DNA chains in which direction?

• 5′ → 3′ direction by adding nucleotides to the 3′-OH
• 3′ → 5′ direction by adding nucleotides to the 5′-phosphate
• Both directions alternately on each replication fork
• Random insertion independent of strand polarity

Correct Answer: 5′ → 3′ direction by adding nucleotides to the 3′-OH

Q4. Which statement correctly contrasts leading and lagging strand synthesis?

• Leading strand is synthesized continuously; lagging strand is synthesized as Okazaki fragments
• Both strands are synthesized continuously without fragments
• Lagging strand is synthesized continuously; leading strand as short fragments
• Only the leading strand requires primers

Correct Answer: Leading strand is synthesized continuously; lagging strand is synthesized as Okazaki fragments

Q5. Which enzyme relaxes DNA supercoiling ahead of the replication fork?

• Topoisomerase (DNA gyrase in bacteria)
• Helicase
• Primase
• DNA ligase

Correct Answer: Topoisomerase (DNA gyrase in bacteria)

Q6. What is the primary role of primase during DNA replication?

• Synthesize short RNA primers complementary to the template
• Remove RNA primers and replace them with DNA
• Seal nicks between Okazaki fragments
• Unwind the double helix at origins

Correct Answer: Synthesize short RNA primers complementary to the template

Q7. Which exonuclease activity provides DNA polymerase proofreading to remove misincorporated nucleotides?

• 3′ → 5′ exonuclease activity
• 5′ → 3′ exonuclease activity
• Endonuclease cleavage at mismatch sites only
• No exonuclease activity is involved in proofreading

Correct Answer: 3′ → 5′ exonuclease activity

Q8. Telomerase extends chromosome ends by which mechanism?

• Using an intrinsic RNA template to add telomeric repeats to the 3′ end
• Directly synthesizing DNA without an RNA template
• Filling gaps left after removal of RNA primers via DNA ligase
• Cleaving and rejoining telomeric DNA to prevent shortening

Correct Answer: Using an intrinsic RNA template to add telomeric repeats to the 3′ end

Q9. How many origins of replication are typical for bacterial chromosomes?

• A single origin of replication
• Multiple origins evenly spaced
• One origin per gene
• No defined origin; replication initiates randomly

Correct Answer: A single origin of replication

Q10. During transcription, which DNA strand serves as the template for RNA synthesis?

• The antisense (template) strand
• The coding (sense) strand
• Both strands simultaneously
• Neither; RNA is synthesized de novo from nucleotides without a template

Correct Answer: The antisense (template) strand

Q11. Which statement correctly contrasts prokaryotic and eukaryotic RNA polymerases?

• Prokaryotes have a single RNA polymerase requiring a sigma factor; eukaryotes have three main RNA polymerases
• Eukaryotes have one RNA polymerase and require sigma factors
• Both prokaryotes and eukaryotes use identical multisubunit RNA polymerases
• Prokaryotes use only reverse transcriptase for transcription

Correct Answer: Prokaryotes have a single RNA polymerase requiring a sigma factor; eukaryotes have three main RNA polymerases

Q12. In bacteria, the sigma factor recognizes which promoter elements to initiate transcription?

• The −10 (Pribnow box) and −35 consensus sequences
• The TATA box at −25 only
• The poly(A) signal downstream of transcription start
• GC-rich hairpin structures in mRNA

Correct Answer: The −10 (Pribnow box) and −35 consensus sequences

Q13. What modification is added to the 5′ end of most eukaryotic mRNAs during processing?

• A 7-methylguanosine (5′) cap
• A poly(A) tail at the 5′ end
• Removal of the first exon
• Methylation of the 5′ phosphate on uracils

Correct Answer: A 7-methylguanosine (5′) cap

Q14. Which ribonucleoprotein complex catalyzes splicing of pre-mRNA introns in eukaryotes?

• snRNPs (small nuclear ribonucleoproteins) within the spliceosome
• Ribosomes during translation
• Telomerase holoenzyme
• RNA-induced silencing complex (RISC)

Correct Answer: snRNPs (small nuclear ribonucleoproteins) within the spliceosome

Q15. Which property best describes the genetic code?

• Degenerate (redundant) with multiple codons coding for the same amino acid
• Non-degenerate with exactly one codon per amino acid
• Ambiguous because a codon can code for different amino acids depending on tRNA
• Reads in overlapping frames for all genes

Correct Answer: Degenerate (redundant) with multiple codons coding for the same amino acid

Q16. What is the canonical start codon in most mRNAs that initiates translation?

• AUG (codes for methionine)
• UAA
• UGA
• AAA

Correct Answer: AUG (codes for methionine)

Q17. Which of the following is a stop (nonsense) codon signaling termination of translation?

• UAA
• AUG
• GAA
• UAC

Correct Answer: UAA

Q18. Which enzyme charges tRNA molecules with their corresponding amino acids?

• Aminoacyl-tRNA synthetase
• Peptidyl transferase
• RNA polymerase
• Ribonuclease P

Correct Answer: Aminoacyl-tRNA synthetase

Q19. During elongation on the ribosome, where does an incoming aminoacyl-tRNA first bind?

• The A (aminoacyl) site
• The P (peptidyl) site
• The E (exit) site
• The R site specific to eukaryotes only

Correct Answer: The A (aminoacyl) site

Q20. Which antibiotic inhibits the bacterial 50S ribosomal peptidyl transferase, blocking peptide bond formation?

• Chloramphenicol
• Tetracycline
• Aminoglycosides (e.g., streptomycin)
• Polymyxin B

Correct Answer: Chloramphenicol

Q21. Which enzyme synthesizes DNA from an RNA template, a key step in retroviral replication?

• Reverse transcriptase
• DNA-dependent RNA polymerase
• Telomerase reverse RNA polymerase
• RNase H

Correct Answer: Reverse transcriptase

Q22. Which thermostable DNA polymerase is commonly used for the extension step in PCR?

• Taq DNA polymerase
• DNA polymerase I from E. coli
• Reverse transcriptase
• DNA ligase

Correct Answer: Taq DNA polymerase

Q23. A single-nucleotide deletion in a coding region typically produces which type of mutation?

• Frameshift mutation
• Silent mutation
• Conservative missense mutation only
• Duplication without reading frame change

Correct Answer: Frameshift mutation

Q24. Which mutation converts a codon for an amino acid into a stop codon, prematurely terminating translation?

• Nonsense mutation
• Missense mutation
• Silent mutation
• Frameshift mutation only

Correct Answer: Nonsense mutation

Q25. Which epigenetic modification of DNA is most directly associated with transcriptional repression?

• DNA methylation (e.g., CpG methylation)
• Histone acetylation
• Histone phosphorylation activating chromatin
• 5′ capping of mRNA

Correct Answer: DNA methylation (e.g., CpG methylation)

Q26. Histone acetylation typically affects chromatin how, and what is the functional outcome?

• Loosens chromatin structure and increases transcription
• Tightens chromatin structure and decreases transcription
• Causes DNA methylation directly
• Promotes immediate mRNA degradation

Correct Answer: Loosens chromatin structure and increases transcription

Q27. Which process is an exception to the classic “DNA → RNA → protein” central dogma?

• Reverse transcription (RNA → DNA)
• Transcription (DNA → RNA)
• Translation (RNA → protein)
• DNA replication (DNA → DNA)

Correct Answer: Reverse transcription (RNA → DNA)

Q28. In recombinant DNA cloning, which enzyme covalently joins DNA fragments together?

• DNA ligase
• Restriction endonuclease
• Topoisomerase
• Reverse transcriptase

Correct Answer: DNA ligase

Q29. Zidovudine (AZT) is an antiviral nucleoside analog that acts by which mechanism?

• Chain termination after incorporation by reverse transcriptase
• Inhibition of ribosomal translocation during translation
• Intercalation into DNA causing frameshift mutations
• Blocking DNA ligase activity during replication

Correct Answer: Chain termination after incorporation by reverse transcriptase

Q30. Which enzyme unwinds the DNA double helix at the replication fork to allow replication to proceed?

• Helicase
• DNA ligase
• DNA polymerase
• Primase

Correct Answer: Helicase

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