100 Molecular Biology MCQs with Answers and Explanations

These MCQs are important for CBSE/State Boards, NEET-UG/PG, Medical Science Entrances, and Competitive Exams worldwide.

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Q1. The sugar present in DNA is:

A. Ribose
Explanation: Incorrect. Ribose has a 2′-OH and occurs in RNA, not DNA.

B. Deoxyribose
Answer: Correct. DNA contains deoxyribose lacking a 2′-hydroxyl group.

C. Fructose
Explanation: Incorrect. Fructose is not a component of nucleic acids.

D. Glucose
Explanation: Incorrect. Glucose is a hexose used in energy metabolism, not nucleic acids.

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Q2. Chargaff’s rule states that in double-stranded DNA:

A. A = G and C = T
Explanation: Incorrect. Purines do not equal each other universally.

B. A = T and G = C
Answer: Correct. Complementary base pairing makes A equal T and G equal C.

C. A + T = G + C in all species
Explanation: Incorrect. A+T and G+C vary among species.

D. A/T ratio is constant across species
Explanation: Incorrect. The A/T ratio varies by organism; only A=T holds within a molecule.

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Q3. The bond between base pairs in DNA is primarily:

A. Phosphodiester bond
Explanation: Incorrect. Phosphodiester bonds link nucleotides along a strand, not between bases.

B. Glycosidic bond
Explanation: Incorrect. Glycosidic bonds join base to sugar.

C. Hydrogen bond
Answer: Correct. Hydrogen bonds mediate A–T (2) and G–C (3) pairing.

D. Ionic bond
Explanation: Incorrect. Ionic interactions are not the main force for specific base pairing.

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Q4. Which enzyme unwinds the DNA helix ahead of the replication fork?

A. DNA polymerase III
Explanation: Incorrect. DNA polymerase III synthesizes DNA, it does not unwind the helix.

B. Helicase
Answer: Correct. Helicase breaks hydrogen bonds to separate strands.

C. Ligase
Explanation: Incorrect. Ligase seals nicks in the sugar–phosphate backbone.

D. Topoisomerase I
Explanation: Incorrect. Topoisomerase relieves supercoiling; it does not separate base pairs.

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Q5. Okazaki fragments are formed:

A. On the leading strand only
Explanation: Incorrect. Leading strand is synthesized continuously.

B. On both strands
Explanation: Incorrect. Only the lagging strand is discontinuous.

C. On the lagging strand
Answer: Correct. Lagging strand synthesis is discontinuous, producing Okazaki fragments.

D. Only in eukaryotes
Explanation: Incorrect. They form in both prokaryotes and eukaryotes.

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Q6. The primer for DNA synthesis in cells is usually:

A. DNA made by primase
Explanation: Incorrect. Primase synthesizes short RNA, not DNA, primers.

B. RNA made by primase
Answer: Correct. Primase lays down RNA primers to initiate DNA polymerization.

C. Protein primer
Explanation: Incorrect. Protein primers are not used in chromosomal DNA replication.

D. Deoxynucleoside triphosphate
Explanation: Incorrect. dNTPs are substrates, not primers.

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Q7. Proofreading activity in many DNA polymerases is:

A. 5’→3′ exonuclease
Explanation: Incorrect. 5’→3′ exonuclease removes primers, not proofreading.

B. 3’→5′ exonuclease
Answer: Correct. Proofreading removes misincorporated nucleotides.

C. Endonuclease
Explanation: Incorrect. Endonucleases cut within DNA, not proofreading.

D. Helicase
Explanation: Incorrect. Helicase unwinds DNA, unrelated to proofreading.

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Q8. In eukaryotes, telomere length is maintained by:

A. DNA ligase
Explanation: Incorrect. Ligase seals nicks but does not extend telomeres.

B. Telomerase
Answer: Correct. Telomerase is a reverse transcriptase that extends telomeric repeats.

C. Topoisomerase II
Explanation: Incorrect. Topoisomerase II relieves supercoils, not telomeres.

D. RNA polymerase II
Explanation: Incorrect. RNAP II transcribes mRNA, not telomeres.

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Q9. The TATA box in eukaryotic promoters is recognized by:

A. TFIID (TBP)
Answer: Correct. TBP binds the TATA box to initiate transcription.

B. RNA polymerase I
Explanation: Incorrect. RNAP I transcribes rRNA genes, not TATA.

C. Sigma factor
Explanation: Incorrect. Sigma factors are bacterial promoter recognition proteins.

D. Enhancers
Explanation: Incorrect. Enhancers are DNA sequences, not proteins.

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Q10. In bacteria, transcription termination by rho factor requires:

A. A hairpin followed by U-rich tract
Explanation: Incorrect. That is intrinsic termination.

B. ATP-dependent helicase activity
Answer: Correct. Rho uses ATP to dislodge RNAP.

C. TFIID binding
Explanation: Incorrect. TFIID is a eukaryotic factor.

D. Poly(A) polymerase
Explanation: Incorrect. Polyadenylation occurs in eukaryotic mRNAs.

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Q11. The 5′ cap of eukaryotic mRNA is:

A. Poly(A) tail
Explanation: Incorrect. The poly(A) tail is at the 3′ end.

B. 7-methylguanosine linked 5′–5′ triphosphate
Answer: Correct. This unique linkage protects mRNA and aids translation.

C. 5-methylcytosine
Explanation: Incorrect. This is DNA methylation.

D. 3′ CCA addition
Explanation: Incorrect. 3′ CCA is added to tRNA, not mRNA.

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Q12. Introns are removed from pre-mRNA by:

A. Spliceosome
Answer: Correct. snRNPs and proteins form the spliceosome for splicing.

B. Ribosome
Explanation: Incorrect. Ribosomes translate mRNA, not splice it.

C. Proteasome
Explanation: Incorrect. Proteasomes degrade proteins.

D. Exosome
Explanation: Incorrect. The exosome degrades RNA, not splice it.

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Q13. The wobble hypothesis explains:

A. Frameshift mutations
Explanation: Incorrect. Frameshifts result from indels.

B. Redundancy of the genetic code at the third base
Answer: Correct. Wobble allows flexibility at codon’s third base.

C. Initiation of translation
Explanation: Incorrect. Initiation is different.

D. Transcription termination
Explanation: Incorrect. Wobble does not govern termination.

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Q14. Shine–Dalgarno sequence is found in:

A. Eukaryotic mRNA
Explanation: Incorrect. Eukaryotes use Kozak sequence.

B. Prokaryotic mRNA
Answer: Correct. It aligns mRNA with 16S rRNA.

C. tRNA
Explanation: Incorrect. Not in tRNA.

D. rRNA only
Explanation: Incorrect. Shine–Dalgarno is on mRNA.

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Q15. Peptidyl transferase activity is a function of:

A. Protein enzyme in the large subunit
Explanation: Incorrect. The catalytic site is RNA.

B. rRNA (ribozyme) of the large subunit
Answer: Correct. The ribosome is a ribozyme.

C. Small subunit protein
Explanation: Incorrect. The small subunit decodes.

D. EF-Tu only
Explanation: Incorrect. EF-Tu delivers aa-tRNA, not catalysis.

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Q16. An operon is best described as:

A. A single eukaryotic gene with many introns
Explanation: Incorrect. Eukaryotic genes are monocistronic.

B. A cluster of bacterial genes under one promoter
Answer: Correct. Operons coordinate gene expression.

C. A transcription factor
Explanation: Incorrect. TFs regulate operons.

D. A noncoding RNA
Explanation: Incorrect. Operons usually encode proteins.

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Q17. In the lac operon, the inducer allolactose binds to:

A. CAP (CRP)
Explanation: Incorrect. CAP binds cAMP.

B. RNA polymerase
Explanation: Incorrect. RNAP does not bind allolactose.

C. Lac repressor
Answer: Correct. Allolactose inactivates repressor to allow transcription.

D. Operator DNA
Explanation: Incorrect. The inducer binds the repressor, not DNA.

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Q18. cAMP–CAP complex in the lac operon:

A. Represses transcription in low glucose
Explanation: Incorrect. It activates transcription.

B. Activates transcription when glucose is scarce
Answer: Correct. cAMP–CAP recruits RNAP to promoter.

C. Is unnecessary for lactose metabolism
Explanation: Incorrect. It enhances expression.

D. Binds operator to block RNAP
Explanation: Incorrect. It binds near promoter, not operator.

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Q19. A mutation changing a codon to a stop codon is:

A. Missense
Explanation: Incorrect. Missense changes amino acid.

B. Nonsense
Answer: Correct. Nonsense mutations create premature stop codons.

C. Silent
Explanation: Incorrect. Silent does not change amino acid.

D. Frameshift
Explanation: Incorrect. Frameshift involves indels.

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Q20. Which DNA repair corrects thymine dimers using light energy in some organisms?

A. Nucleotide excision repair
Explanation: Incorrect. NER excises dimers but not light-dependent.

B. Photoreactivation
Answer: Correct. Photolyase splits dimers using visible light.

C. Base excision repair
Explanation: Incorrect. BER fixes small lesions.

D. Mismatch repair
Explanation: Incorrect. MMR fixes replication errors.

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Q21. Which technique amplifies specific DNA sequences in vitro?

A. Northern blotting
Explanation: Incorrect. Northern blot detects RNA.

B. PCR
Answer: Correct. PCR amplifies DNA exponentially.

C. SDS-PAGE
Explanation: Incorrect. Separates proteins.

D. ELISA
Explanation: Incorrect. Detects antigens/antibodies.

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Q22. Taq polymerase is isolated from:

A. Thermus aquaticus
Answer: Correct. Thermophilic bacterium provides thermostable enzyme.

B. Escherichia coli
Explanation: Incorrect. E. coli enzymes are not thermostable.

C. Saccharomyces cerevisiae
Explanation: Incorrect. Yeast enzymes differ.

D. Homo sapiens
Explanation: Incorrect. Human polymerases are not used in PCR.

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Q23. A vector with genes for β-galactosidase allows:

A. Blue-white screening
Answer: Correct. Disruption of lacZ enables color screening.

B. Replica plating
Explanation: Incorrect. Used for auxotrophy testing.

C. Southern blotting
Explanation: Incorrect. Detects DNA sequences.

D. RFLP typing
Explanation: Incorrect. Not the main role.

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Q24. Restriction enzymes typically recognize:

A. Random long sequences
Explanation: Incorrect. They are sequence-specific.

B. Specific palindromic DNA sequences
Answer: Correct. Restriction enzymes cut at palindromic sites.

C. RNA hairpins
Explanation: Incorrect. They act on DNA, not RNA.

D. Protein motifs
Explanation: Incorrect. They do not cut proteins.

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Q25. Sticky ends are useful because they:

A. Are resistant to ligases
Explanation: Incorrect. They ligate easily.

B. Facilitate directional cloning via base pairing
Answer: Correct. Overhangs guide insertion orientation.

C. Prevent recombination
Explanation: Incorrect. They promote recombination.

D. Block PCR
Explanation: Incorrect. They do not block PCR.

Q26. cDNA is synthesized from mRNA using:

A. DNA ligase
Explanation: Incorrect. Ligase joins DNA fragments.

B. Reverse transcriptase
Answer: Correct. Reverse transcriptase converts RNA to DNA.

C. RNase H only
Explanation: Incorrect. RNase H degrades RNA, does not synthesize DNA.

D. Topoisomerase
Explanation: Incorrect. Alters DNA topology, not reverse transcription.

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Q27. DNA migrates in agarose gel towards the:

A. Cathode due to positive charge
Explanation: Incorrect. DNA is negatively charged.

B. Anode due to negative backbone
Answer: Correct. Phosphate backbone is negative, DNA moves to anode.

C. Cathode due to negative backbone
Explanation: Incorrect. Opposite direction.

D. Does not move
Explanation: Incorrect. DNA moves under electric field.

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Q28. Sanger sequencing relies on:

A. ddNTP chain terminators
Answer: Correct. Dideoxynucleotides terminate DNA synthesis.

B. RNase P
Explanation: Incorrect. RNase P processes tRNA.

C. Mass spectrometry of peptides
Explanation: Incorrect. Proteomics, not sequencing.

D. RNA-seq
Explanation: Incorrect. RNA-seq is next-gen sequencing.

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Q29. Which blot detects proteins?

A. Southern
Explanation: Incorrect. Southern detects DNA.

B. Northern
Explanation: Incorrect. Northern detects RNA.

C. Western
Answer: Correct. Western blot uses antibodies to detect proteins.

D. Eastern
Explanation: Incorrect. Not standard.

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Q30. CRISPR–Cas9 requires a:

A. Guide RNA and PAM
Answer: Correct. gRNA directs Cas9 to DNA adjacent to PAM.

B. DNA primer and ligase
Explanation: Incorrect. Not the targeting mechanism.

C. Only dsDNA breaks without guide
Explanation: Incorrect. Cas9 requires guide RNA.

D. Reverse transcriptase
Explanation: Incorrect. Not involved in CRISPR.

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Q31. Real-time qPCR monitors amplification by:

A. Antibody staining
Explanation: Incorrect. Antibodies are not used.

B. Fluorescence during cycles
Answer: Correct. qPCR uses fluorescent dyes or probes.

C. Gel staining after run
Explanation: Incorrect. That’s end-point PCR.

D. Electron microscopy
Explanation: Incorrect. Not used in PCR.

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Q32. A housekeeping gene is used in qPCR for:

A. Increasing mutation rate
Explanation: Incorrect. Not a function.

B. Normalization of expression levels
Answer: Correct. Serves as internal control.

C. Inducing apoptosis
Explanation: Incorrect. Not related.

D. Sequencing adapters
Explanation: Incorrect. Not a role in qPCR.

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Q33. A plasmid origin of replication determines:

A. Antibiotic resistance
Explanation: Incorrect. Resistance genes provide selection.

B. Copy number and host range
Answer: Correct. ori controls replication in host.

C. Promoter strength
Explanation: Incorrect. Promoter regulates transcription, not replication.

D. Restriction map size only
Explanation: Incorrect. ori has functional role.

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Q34. An expression vector must contain:

A. Origin only
Explanation: Incorrect. Insufficient for expression.

B. Promoter and ribosome-binding/translation signals
Answer: Correct. These are required for expression of inserted gene.

C. Telomeres
Explanation: Incorrect. Telomeres are in linear chromosomes.

D. CRISPR array
Explanation: Incorrect. Not required for expression.

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Q35. DNA barcoding commonly uses which gene in animals?

A. 16S rRNA
Explanation: Incorrect. Common for bacteria.

B. COI (cytochrome c oxidase I)
Answer: Correct. Standard gene for animal barcoding.

C. GAPDH
Explanation: Incorrect. Housekeeping gene, not barcoding.

D. HBB
Explanation: Incorrect. Hemoglobin beta is not used for barcoding.

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Q36. Next-generation sequencing differs from Sanger by:

A. Reading single molecules only
Explanation: Incorrect. Not all platforms.

B. Massively parallel sequencing of many fragments
Answer: Correct. NGS sequences millions simultaneously.

C. No need for DNA polymerase
Explanation: Incorrect. Polymerases are still used.

D. Only RNA templates
Explanation: Incorrect. DNA is also sequenced.

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Q37. DNA microarrays primarily measure:

A. Protein abundance
Explanation: Incorrect. Proteomics, not arrays.

B. mRNA expression levels
Answer: Correct. Hybridization intensity reflects transcript abundance.

C. Metabolite concentrations
Explanation: Incorrect. Measured by metabolomics.

D. Chromosome numbers only
Explanation: Incorrect. Not the main use.

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Q38. RNA interference (RNAi) uses:

A. dsRNA to trigger mRNA degradation
Answer: Correct. siRNA and miRNA silence mRNA via RISC.

B. ssDNA to cut proteins
Explanation: Incorrect. ssDNA does not cut proteins.

C. rRNA to block replication
Explanation: Incorrect. rRNA is structural.

D. Lipids to digest RNA
Explanation: Incorrect. Not possible.

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Q39. A selectable marker in cloning is used to:

A. Visualize DNA bands
Explanation: Incorrect. Stains do that.

B. Select cells that carry the vector
Answer: Correct. Resistance markers help identify transformed cells.

C. Induce transformation
Explanation: Incorrect. Marker does not induce.

D. Sequence the insert
Explanation: Incorrect. Not its role.

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Q40. Gibson assembly enables:

A. Random ligation only
Explanation: Incorrect. Not random.

B. Seamless joining of overlapping DNA fragments
Answer: Correct. Exonuclease, polymerase, ligase assemble DNA fragments.

C. RNA splicing
Explanation: Incorrect. Splicing is RNA process.

D. Protein ligation
Explanation: Incorrect. Not for proteins.

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Q41. Histone acetylation generally:

A. Condenses chromatin and represses
Explanation: Incorrect. Opposite effect.

B. Relaxes chromatin and activates
Answer: Correct. Acetylation opens chromatin.

C. Degrades histones
Explanation: Incorrect. Does not degrade.

D. Prevents replication
Explanation: Incorrect. Replication still occurs.

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Q42. DNA methylation at CpG islands often leads to:

A. Gene activation
Explanation: Incorrect. Usually represses.

B. Gene silencing
Answer: Correct. CpG methylation blocks transcription.

C. Increased recombination
Explanation: Incorrect. Not a direct effect.

D. RNA editing
Explanation: Incorrect. Post-transcriptional.

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Q43. Enhancers are:

A. Protein repressors
Explanation: Incorrect. Enhancers are DNA, not proteins.

B. Distant DNA elements that increase transcription
Answer: Correct. Enhancers act at a distance.

C. RNA polymerases
Explanation: Incorrect. RNAP is not enhancer.

D. Promoter cores only
Explanation: Incorrect. Distinct from promoters.

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Q44. Mediator complex:

A. Directly synthesizes RNA
Explanation: Incorrect. RNAP II synthesizes.

B. Bridges transcription factors and RNAP II
Answer: Correct. Mediator links regulatory signals.

C. Degrades mRNA
Explanation: Incorrect. Exosome degrades.

D. Replicates DNA
Explanation: Incorrect. Polymerases replicate DNA.

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Q45. Heterochromatin is characterized by:

A. High gene expression
Explanation: Incorrect. It is silent.

B. Hypomethylated DNA
Explanation: Incorrect. Usually hypermethylated.

C. H3K9 methylation and compaction
Answer: Correct. Hallmark of heterochromatin.

D. Open DNase I sensitivity
Explanation: Incorrect. Resistant to DNase.

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Q46. Insulators (CTCF sites) function to:

A. Enhance transcription globally
Explanation: Incorrect. Not their role.

B. Block enhancer–promoter communication across domains
Answer: Correct. Insulators partition chromatin.

C. Promote translation
Explanation: Incorrect. Translation is cytoplasmic.

D. Methylate DNA
Explanation: Incorrect. Not enzymes.

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Q47. X-chromosome inactivation in females involves:

A. Xist lncRNA coating
Answer: Correct. Xist coats and silences one X.

B. Y-linked gene expression increase
Explanation: Incorrect. Not involved.

C. DNA recombination between X and Y
Explanation: Incorrect. Not mechanism.

D. Histone acetylation only
Explanation: Incorrect. Multiple marks are involved.

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Q48. miRNAs typically:

A. Increase target protein translation
Explanation: Incorrect. Usually repress.

B. Bind 3′ UTR to repress translation or degrade mRNA
Answer: Correct. miRNAs regulate gene expression.

C. Edit DNA sequences
Explanation: Incorrect. Not their function.

D. Replicate independently
Explanation: Incorrect. Encoded in genome.

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Q49. Operons are rare in eukaryotes because:

A. mRNA capping prevents it
Explanation: Incorrect. Not the reason.

B. Eukaryotic mRNAs are generally monocistronic
Answer: Correct. One mRNA → one protein.

C. No promoters exist
Explanation: Incorrect. Promoters exist.

D. Ribosomes cannot bind mRNA
Explanation: Incorrect. Ribosomes do bind.

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Q50. Kozak consensus sequence facilitates:

A. DNA replication start
Explanation: Incorrect. Replication origins differ.

B. Eukaryotic translation initiation
Answer: Correct. Kozak enhances AUG recognition.

C. Splice site recognition
Explanation: Incorrect. GU–AG defines introns.

D. Polyadenylation
Explanation: Incorrect. Uses AAUAAA signal.

Q51. Ubiquitination of a protein commonly signals:

A. Nuclear import
Explanation: Incorrect. NLS controls import, not ubiquitin.

B. Proteasomal degradation
Answer: Correct. Polyubiquitin chains target proteins to proteasome.

C. mRNA export
Explanation: Incorrect. Not its role.

D. DNA replication
Explanation: Incorrect. Replication uses polymerases.

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Q52. Epigenetic inheritance refers to:

A. Changes in DNA sequence
Explanation: Incorrect. Epigenetics does not alter sequence.

B. Heritable chromatin/RNA states without DNA change
Answer: Correct. Marks like DNA methylation can be inherited.

C. Only mitochondrial genes
Explanation: Incorrect. Not limited to mitochondria.

D. Protein mutations
Explanation: Incorrect. Mutations are genetic, not epigenetic.

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Q53. The lac operon operator is bound by:

A. RNAP core enzyme
Explanation: Incorrect. RNAP binds the promoter.

B. LacI repressor
Answer: Correct. LacI binds operator to repress transcription.

C. CAP alone
Explanation: Incorrect. CAP binds near promoter as activator.

D. Rho factor
Explanation: Incorrect. Rho terminates transcription.

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Q54. Attenuation regulates the trp operon via:

A. DNA methylation
Explanation: Incorrect. Not the main mechanism.

B. Coupling of transcription and translation in bacteria
Answer: Correct. Leader peptide translation affects RNA structure.

C. Alternative splicing
Explanation: Incorrect. That’s eukaryotic.

D. Protein degradation
Explanation: Incorrect. Not involved.

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Q55. Allosteric effectors of transcription factors:

A. Bind DNA directly
Explanation: Incorrect. Effectors bind proteins.

B. Bind proteins to change their activity
Answer: Correct. Effectors alter conformation of TFs.

C. Only increase activity
Explanation: Incorrect. They may repress too.

D. Are always nucleotides
Explanation: Incorrect. Can be many small molecules.

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Q56. DNase I hypersensitive sites indicate:

A. Highly condensed chromatin
Explanation: Incorrect. Condensed is DNase-resistant.

B. Accessible regulatory regions
Answer: Correct. DNase hypersensitivity marks open chromatin.

C. Replication origins only
Explanation: Incorrect. Not specific to origins.

D. Centromeric heterochromatin
Explanation: Incorrect. Centromeres are resistant.

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Q57. SWI/SNF complexes:

A. Remodel nucleosomes using ATP
Answer: Correct. They reposition nucleosomes.

B. Synthesize DNA
Explanation: Incorrect. Polymerases do this.

C. Degrade histones
Explanation: Incorrect. Proteasome degrades proteins.

D. Cap mRNA
Explanation: Incorrect. Nuclear process unrelated.

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Q58. CpG island promoters are often:

A. Methylated in housekeeping genes
Explanation: Incorrect. They are usually unmethylated.

B. Unmethylated and active
Answer: Correct. Active promoters are unmethylated.

C. Absent from vertebrates
Explanation: Incorrect. Common in vertebrate genomes.

D. Only in bacteria
Explanation: Incorrect. CpG islands are eukaryotic.

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Q59. Histone H1 is associated with:

A. Nucleosome core particle
Explanation: Incorrect. Core has H2A, H2B, H3, H4.

B. Linker DNA and higher-order compaction
Answer: Correct. H1 binds linker DNA to compact chromatin.

C. DNA replication origins
Explanation: Incorrect. Not a role of H1.

D. RNA splicing
Explanation: Incorrect. Unrelated.

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Q60. Topoisomerase II inhibitors (e.g., etoposide) primarily:

A. Increase supercoiling resolution
Explanation: Incorrect. They block resolution.

B. Stabilize double-strand break intermediates causing cytotoxicity
Answer: Correct. Leads to DNA breaks and cell death.

C. Act as polymerases
Explanation: Incorrect. Not enzymes of synthesis.

D. Methylate DNA
Explanation: Incorrect. Not involved.

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Q61. Synonymous mutation:

A. Changes amino acid
Explanation: Incorrect. That’s missense.

B. Changes codon without changing amino acid
Answer: Correct. Degeneracy allows same amino acid.

C. Introduces a frameshift
Explanation: Incorrect. Frameshifts are indels.

D. Always harmful
Explanation: Incorrect. Often neutral.

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Q62. A transition mutation is:

A. Purine to pyrimidine
Explanation: Incorrect. That’s transversion.

B. Purine to purine or pyrimidine to pyrimidine
Answer: Correct. Transition is within same base class.

C. Any insertion
Explanation: Incorrect. Insertions are not base substitutions.

D. Triplet repeat expansion only
Explanation: Incorrect. Different mechanism.

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Q63. Ames test assesses:

A. Antibiotic resistance
Explanation: Incorrect. Not its role.

B. Mutagenicity using bacterial reversion
Answer: Correct. Detects mutagens by His+ reversion.

C. Protein stability
Explanation: Incorrect. Not tested.

D. Virus infectivity
Explanation: Incorrect. Not the aim.

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Q64. Mitochondrial DNA is inherited:

A. Maternally in most animals
Answer: Correct. mtDNA usually comes from mother.

B. Paternally
Explanation: Incorrect. Rarely transmitted.

C. Equally from both parents
Explanation: Incorrect. Not biparental.

D. Randomly from neighbors
Explanation: Incorrect. Not possible.

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Q65. Exons are:

A. Noncoding regions removed
Explanation: Incorrect. Introns are removed.

B. Coding or UTR segments retained in mRNA
Answer: Correct. Exons remain in mature mRNA.

C. Only protein-coding regions
Explanation: Incorrect. Exons may include UTRs.

D. Intronic repeats
Explanation: Incorrect. Not exons.

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Q66. A haplotype is best defined as:

A. A single allele
Explanation: Incorrect. More than one allele.

B. A set of linked alleles on a chromosome segment
Answer: Correct. Haplotype = combination of alleles.

C. A phenotype
Explanation: Incorrect. Phenotype = traits.

D. Aneuploidy
Explanation: Incorrect. Chromosome number change.

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Q67. Copy number variation (CNV) refers to:

A. Only SNP changes
Explanation: Incorrect. SNPs are single bases.

B. Segmental duplications/deletions altering dosage
Answer: Correct. CNVs change DNA dosage.

C. Point mutations
Explanation: Incorrect. Not CNV.

D. Methylation status
Explanation: Incorrect. That’s epigenetic.

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Q68. Microsatellites are:

A. Long interspersed elements
Explanation: Incorrect. Those are LINEs.

B. Short tandem repeats (1–6 bp units)
Answer: Correct. STRs used in forensics.

C. rRNA genes
Explanation: Incorrect. Not microsatellites.

D. Mitochondrial repeats only
Explanation: Incorrect. Found in nuclear DNA too.

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Q69. Nonsense-mediated decay targets mRNAs with:

A. No poly(A) tail
Explanation: Incorrect. Not specific to NMD.

B. Premature stop codons and exon–junction signals
Answer: Correct. PTCs trigger NMD pathway.

C. Too long 5′ UTR
Explanation: Incorrect. Not the trigger.

D. No cap only
Explanation: Incorrect. Not NMD mechanism.

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Q70. The genetic code is:

A. Overlapping
Explanation: Incorrect. Code is non-overlapping.

B. Nearly universal and non-overlapping
Answer: Correct. Triplet code applies across life.

C. Ambiguous
Explanation: Incorrect. Each codon = one amino acid.

D. Two-letter
Explanation: Incorrect. Code uses 4 bases.

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Q71. Frameshift mutations are often caused by:

A. Synonymous substitutions
Explanation: Incorrect. They don’t shift frame.

B. Insertions or deletions not in multiples of three
Answer: Correct. Alters reading frame.

C. Promoter methylation
Explanation: Incorrect. Epigenetic effect, not frame.

D. Histone acetylation
Explanation: Incorrect. Unrelated.

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Q72. A genetic map unit (centimorgan) corresponds to:

A. 1% recombination frequency
Answer: Correct. 1 cM ≈ 1% recombination.

B. 1% GC content
Explanation: Incorrect. Not map unit.

C. 1 Mb distance
Explanation: Incorrect. Physical distance varies.

D. 1 gene
Explanation: Incorrect. Not defined per gene.

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Q73. Horizontal gene transfer includes:

A. Transformation, transduction, conjugation
Answer: Correct. Mechanisms of HGT.

B. Mitosis
Explanation: Incorrect. Vertical inheritance.

C. Meiosis
Explanation: Incorrect. Sexual reproduction.

D. Binary fission only
Explanation: Incorrect. Not HGT.

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Q74. Transposons move via:

A. Only DNA replication forks
Explanation: Incorrect. Not limited to forks.

B. Cut-and-paste DNA or copy-and-paste via RNA intermediates
Answer: Correct. Two main mechanisms.

C. Protein channels only
Explanation: Incorrect. Proteins assist, but not channels.

D. Membrane vesicles
Explanation: Incorrect. Not mechanism.

---

Q75. Prions are:

A. Infectious proteins lacking nucleic acids
Answer: Correct. Misfolded proteins propagate disease.

B. RNA viruses
Explanation: Incorrect. They have genomes.

C. Retrotransposons
Explanation: Incorrect. Not proteins.

D. Bacteriophages
Explanation: Incorrect. Phages infect bacteria.

Q76. A gene knockout in mice is commonly achieved by:

A. Random mutagenesis only
Explanation: Incorrect. Less precise.

B. Homologous recombination or CRISPR
Answer: Correct. Targeted disruption removes gene function.

C. RNA editing by ADAR
Explanation: Incorrect. Edits RNA, not DNA.

D. Protein ubiquitination
Explanation: Incorrect. Post-translational modification.

---

Q77. A linkage disequilibrium (LD) block reflects:

A. Random assortment
Explanation: Incorrect. LD is non-random.

B. Non-random association of alleles in a region
Answer: Correct. LD keeps alleles together.

C. Only coding regions
Explanation: Incorrect. Spans coding and noncoding.

D. Mitochondrial genes
Explanation: Incorrect. mtDNA separate.

---

Q78. Genome-wide association studies (GWAS) detect:

A. Causation directly
Explanation: Incorrect. GWAS shows correlation.

B. Statistical association between variants and traits
Answer: Correct. SNP-trait links identified.

C. Only rare variants
Explanation: Incorrect. Often common variants.

D. Protein structures
Explanation: Incorrect. Structural biology is different.

---

Q79. A balanced translocation typically:

A. Changes copy number of genes
Explanation: Incorrect. No net gain/loss.

B. Moves segments without net gain/loss
Answer: Correct. Dosage remains the same.

C. Always lethal
Explanation: Incorrect. Many carriers survive.

D. Only happens in bacteria
Explanation: Incorrect. Occurs in eukaryotes.

---

Q80. Sickle-cell trait persists due to:

A. Genetic drift only
Explanation: Incorrect. Selection is major factor.

B. Heterozygote advantage against malaria
Answer: Correct. Protects carriers from severe malaria.

C. Founder effect only
Explanation: Incorrect. Not the main driver.

D. Neutral evolution
Explanation: Incorrect. Maintained by selection.

---

Q81. DNA mismatch repair recognizes:

A. Mature mRNA cap
Explanation: Incorrect. That’s RNA.

B. Replication errors like mispaired bases and small loops
Answer: Correct. MMR fixes replication mistakes.

C. Double-strand breaks only
Explanation: Incorrect. Fixed by HR/NHEJ.

D. Telomere overhangs only
Explanation: Incorrect. Not MMR role.

---

Q82. Base excision repair starts with:

A. Endonuclease nicking bulky lesion
Explanation: Incorrect. That’s NER.

B. DNA glycosylase removing damaged base
Answer: Correct. Glycosylase excises base.

C. Helicase unwinding lesion
Explanation: Incorrect. Not BER step.

D. DNA ligase only
Explanation: Incorrect. Acts later.

---

Q83. Nucleotide excision repair is crucial for removing:

A. Oxidized bases only
Explanation: Incorrect. BER removes those.

B. Bulky adducts like thymine dimers
Answer: Correct. NER excises helix-distorting lesions.

C. Correctly paired bases
Explanation: Incorrect. Not removed.

D. Telomeric repeats
Explanation: Incorrect. Not lesions.

---

Q84. Homologous recombination requires:

A. Sister chromatid or homologous template
Answer: Correct. Uses homology for repair.

B. No template
Explanation: Incorrect. That’s NHEJ.

C. Only ligase
Explanation: Incorrect. Insufficient.

D. Only transposase
Explanation: Incorrect. Not mechanism.

---

Q85. p53 primarily acts as a:

A. Helicase
Explanation: Incorrect. Does not unwind DNA.

B. Transcription factor inducing cell cycle arrest and apoptosis
Answer: Correct. Tumor suppressor via p21, BAX.

C. Ribosomal protein
Explanation: Incorrect. Not a ribosomal subunit.

D. DNA polymerase
Explanation: Incorrect. Not a polymerase.

---

Q86. Oncogenes typically are:

A. Loss-of-function in tumor suppressors
Explanation: Incorrect. That defines tumor suppressors.

B. Gain-of-function mutations increasing cell growth
Answer: Correct. Proto-oncogenes become oncogenes.

C. Neutral polymorphisms only
Explanation: Incorrect. They affect growth.

D. Mitochondrial genes only
Explanation: Incorrect. Can be nuclear.

---

Q87. Reverse transcriptase activity is inherent in:

A. Telomerase
Answer: Correct. Extends telomeres using RNA template.

B. DNA ligase
Explanation: Incorrect. Seals nicks.

C. RNA polymerase II
Explanation: Incorrect. Transcribes mRNA.

D. Topoisomerase I
Explanation: Incorrect. Relieves supercoils.

---

Q88. A polycistronic mRNA contains:

A. Multiple coding sequences each with its own start/stop
Answer: Correct. Common in prokaryotes.

B. Only one ORF
Explanation: Incorrect. That’s monocistronic.

C. Only introns
Explanation: Incorrect. Bacteria lack introns usually.

D. Only UTRs
Explanation: Incorrect. Has coding regions too.

---

Q89. Ribosomal A site binds:

A. Peptidyl-tRNA
Explanation: Incorrect. That’s in P site.

B. Aminoacyl-tRNA
Answer: Correct. A site accepts new aa-tRNA.

C. Ejected tRNA
Explanation: Incorrect. E site holds exit tRNA.

D. mRNA cap
Explanation: Incorrect. Cap is in mRNA, not ribosome.

---

Q90. The start codon AUG codes for:

A. Methionine in eukaryotes and fMet in bacteria
Answer: Correct. AUG initiates translation.

B. Valine in all organisms
Explanation: Incorrect. Valine codons are GUU/GUC/GUA/GUG.

C. Isoleucine
Explanation: Incorrect. AUU/AUC/AUA.

D. Stop
Explanation: Incorrect. AUG is not stop codon.

---

Q91. Which RNA polymerase transcribes rRNA (28S, 18S, 5.8S) in eukaryotes?

A. RNA Pol I
Answer: Correct. Transcribes major rRNAs in nucleolus.

B. RNA Pol II
Explanation: Incorrect. Transcribes mRNA.

C. RNA Pol III
Explanation: Incorrect. Transcribes tRNA and 5S rRNA.

D. Mitochondrial RNAP only
Explanation: Incorrect. Only for mtDNA.

---

Q92. A 3′ poly(A) tail in mRNA:

A. Decreases stability
Explanation: Incorrect. It stabilizes.

B. Enhances stability and translation
Answer: Correct. Aids export and translation.

C. Is added co-transcriptionally by RNAP II
Explanation: Incorrect. Added post-transcriptionally.

D. Occurs only in bacteria
Explanation: Incorrect. Mostly in eukaryotes.

---

Q93. Ribozyme is:

A. A protein enzyme in ribosome
Explanation: Incorrect. Catalytic center is RNA.

B. An RNA molecule with catalytic activity
Answer: Correct. Example: peptidyl transferase.

C. A DNA enzyme only
Explanation: Incorrect. Deoxyribozymes are engineered.

D. A lipoprotein
Explanation: Incorrect. Not definition.

---

Q94. Allosteric inhibitors of enzymes often:

A. Bind active site only
Explanation: Incorrect. Competitive inhibitors do this.

B. Bind regulatory sites and change conformation
Answer: Correct. They alter enzyme activity indirectly.

C. Increase Vmax always
Explanation: Incorrect. Usually decrease activity.

D. Convert competitive to noncompetitive invariably
Explanation: Incorrect. Mechanisms vary.

---

Q95. A northern blot probe must be:

A. Protein
Explanation: Incorrect. Probes are nucleic acids.

B. Nucleic acid complementary to target RNA
Answer: Correct. Hybridizes with RNA.

C. Lipid
Explanation: Incorrect. Lipids don’t hybridize.

D. Carbohydrate
Explanation: Incorrect. Carbs don’t hybridize.

---

Q96. In bacteria, sigma factor’s main role is to:

A. Elongate mRNA
Explanation: Incorrect. Core RNAP elongates.

B. Recognize promoter sequences for initiation
Answer: Correct. Sigma directs RNAP to promoter.

C. Terminate transcription
Explanation: Incorrect. Rho or intrinsic signals terminate.

D. Repair DNA
Explanation: Incorrect. Not a sigma function.

---

Q97. Southern blotting involves:

A. Transfer of DNA from gel to membrane
Answer: Correct. DNA fragments transferred for hybridization.

B. Transfer of protein to membrane
Explanation: Incorrect. That’s Western blot.

C. RNA to membrane
Explanation: Incorrect. That’s Northern blot.

D. Only in situ hybridization
Explanation: Incorrect. Different technique.

---

Q98. A tautomeric shift in bases can cause:

A. Increased recombination
Explanation: Incorrect. Not main effect.

B. Mispairing during replication
Answer: Correct. Rare tautomers mispair, causing mutations.

C. Protein denaturation
Explanation: Incorrect. Proteins not affected.

D. RNA splicing
Explanation: Incorrect. Not related.

---

Q99. Helicase activity requires:

A. ATP hydrolysis
Answer: Correct. Helicases are ATPases that unwind DNA.

B. No energy
Explanation: Incorrect. Energy is required.

C. GTP binding only
Explanation: Incorrect. Most use ATP.

D. Light energy
Explanation: Incorrect. Light used by photolyase, not helicase.

---

Q100. An anticodon is located on:

A. mRNA
Explanation: Incorrect. Codons are on mRNA.

B. tRNA
Answer: Correct. Anticodon pairs with codon during translation.

C. rRNA
Explanation: Incorrect. rRNA is structural/catalytic.

D. DNA
Explanation: Incorrect. DNA is the template.