Structure of DNA MCQs With Answer

Understanding the Structure of DNA MCQs With Answer is essential for B.Pharm students preparing for pharmacology, molecular biology and genetics exams. This concise guide of Structure of DNA MCQs With Answer covers key concepts such as nucleotide composition, base pairing, sugar-phosphate backbone, antiparallel strands, major and minor grooves, and DNA replication basics. Clear, exam-focused questions help reinforce biochemical principles relevant to drug action, pharmacogenomics, and therapeutic design. Emphasis on pictorial models, hydrogen bonding, and DNA topology improves conceptual clarity and application in pharmaceutical contexts. Use these multiple-choice questions to strengthen recall and problem-solving skills. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which components make up a DNA nucleotide?

• A nitrogenous base, a pentose sugar and a phosphate group
• Two antiparallel strands and a phosphate backbone
• Amino acid, phosphate and ribose
• Deoxyribose, histone and base pair

Correct Answer: A nitrogenous base, a pentose sugar and a phosphate group

Q2. Which sugar is present in DNA?

• Ribose
• Deoxyribose
• Fructose
• Glucose

Correct Answer: Deoxyribose

Q3. Which bases are purines in DNA?

• Adenine and Cytosine
• Guanine and Thymine
• Adenine and Guanine
• Cytosine and Thymine

Correct Answer: Adenine and Guanine

Q4. How many hydrogen bonds form between adenine and thymine?

• One
• Two
• Three
• Four

Correct Answer: Two

Q5. Which rule states that DNA from any cell of any organism has a 1:1 ratio of purines to pyrimidines?

• Franklin’s rule
• Chargaff’s rules
• Watson-Crick parity
• Mendel’s rule

Correct Answer: Chargaff’s rules

Q6. DNA strands are described as antiparallel. What does this mean?

• Both strands run 5′ to 3′ in the same direction
• One strand runs 5′ to 3′ while the complementary runs 3′ to 5′
• Strands are in parallel but have opposite bases
• Strands are covalently linked end-to-end

Correct Answer: One strand runs 5′ to 3′ while the complementary runs 3′ to 5′

Q7. What type of bond connects the sugar and phosphate in the DNA backbone?

• Hydrogen bond
• Phosphodiester bond
• Peptide bond
• Glycosidic bond

Correct Answer: Phosphodiester bond

Q8. Which bond links a base to the deoxyribose sugar?

• Phosphodiester bond
• Hydrogen bond
• N-glycosidic bond
• Peptide bond

Correct Answer: N-glycosidic bond

Q9. The major groove of DNA is important because:

• It is where phosphates are exposed for enzyme binding
• It allows sequence-specific protein binding and recognition
• It is narrower and less accessible than the minor groove
• It only contains thymine residues

Correct Answer: It allows sequence-specific protein binding and recognition

Q10. Which form of DNA is the classic right-handed double helix described by Watson and Crick?

• Z-DNA
• A-DNA
• B-DNA
• Single-stranded DNA

Correct Answer: B-DNA

Q11. Which of the following is characteristic of Z-DNA?

• Right-handed helix with wide major groove
• Left-handed helix with alternating syn-anti bases
• Double helix with RNA-like sugar pucker
• Triple-stranded structure

Correct Answer: Left-handed helix with alternating syn-anti bases

Q12. The hyperchromic effect refers to:

• Decrease in UV absorbance on DNA denaturation
• Increase in UV absorbance when DNA strands separate
• Enhanced fluorescence of bound drugs
• Reduction of base pairing during replication

Correct Answer: Increase in UV absorbance when DNA strands separate

Q13. Which base pairing is most stable and contributes most to higher melting temperature (Tm)?

• A-T pairs because they have two hydrogen bonds
• G-C pairs because they have three hydrogen bonds
• Pyrimidine-pyrimidine pairs
• Base-sugar interactions

Correct Answer: G-C pairs because they have three hydrogen bonds

Q14. Who provided key X-ray diffraction images that helped reveal the DNA double helix?

• James Watson
• Rosalind Franklin
• Frederick Sanger
• Linus Pauling

Correct Answer: Rosalind Franklin

Q15. Watson and Crick proposed that the strands of DNA are held together primarily by:

• Phosphodiester bonds between bases
• Hydrogen bonds between complementary bases
• Disulfide bridges
• Hydrophobic interactions between phosphates

Correct Answer: Hydrogen bonds between complementary bases

Q16. Which of the following best describes the sugar pucker in B-DNA?

• C3′-endo conformation typical of RNA
• C2′-endo conformation typical of B-DNA
• Planar sugar without pucker
• Alternating C1′-endo/C4′-endo

Correct Answer: C2′-endo conformation typical of B-DNA

Q17. Intercalating agents such as doxorubicin primarily affect DNA by:

• Forming covalent bonds with purines
• Inserting between base pairs and distorting the helix
• Cleaving phosphodiester bonds
• Methylating cytosine residues

Correct Answer: Inserting between base pairs and distorting the helix

Q18. DNA supercoiling is important because it:

• Prevents DNA replication
• Helps compact DNA and affects transcription and replication
• Only occurs in single-stranded viruses
• Destroys base pairing

Correct Answer: Helps compact DNA and affects transcription and replication

Q19. Which enzyme relieves supercoiling ahead of a replication fork?

• DNA ligase
• Helicase
• Topoisomerase
• Primase

Correct Answer: Topoisomerase

Q20. Methylation of cytosine in CpG islands typically affects DNA by:

• Increasing transcriptional activity
• Decreasing gene expression via transcriptional repression
• Cleaving DNA strands
• Converting cytosine to uracil

Correct Answer: Decreasing gene expression via transcriptional repression

Q21. The directionality of a DNA strand is defined by:

• The orientation of nitrogenous bases only
• The free hydroxyl group at the 3′ end and the phosphate at the 5′ end
• The presence of methyl groups on bases
• The sequence of amino acids bound to DNA

Correct Answer: The free hydroxyl group at the 3′ end and the phosphate at the 5′ end

Q22. Which technique directly demonstrated the helical parameters of DNA used by Watson and Crick?

• Northern blotting
• X-ray diffraction
• Mass spectrometry
• Chromatography

Correct Answer: X-ray diffraction

Q23. What is the role of histones in chromosomal DNA structure?

• They act as enzymes to replicate DNA
• They package DNA into nucleosomes facilitating compaction
• They provide the phosphate backbone
• They serve as primers for replication

Correct Answer: They package DNA into nucleosomes facilitating compaction

Q24. Which of the following statements about the minor groove is true?

• It is the main site for sequence-specific recognition by most transcription factors
• It is narrower and often binds small ligands and drugs
• It contains only purines
• It has a higher number of hydrogen bonds than the major groove

Correct Answer: It is narrower and often binds small ligands and drugs

Q25. Base stacking interactions in DNA primarily involve:

• Hydrogen bonds between stacked bases
• Van der Waals and hydrophobic interactions between adjacent bases
• Covalent links between purines and pyrimidines
• Electrostatic attraction between phosphates

Correct Answer: Van der Waals and hydrophobic interactions between adjacent bases

Q26. In terms of pharmacology, sequence-specific binding to the major groove is important for:

• General DNA condensation
• Regulatory proteins and some drug-DNA interactions affecting gene expression
• Hydrolyzing DNA during digestion
• Generating RNA primers

Correct Answer: Regulatory proteins and some drug-DNA interactions affecting gene expression

Q27. Which nucleotide modification is a common epigenetic mark in eukaryotic DNA?

• Uracil incorporation
• 5-methylcytosine
• 6-oxo-adenine
• Thymine phosphorylation

Correct Answer: 5-methylcytosine

Q28. The stability of short DNA duplexes for primer design is most influenced by:

• Only the length, regardless of sequence
• GC content and nearest-neighbor base stacking interactions
• The presence of histones
• The pH only

Correct Answer: GC content and nearest-neighbor base stacking interactions

Q29. Which statement about single-stranded DNA (ssDNA) is correct?

• ssDNA forms a stable double helix identical to B-DNA
• ssDNA is more susceptible to nuclease degradation and forms secondary structures
• ssDNA contains ribose instead of deoxyribose
• ssDNA has more hydrogen bonds per base than dsDNA

Correct Answer: ssDNA is more susceptible to nuclease degradation and forms secondary structures

Q30. During DNA denaturation, which factor increases the melting temperature (Tm)?

• Lower salt concentration
• Higher GC content
• Presence of denaturants like urea
• Shorter duplex length

Correct Answer: Higher GC content

Q31. Which chemical group distinguishes deoxyribose from ribose?

• Hydroxyl group at 2′ position present in deoxyribose
• Hydrogen at 2′ position in deoxyribose (no 2′ OH)
• Extra phosphate on deoxyribose
• Amino group on deoxyribose

Correct Answer: Hydrogen at 2′ position in deoxyribose (no 2′ OH)

Q32. A typical turn of the B-DNA helix contains approximately how many base pairs?

• 5 base pairs
• 10.5 base pairs
• 20 base pairs
• 3 base pairs

Correct Answer: 10.5 base pairs

Q33. In double-stranded DNA, where are the hydrophobic bases located?

• Exposed on the outside interacting with water
• Buried in the interior between the two sugar-phosphate backbones
• Attached to histones on the exterior
• Only at the ends of the molecule

Correct Answer: Buried in the interior between the two sugar-phosphate backbones

Q34. Which structural feature allows DNA polymerases to add nucleotides only in the 5’→3′ direction?

• Presence of 3′ phosphate group
• Requirement of a free 3′ hydroxyl on the primer strand to form a phosphodiester bond
• Polymerase active site binds 5′ hydroxyl
• Polymerase reads template in 5’→3′ direction

Correct Answer: Requirement of a free 3′ hydroxyl on the primer strand to form a phosphodiester bond

Q35. Which phenomenon explains easier strand separation at replication origins?

• High GC content making helix weaker
• High AT-rich regions that are easier to melt
• Increased methylation that breaks bonds
• Binding of topoisomerase only

Correct Answer: High AT-rich regions that are easier to melt

Q36. Which molecular interaction is least important for maintaining DNA double helix stability?

• Base stacking interactions
• Hydrogen bonds between bases
• Electrostatic repulsion between phosphates
• Hydrophobic interactions of bases

Correct Answer: Electrostatic repulsion between phosphates

Q37. DNA-binding drugs that preferentially bind GC-rich regions may influence which pharmacological aspect?

• Protein glycosylation
• Transcription of GC-rich promoters and regulatory sequences
• Membrane fluidity
• mRNA splicing accuracy

Correct Answer: Transcription of GC-rich promoters and regulatory sequences

Q38. Which term describes localized unwinding of DNA due to negative supercoiling?

• Denaturation bubble
• Chromatin remodelling
• Replication fork collapse
• Histone eviction

Correct Answer: Denaturation bubble

Q39. What is the effect of increasing ionic strength (salt concentration) on DNA duplex stability?

• It destabilizes duplex by screening hydrogen bonds
• It stabilizes duplex by shielding negative phosphate charges
• It converts DNA to RNA
• It breaks phosphodiester bonds

Correct Answer: It stabilizes duplex by shielding negative phosphate charges

Q40. Which structural change occurs during DNA denaturation?

• Conversion of single-stranded DNA to double helix
• Separation of complementary strands and loss of secondary structure
• Formation of new covalent bonds between bases
• Addition of methyl groups to the backbone

Correct Answer: Separation of complementary strands and loss of secondary structure

Q41. In the context of DNA topology, negative supercoiling:

• Introduces additional turns in the right-handed helix
• Underwinds DNA facilitating strand separation for replication and transcription
• Prevents unwinding of the helix
• Only occurs in RNA molecules

Correct Answer: Underwinds DNA facilitating strand separation for replication and transcription

Q42. Which method quantifies base composition and can confirm Chargaff’s rules experimentally?

• Gel electrophoresis
• High-performance liquid chromatography (HPLC) of nucleotides
• Light microscopy
• Western blotting

Correct Answer: High-performance liquid chromatography (HPLC) of nucleotides

Q43. Which of the following best describes the role of DNA grooves in drug design?

• Grooves are irrelevant because drugs only bind covalently
• Major and minor grooves provide specific chemical signatures for small molecule or protein binding
• Only the sugar-phosphate backbone is targeted by drugs
• Grooves prevent any ligand from accessing bases

Correct Answer: Major and minor grooves provide specific chemical signatures for small molecule or protein binding

Q44. The term “Watson-Crick base pairing” refers to:

• Specific hydrogen-bond interactions between complementary bases A-T and G-C
• Stacking interactions between adjacent bases
• Random base pairing in mutations
• Peptide bonding between amino acids

Correct Answer: Specific hydrogen-bond interactions between complementary bases A-T and G-C

Q45. Which is a direct consequence of the lack of 2′ OH in DNA compared to RNA?

• DNA is more chemically reactive than RNA
• DNA is less prone to hydrolysis and more chemically stable than RNA
• DNA cannot form double helices
• DNA contains uracil instead of thymine

Correct Answer: DNA is less prone to hydrolysis and more chemically stable than RNA

Q46. Which molecular feature allows certain proteins to read base sequence without breaking hydrogen bonds?

• Access to the sugar-phosphate backbone only
• Recognition of base-specific hydrogen bond donors and acceptors exposed in the major groove
• Cleavage of phosphodiester bonds to sample bases
• Binding exclusively to minor groove which shows no sequence information

Correct Answer: Recognition of base-specific hydrogen bond donors and acceptors exposed in the major groove

Q47. Which of the following describes an intercalator’s effect on DNA replication?

• Enhances replication fidelity by stabilizing primer binding
• Distorts helix, causing frameshift mutations and replication blocks
• Specifically removes thymine dimers
• Methylates guanine to increase replication speed

Correct Answer: Distorts helix, causing frameshift mutations and replication blocks

Q48. Which statement best explains why GC-rich genomes are often more thermally stable?

• G and C are larger molecules and thus heavier
• G-C base pairs form three hydrogen bonds and stronger stacking interactions
• Guanine contains sulfur that stabilizes DNA
• GC-rich DNA has fewer phosphates

Correct Answer: G-C base pairs form three hydrogen bonds and stronger stacking interactions

Q49. DNA topology can be experimentally altered by drugs that inhibit:

• DNA glycosylases
• Topoisomerases, affecting supercoiling and replication
• RNA polymerases only
• Ribosomal function

Correct Answer: Topoisomerases, affecting supercoiling and replication

Q50. Which experimental observation would most directly indicate DNA helix unwinding at a replication origin in vivo?

• Increased histone H1 binding
• Local accumulation of single-stranded binding proteins and replication protein A (RPA)
• Greater methylation at the origin
• Reduction of RNA primers

Correct Answer: Local accumulation of single-stranded binding proteins and replication protein A (RPA)

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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