Part 2: Structure of DNA and RNA (25 MCQs)
Part 2: Structure of DNA and RNA (25 MCQs)
Q26. The double helix model of DNA was proposed by:
a) Hershey and Chase
b) Watson and Crick
c) Avery and MacLeod
d) Franklin and Wilkins
Answer: b
- b) Correct: Watson & Crick (1953) proposed the famous double helix model.
- d) Rosalind Franklin & Wilkins provided X-ray diffraction data.
- a, c) Proved genetic material, not structure.
Q27. The diameter of DNA double helix is approximately:
a) 1 Å
b) 2 nm
c) 10 nm
d) 20 µm
Answer: b
- b) Correct: DNA double helix has 2 nm diameter.
- a) 1 Å = bond length.
- c, d) Wrong by orders of magnitude.
Q28. The two DNA strands are held together by:
a) Ionic bonds
b) Hydrogen bonds
c) Phosphodiester bonds
d) Peptide bonds
Answer: b
- b) Correct: Hydrogen bonds between complementary bases (A=T, G≡C).
- c) Within a single strand, not between.
Q29. Which of the following is NOT a nitrogenous base in DNA?
a) Adenine
b) Thymine
c) Uracil
d) Guanine
Answer: c
- c) Correct: Uracil is found in RNA, not DNA.
Q30. Chargaff’s rule states that:
a) A = T, G = C
b) A + T = G + C
c) A + T ≠ G + C
d) Purines + Pyrimidines = 1
Answer: a
- a) Correct: Chargaff’s rule → A pairs with T, G pairs with C.
Q31. In DNA, adenine pairs with thymine by:
a) One H-bond
b) Two H-bonds
c) Three H-bonds
d) Four H-bonds
Answer: b
- b) Correct: A=T forms 2 hydrogen bonds.
Q32. Guanine pairs with cytosine through:
a) Two H-bonds
b) Three H-bonds
c) Four H-bonds
d) One H-bond
Answer: b
- b) Correct: G≡C forms 3 hydrogen bonds, stronger than A=T.
Q33. DNA has polarity because:
a) Of nitrogenous bases
b) Of 5′ and 3′ ends with phosphate and hydroxyl groups
c) Of hydrogen bonds
d) Of ribose sugar
Answer: b
- b) Correct: Polarity arises due to 5′ phosphate and 3′ –OH orientation.
Q34. Which form of DNA is most common in living cells?
a) A-DNA
b) B-DNA
c) Z-DNA
d) H-DNA
Answer: b
- b) Correct: B-DNA is normal right-handed double helix in cells.
Q35. Z-DNA differs from B-DNA because it:
a) Is right-handed
b) Is left-handed
c) Has only A=T pairs
d) Has no sugar-phosphate backbone
Answer: b
- b) Correct: Z-DNA is left-handed helix, rare.
Q36. The distance between two base pairs in DNA is:
a) 0.34 nm
b) 3.4 nm
c) 2 nm
d) 34 nm
Answer: a
- a) Correct: Distance between two base pairs = 0.34 nm.
Q37. One complete turn of B-DNA contains:
a) 5 base pairs
b) 10 base pairs
c) 20 base pairs
d) 100 base pairs
Answer: b
- b) Correct: One turn = 10 base pairs (~3.4 nm length).
Q38. The backbone of DNA is made up of:
a) Sugar and phosphate
b) Base pairs
c) Proteins and lipids
d) RNA
Answer: a
- a) Correct: Sugar-phosphate backbone + nitrogen bases.
Q39. Which of the following is a purine base?
a) Adenine
b) Thymine
c) Cytosine
d) Uracil
Answer: a
- a) Correct: Purines = Adenine & Guanine.
- b, c, d) Pyrimidines.
Q40. In DNA, the amount of purines is always equal to:
a) Thymine
b) Pyrimidines
c) Cytosine
d) Uracil
Answer: b
- b) Correct: Chargaff’s rule: Purines (A+G) = Pyrimidines (T+C).
Q41. The sugar in DNA is:
a) Ribose
b) Deoxyribose
c) Glucose
d) Fructose
Answer: b
- b) Correct: DNA contains deoxyribose (lacks 2′-OH).
Q42. The sugar in RNA is:
a) Deoxyribose
b) Ribose
c) Fructose
d) Glucose
Answer: b
- b) Correct: RNA contains ribose sugar with 2′-OH group.
Q43. Which nitrogenous base is unique to RNA?
a) Adenine
b) Uracil
c) Guanine
d) Thymine
Answer: b
- b) Correct: RNA contains uracil instead of thymine.
Q44. Which RNA carries genetic information from DNA to ribosomes?
a) tRNA
b) rRNA
c) mRNA
d) snRNA
Answer: c
- c) Correct: mRNA = messenger RNA, carries genetic code.
Q45. Which RNA is involved in protein synthesis as adaptor molecule?
a) mRNA
b) rRNA
c) tRNA
d) snRNA
Answer: c
- c) Correct: tRNA carries amino acids → adaptor role.
Q46. Which RNA forms structural and catalytic part of ribosome?
a) mRNA
b) rRNA
c) tRNA
d) siRNA
Answer: b
- b) Correct: rRNA = ribosomal RNA, structural + catalytic role.
Q47. Which statement about DNA and RNA is true?
a) DNA has thymine, RNA has uracil
b) DNA is more stable, RNA is less stable
c) DNA is double-stranded, RNA mostly single-stranded
d) All of the above
Answer: d
- d) Correct: All statements are true differences.
Q48. The pitch of DNA helix (length of one complete turn) is:
a) 0.34 nm
b) 3.4 nm
c) 2 nm
d) 34 nm
Answer: b
- b) Correct: One complete turn = 3.4 nm.
Q49. Which form of DNA is left-handed?
a) A-DNA
b) B-DNA
c) Z-DNA
d) H-DNA
Answer: c
- c) Correct: Z-DNA = left-handed helix.
Q50. Who gave X-ray diffraction data crucial for DNA model?
a) Franklin and Wilkins
b) Watson and Crick
c) Hershey and Chase
d) Avery and MacLeod
Answer: a
- a) Correct: Rosalind Franklin & Maurice Wilkins → X-ray diffraction data, basis of Watson & Crick model.
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