Part 6 — Nucleic Acids: DNA, RNA, Bases, Structure, Functions (Q126–150)

Part 6 — Nucleic Acids: DNA, RNA, Bases, Structure, Functions (Q126–150)

Q126.

The monomers of nucleic acids are:
A. Amino acids
B. Nucleotides ✅
C. Monosaccharides
D. Fatty acids

Explanation:

• A. Amino acids: Proteins.
• B. Nucleotides (Correct): DNA & RNA are polymers of nucleotides.
• C. Monosaccharides: Carbohydrates.
• D. Fatty acids: Lipids.

Q127.

Which sugar is present in DNA?
A. Ribose
B. Deoxyribose ✅
C. Glucose
D. Fructose

Explanation:

• A. Ribose: In RNA.
• B. Deoxyribose (Correct): Sugar in DNA nucleotides.
• C. Glucose: Monosaccharide, not DNA sugar.
• D. Fructose: Fruit sugar.

Q128.

Which base is found only in RNA?
A. Thymine
B. Uracil ✅
C. Cytosine
D. Adenine

Explanation:

• A. Thymine: DNA only.
• B. Uracil (Correct): Replaces thymine in RNA.
• C & D. Present in both DNA & RNA.

Q129.

Which are purine bases?
A. Adenine & Guanine ✅
B. Cytosine & Thymine
C. Uracil & Cytosine
D. Thymine & Uracil

Explanation:

• **A. Correct — two-ring structure.
• B, C, D. Pyrimidines (single-ring).

Q130.

Chargaff’s rule states:
A. A + G = T + C
B. A = T; G = C ✅
C. A = C; T = G
D. A + T = G + C

Explanation:

• **A. Wrong equality.
• **B. Correct — base pairing principle.
• C/D. Not true for double helix.

Q131.

The bond between nitrogenous base and sugar is:
A. Hydrogen bond
B. Glycosidic bond ✅
C. Peptide bond
D. Phosphodiester bond

Explanation:

• **A. Hydrogen bonds: between complementary bases.
• B. Glycosidic bond (Correct): Between sugar & base.
• **C. Peptide: proteins.
• **D. Phosphodiester: links nucleotides.

Q132.

The DNA backbone is made of:
A. Base pairs
B. Sugar-phosphate groups ✅
C. Amino acids
D. Fatty acids

Explanation:

• **A. Base pairs: in the middle of helix.
• **B. Correct — alternating sugar & phosphate form backbone.
• C/D. Not part of DNA backbone.

Q133.

The two DNA strands are held together by:
A. Covalent bonds
B. Hydrogen bonds ✅
C. Peptide bonds
D. Ionic bonds

Explanation:

• **A. Covalent: strong bonds in backbone.
• **B. Correct — H-bonds between bases stabilize double helix.
• **C. Peptide: proteins.
• **D. Ionic: not here.

Q134.

Which form of DNA is the most common under physiological conditions?
A. A-DNA
B. B-DNA ✅
C. Z-DNA
D. C-DNA

Explanation:

• A. A-DNA: Dehydrated form.
• B. B-DNA (Correct): Most common in cells.
• **C. Z-DNA: left-handed, rare.
• **D. C-DNA: not physiological.

Q135.

Number of base pairs per turn in B-DNA is:
A. 8
B. 10 ✅
C. 12
D. 14

Explanation:

• **A. 8: A-DNA.
• B. 10 (Correct): B-DNA standard → 10 bp per turn.
• C/D. Not typical values.

Q136.

Which RNA carries genetic code from DNA to ribosome?
A. tRNA
B. mRNA ✅
C. rRNA
D. snRNA

Explanation:

• **A. tRNA: brings amino acids.
• B. mRNA (Correct): Messenger RNA.
• **C. rRNA: structural/ribosomal.
• **D. snRNA: splicing.

Q137.

Which RNA carries amino acids to ribosomes during translation?
A. rRNA
B. tRNA ✅
C. mRNA
D. snRNA

Explanation:

• **A. rRNA: structural.
• B. tRNA (Correct): Transfer RNA delivers amino acids.
• **C. mRNA: coding info.
• **D. snRNA: splicing.

Q138.

Which RNA forms catalytic part of ribosome?
A. rRNA ✅
B. mRNA
C. tRNA
D. siRNA

Explanation:

• A. rRNA (Correct): Ribozymes catalyze peptide bond formation.
• **B. mRNA: coding info.
• **C. tRNA: transfer role.
• **D. siRNA: gene silencing.

Q139.

DNA replication is:
A. Conservative
B. Semiconservative ✅
C. Dispersive only
D. Random

Explanation:

• **A. Conservative: one new, one old intact. False.
• B. Semiconservative (Correct): Each new DNA has one old + one new strand.
• **C. Dispersive: fragments of old/new, not true model.
• **D. Random: incorrect.

Q140.

Which enzyme unwinds the DNA helix?
A. DNA ligase
B. DNA helicase ✅
C. DNA polymerase
D. Primase

Explanation:

• **A. Ligase: joins Okazaki fragments.
• B. Helicase (Correct): Unwinds double helix.
• **C. Polymerase: adds nucleotides.
• **D. Primase: RNA primers.

Q141.

Which enzyme joins Okazaki fragments?
A. DNA helicase
B. DNA ligase ✅
C. Primase
D. Gyrase

Explanation:

• **A. Helicase: unwinds.
• B. Ligase (Correct): Seals nicks.
• **C. Primase: RNA primers.
• **D. Gyrase: relieves supercoiling.

Q142.

Which nitrogenous base is absent in DNA?
A. Adenine
B. Thymine
C. Uracil ✅
D. Guanine

Explanation:

• A, B, D: All present in DNA.
• C. Uracil (Correct): Found only in RNA.

Q143.

In RNA, the sugar is:
A. Ribose ✅
B. Deoxyribose
C. Glucose
D. Xylose

Explanation:

• A. Ribose (Correct): Pentose sugar of RNA.
• **B. Deoxyribose: DNA.
• C/D. Not correct for RNA.

Q144.

Who proposed the double helix model of DNA?
A. Watson and Crick ✅
B. Franklin and Wilkins
C. Meselson and Stahl
D. Avery and Griffith

Explanation:

• A. Watson & Crick (Correct): Proposed in 1953.
• **B. Franklin & Wilkins: X-ray data.
• **C. Meselson & Stahl: replication experiment.
• **D. Avery/Griffith: transforming principle.

Q145.

Hydrogen bonds in A–T base pairs are:
A. 1
B. 2 ✅
C. 3
D. 4

Explanation:

• **A. 1: not correct.
• B. 2 (Correct): Adenine–Thymine forms 2 hydrogen bonds.
• **C. 3: G–C pair.
• **D. 4: none.

Q146.

Hydrogen bonds in G–C base pairs are:
A. 1
B. 2
C. 3 ✅
D. 4

Explanation:

• A/B: Too few.
• C. 3 (Correct): Guanine–Cytosine pairs have 3 H-bonds.
• **D. 4: none.

Q147.

Which nucleic acid acts as genetic material in some viruses?
A. Only DNA
B. Only RNA
C. Either DNA or RNA ✅
D. Both simultaneously

Explanation:

• A/B. Too narrow.
• **C. Correct — viruses may have DNA (adenoviruses) or RNA (retroviruses) as genetic material.
• **D. Both: not typical.

Q148.

Which base pairing is correct in RNA?
A. Adenine–Thymine
B. Adenine–Uracil ✅
C. Guanine–Thymine
D. Cytosine–Uracil

Explanation:

• **A. A–T: DNA only.
• B. A–U (Correct): RNA pairing.
• C/D. Incorrect.

Q149.

Which RNA has a cloverleaf structure?
A. mRNA
B. tRNA ✅
C. rRNA
D. hnRNA

Explanation:

• **A. mRNA: linear.
• B. tRNA (Correct): Cloverleaf secondary structure.
• **C. rRNA: complex, not cloverleaf.
• **D. hnRNA: precursor to mRNA.

Q150.

Which nitrogenous base is methylated in DNA for regulation?
A. Cytosine ✅
B. Adenine
C. Thymine
D. Uracil

Explanation:

• A. Cytosine (Correct): DNA methylation (at C5) regulates gene expression.
• **B. Adenine: rare methylation in eukaryotes.
• C/D. Not primary targets.

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