Free Cell Structure and Function MCQs with Answers

• A. 4
• B. 16
• C. 8
• D. 64

Explanation: Each codon = 3 bases. With 2 possible nucleotides (A and C): 2³ = 8 codons.

• A. DNA proof‑reading
• B. DNA repair
• C. Unwinding
• D. Annealing

Explanation: DNA polymerase I has 3′ → 5′ exonuclease activity allowing proofreading and correction of errors in the new strand.

• A. Calcium chloride
• B. Cesium chloride
• C. Calcium carbide
• D. Silicon carbide

Explanation: Meselson and Stahl used density gradient centrifugation in CsCl with isotopes of nitrogen (¹⁵N and ¹⁴N) to prove semi conservative DNA…

• A. Addition of primer
• B. Unwinding
• C. Polymerization
• D. Rewinding

Explanation: Helicase unwinds the double helix, separating the strands to allow replication.

• A. Adenine and guanine
• B. Cytosine and uracil
• C. Cytosine and guanine
• D. All of the above

Explanation: The genetic code includes all 4 bases (A, U/T, G, C) in different triplet combinations to form codons.

• A. Serine and isoleucine
• B. Cysteine and valine
• C. Tyrosine and tryptophan
• D. Phenylalanine and methionine

Explanation: Har Gobind Khorana used synthetic RNA to decipher codons. He first identified codons for cysteine (UGU, UGC) and valine (GUU, GUC, GUA…

• A. DNA polymerase
• B. Topoisomerase
• C. Unwindase/helicase
• D. Gyrase

Explanation: Helicase separates the two DNA strands by breaking hydrogen bonds.

• A. tRNA
• B. mRNA
• C. rRNA
• D. both (B) and (C)

Explanation: Anticodons are present on tRNA and are complementary to codons on mRNA during translation.

• A. GUAGCUUA
• B. AUUCGAUG
• C. CAUCGAAU
• D. UAAGCUAC

Explanation: In transcription, complementary mRNA is formed. A → U, T → A, C → G, G → C. Hence, ATTCGATG → UAAGCUAC.

• A. Peptidyl transferase reaction
• B. Aminoacyl tRNA binding to A­-site
• C. Translocation
• D. Amino acid activation

Explanation: ATP/GTP is consumed during amino acid activation, tRNA binding, and translocation.

• A. AGT, TAG, UGA
• B. UAG, UGA, UAA
• C. TAG, TAA, TGA
• D. GAT, AAT, AGT

Explanation: The genetic code contains three stop codons: UAG, UAA, and UGA. These terminate translation and are called nonsense codons.

• A. AT­-GC
• B. AG­-CT
• C. AC­-GT
• D. All of these

Explanation: Base pairing in DNA follows Chargaff's rule: A pairs with T and G pairs with C.

• A. Inhibitor
• B. Repressor
• C. Regulator
• D. All of these

Explanation: In the operon model, the regulator gene codes for a (repressor) protein that controls the expression of the operon.

• A. Dispersive
• B. Conservative
• C. Semi­-conservative
• D. Non-conservative

Explanation: Each daughter DNA molecule has one parental and one newly synthesized strand (Meselson-Stahl experiment).

• A. 5' ➡ 3'
• B. 3' ➡ 5'
• C. Both (A) and (B)
• D. Any

Explanation: RNA polymerase always synthesizes RNA in the 5′ → 3′ direction, using the template strand in the 3′ → 5′ direction.

• A. Single form
• B. Tetrad form
• C. Pair form
• D. Identical form

Explanation: Diploid organisms carry chromosomes in homologous pairs, one from each parent, carrying same genes but possibly different alleles.

• A. 10 nucleotides/ sec
• B. 1000 nucleotides/ sec
• C. 100 nucleotides/ sec
• D. 10000 nucleotides/ sec

Explanation: DNA polymerase III adds nucleotides rapidly (~1000/sec) during prokaryotic replication.

• A. DNA ligase
• B. DNA topoisomerase I
• C. DNA gyrase
• D. Helicase

Explanation: Helicase unwinds the DNA double helix by breaking hydrogen bonds, providing single-stranded templates.

• A. Multi-­helix stage
• B. Single­-helix stage
• C. Double ­-helix stage
• D. None of these

Explanation: X-ray crystallography and radiotracers confirmed DNA's double-helical structure held by complementary base pairing.

• A. 5­-bromouracil
• B. 5­-methylcytosine
• C. Guanine
• D. Adenine

Explanation: Spontaneous deamination of 5-methylcytosine creates thymine (C→ T), making CpG sites mutation "hot spots."