All Free Biology MCQs with Answers
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19844 questions · page 719 of 1985
7181. What is the primary role of DNA polymerase I during replication?
- 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.
Correct answer: DNA proof‑reading7182. The medium used by Meselson & Stahl for confirmation of the semi-conservative method was:
- 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 replication.
Correct answer: Cesium chloride7183. First step in DNA replication is:
- A. Addition of primer
- B. Unwinding
- C. Polymerization
- D. Rewinding
Explanation: Helicase unwinds the double helix, separating the strands to allow replication.
Correct answer: Unwinding7184. Genetic code consists of:
- 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.
Correct answer: All of the above7185. Khorana first deciphered the triplet codons of:
- 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, GUG).
Correct answer: Cysteine and valine7186. During DNA replication, the strands separate by:
- A. DNA polymerase
- B. Topoisomerase
- C. Unwindase/helicase
- D. Gyrase
Explanation: Helicase separates the two DNA strands by breaking hydrogen bonds.
Correct answer: Unwindase/helicase7187. An anticodon is an unpaired triplet of bases in an exposed position of :
- 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.
Correct answer: tRNA7188. If the sequence of bases in DNA is ATTCGATG, then the sequence of bases in its transcript will be:
- 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.
Correct answer: UAAGCUAC7189. Which of the following steps of translation does not consume a high energy phosphate bond?
- 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. Peptide bond formation by peptidyl transferase does not require energy.
Correct answer: Peptidyl transferase reaction7190. The codons causing chain termination are :
- 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.
Correct answer: UAG, UGA, UAA