All Free Biology MCQs with Answers
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19844 questions · page 720 of 1985
7191. In DNA, when AGCT occurs, their association is as per which of the following pair?
- 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.
Correct answer: AT-GC7192. In the operon concept, the regulator gene functions as a/an:
- 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. The repressor can be considered an inhibitor because it inhibits transcription when it binds to the operator. Thus, the regulator gene functions as a repressor (and effectively an inhibitor) .
Correct answer: All of these7193. The method of DNA replication in which two strands of DNA separate and synthesize new strands is called:
- 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).
Correct answer: Semi-conservative7194. mRNA is synthesised on the DNA template in which direction?
- 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.
Correct answer: 5' ➡ 3'7195. In a diploid individual, homologous chromosomes are present in:
- 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.
Correct answer: Pair form7196. Rate of synthesis by DNA polymerase III is:
- 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.
Correct answer: 1000 nucleotides/ sec7197. Proteins that help to open up the DNA double helix in front of the replication fork are:
- 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.
Correct answer: Helicase7198. Radiotracer technique shows that DNA is in:
- 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.
Correct answer: Double -helix stage7199. What base is responsible for hot spots of spontaneous point mutations?
- 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."
Correct answer: 5-methylcytosine7200. Which of the following reunites the exon segments after RNA splicing?
- A. RNA polymerase
- B. RNA primase
- C. RNA ligase
- D. RNA protease
Explanation: RNA ligase seals exons after introns are removed, producing a continuous coding sequence.
Correct answer: RNA ligase