Part 7: Gene Expression & Regulation – Lac Operon (25 MCQs)
Part 7: Gene Expression & Regulation – Lac Operon (25 MCQs)
Q151. The lac operon was discovered by:
a) Watson and Crick
b) Jacob and Monod
c) Meselson and Stahl
d) Hershey and Chase
Answer: b
- b) Correct: Jacob and Monod proposed lac operon model in E. coli.
Q152. The lac operon is found in:
a) Eukaryotes
b) Prokaryotes (E. coli)
c) Viruses
d) Plants
Answer: b
- b) Correct: Operons are prokaryotic gene regulation units, not in eukaryotes.
Q153. The lac operon consists of:
a) Structural genes only
b) Structural genes, promoter, operator, regulator
c) Promoter only
d) Operator and enhancer
Answer: b
- b) Correct: Complete operon = regulator gene, promoter, operator, structural genes.
Q154. Which is NOT a structural gene of lac operon?
a) lacZ
b) lacY
c) lacA
d) lacR
Answer: d
- d) Correct: lacR = regulator gene, not structural.
Q155. The lacZ gene codes for:
a) Permease
b) β-galactosidase
c) Transacetylase
d) Repressor
Answer: b
- b) Correct: lacZ → β-galactosidase, breaks lactose into glucose + galactose.
Q156. The lacY gene codes for:
a) Permease
b) β-galactosidase
c) Transacetylase
d) RNA polymerase
Answer: a
- a) Correct: lacY → permease, allows lactose entry into cell.
Q157. The lacA gene codes for:
a) Permease
b) Transacetylase
c) β-galactosidase
d) Regulator protein
Answer: b
- b) Correct: lacA → transacetylase (minor role in detoxification).
Q158. The regulator gene (lacI) codes for:
a) Repressor protein
b) RNA polymerase
c) Promoter
d) β-galactosidase
Answer: a
- a) Correct: lacI → repressor protein that binds operator.
Q159. In the absence of lactose, lac operon is:
a) Induced
b) Repressed
c) Overexpressed
d) Non-functional
Answer: b
- b) Correct: No lactose → repressor binds operator → operon OFF.
Q160. In the presence of lactose, lac operon is:
a) Repressed
b) Induced
c) Inactive
d) Degraded
Answer: b
- b) Correct: Lactose (allolactose form) binds repressor → operon ON.
Q161. Lactose acts as:
a) Substrate only
b) Inducer
c) Both substrate and inducer
d) Repressor
Answer: c
- c) Correct: Lactose = substrate, its derivative allolactose = inducer.
Q162. The operator site is the binding site for:
a) RNA polymerase
b) Repressor protein
c) Lactose
d) Activator protein
Answer: b
- b) Correct: Repressor binds operator, blocking transcription.
Q163. The promoter site is the binding site for:
a) Repressor protein
b) RNA polymerase
c) Lactose
d) Allolactose
Answer: b
- b) Correct: Promoter = site for RNA polymerase binding.
Q164. In the lac operon, which molecule acts as inducer?
a) Glucose
b) Lactose (allolactose form)
c) Galactose
d) RNA polymerase
Answer: b
- b) Correct: Allolactose (isomer of lactose) = inducer.
Q165. When glucose is present, lac operon is:
a) Fully induced
b) Repressed due to catabolite repression
c) Activated
d) Independent of glucose
Answer: b
- b) Correct: Glucose suppresses lac operon (catabolite repression).
Q166. Catabolite activator protein (CAP) functions with:
a) Lactose
b) RNA polymerase
c) cAMP
d) Repressor
Answer: c
- c) Correct: CAP binds with cAMP, then binds promoter to enhance transcription.
Q167. When glucose level is high, cAMP level is:
a) High
b) Low
c) Constant
d) Independent
Answer: b
- b) Correct: High glucose → low cAMP → no CAP binding.
Q168. When glucose is absent and lactose is present, lac operon shows:
a) No expression
b) Maximum expression
c) Partial expression
d) Repression
Answer: b
- b) Correct: Best condition = low glucose + lactose present → operon ON at high level.
Q169. Which of the following is an inducible operon?
a) Lac operon
b) Trp operon
c) Both lac and trp
d) None
Answer: a
- a) Correct: Lac operon = inducible, Trp operon = repressible.
Q170. In absence of tryptophan, the trp operon is:
a) Repressed
b) Active
c) Destroyed
d) Induced by lactose
Answer: b
- b) Correct: No tryptophan → no repressor binding → trp operon ON.
Q171. Lac operon is switched on when:
a) Lactose absent, glucose absent
b) Lactose present, glucose present
c) Lactose present, glucose absent
d) Lactose absent, glucose present
Answer: c
- c) Correct: Best induction when lactose present, glucose absent.
Q172. The polycistronic mRNA of lac operon contains:
a) One gene
b) Two genes
c) Three structural genes (lacZ, lacY, lacA)
d) No genes
Answer: c
- c) Correct: Polycistronic mRNA codes for Z, Y, A proteins.
Q173. In positive control of lac operon, the regulatory protein is:
a) Repressor
b) CAP–cAMP complex
c) RNA polymerase
d) DNA ligase
Answer: b
- b) Correct: CAP–cAMP binds promoter → enhances RNA polymerase binding.
Q174. What happens when both glucose and lactose are absent?
a) Operon fully induced
b) Operon repressed (no substrate)
c) Operon partially active
d) RNA degraded
Answer: b
- b) Correct: No lactose = no substrate → operon OFF.
Q175. The lac operon is an example of:
a) Inducible operon with both negative and positive regulation
b) Repressible operon
c) Constitutive operon
d) Epigenetic regulation
Answer: a
- a) Correct: Lac operon is inducible (ON in presence of lactose) and under both negative (repressor) & positive (CAP–cAMP) control.
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