lac operon in prokaryotes,structure, control regions .pptx

lac operon, concept,molecular biology

1. VIVEKANANDHA Ms.S.Anandhi Assistant Professor PG & Research Department of Biotechnology Vivekanandha Arts and Science College For Women Sankari Molecular Biology : The lac Operon ARTS & SCIENCE COLLEGE FOR WOMEN [An ISO 9001:2015 Certified Institution] (Affiliated to Periyar University, Salem Recognised Under Section 2(f) &12(B) of the UGC Act, 1956) Veerachipalayam , Sankari West (Post) – 637 303, Sankari Tk , Salem Dt., Tamil Nadu PG & RESEARCH DEPARTMENT OF BIOTECHNOLOGY

2. Condition Repressor Lactose cAMP Operon Status No lactose, + glucose Active Absent Low OFF + Lactose, + glucose Inactive Present Low Weak ON + Lactose, - glucose Inactive Present High Strong ON Summary Table

3. Working Mechanism Case 1: No lactose present (Operon OFF) The repressor from the lacI gene binds to the operator . This blocks RNA polymerase , so no transcription of lacZ , lacY , lacA occurs. Enzymes for lactose metabolism are not produced . ➡️ Energy is saved. Case 2: Lactose present (Operon ON) Lactose (or allolactose ) acts as an inducer . It binds to the repressor , changing its shape. The repressor cannot bind to the operator . RNA polymerase transcribes lacZ , lacY , lacA genes. Enzymes are produced → lactose is metabolized. ➡️ Lactose induces the operon. Case 3: Both Glucose and Lactose present The operon works weakly , because glucose represses via catabolite repression . High glucose → low cAMP → CAP (catabolite activator protein) inactive → weak transcription. When glucose is low → cAMP increases → CAP binds → enhances transcription .

4. 1. The Lac Operon is found in which organism? A. Saccharomyces cerevisiae B. E. coli C. Bacillus subtilis D. Streptococcus ✅ Answer: B. E. coli 2. Who proposed the Lac Operon model ? A. Watson and Crick B. Beadle and Tatum C. Jacob and Monod D. Griffith ✅ Answer: C. Jacob and Monod 3. The Lac Operon is an example of which type of gene regulation? A. Repressible operon B. Inducible operon C. Constitutive operon D. Catabolic operon ✅ Answer: B. Inducible operon

5. 4. The regulator gene in the Lac Operon is: A. lacZ B. lacY C. lacA D. lacI ✅ Answer: D. lacI 5. The enzyme β-galactosidase is coded by which gene? A. lacZ B. lacY C. lacA D. lacI ✅ Answer: A. lacZ 6. The operator region of the Lac Operon functions as: A. The site where RNA polymerase binds B. The site where repressor binds C. The site where ribosome binds D. The site of translation ✅ Answer: B. The site where repressor binds 7. In the absence of lactose, the Lac Operon is: A. Switched ON B. Switched OFF C. Constitutively active D. Repressed by CAP ✅ Answer: B. Switched OFF 8. Which molecule acts as an inducer in the Lac Operon? A. Lactose B. Allolactose C. Glucose D. Galactose ✅ Answer: B. Allolactose 9. What is the role of the lacY gene? A. Codes for β-galactosidase B. Codes for permease C. Codes for repressor D. Codes for RNA polymerase ✅ Answer: B. Codes for permease

6. 10. Which of the following inhibits the Lac Operon even when lactose is present? A. Low cAMP levels B. High cAMP levels C. Low glucose levels D. Presence of CAP protein ✅ Answer: A. Low cAMP levels 11. When both lactose and glucose are present, the operon is: A. Fully active B. Partially active C. Inactive D. Mutated ✅ Answer: B. Partially active 12. The CAP– cAMP complex helps in: A. Repression of operon B. Activation of transcription C. DNA replication D. Translation of enzymes ✅ Answer: B. Activation of transcription 13. The Lac Operon is a model for: A. Transcriptional control B. Translational control C. Post-translational control D. Genetic recombination ✅ Answer: A. Transcriptional control 14. The lacA gene codes for: A. β- galactosidase B. Transacetylase C. RNA polymerase D. Permease ✅ Answer: B. Transacetylase