Question

For this assignment you will complete an illustration(s) (by hand) of how the lac operon works. You should be sure to include details in your drawing such as the names of relevant structural genes (lac Z and lac Y) and controlling sequences (operator, promoter, repressor).

1. Show the influence of lactose (in the form of allolactose) on the repressor protein.

2. Show what effect the primary structural genes have on lactose.

3. Show how glucose levels can influence the expression of the lac operon (through the CRP-cAMP complex, etc...).

4. How do cAMP levels influence the expression of the lac operon and what is the mechanism?

5. Your illustrations should be detailed and make it clear how the lac operon works. Feel free to include multiple drawings and written explanations with arrows drawn to make your illustration clear. Also, creativity will be appreciated.

Answer 1

Lac operon

Step-by-step explanation:

Lac operon present in prokaryotic chromosomes whose product involves in transportation and catabolism of lactose

• Lac operon consists of three structural genes called Lac Z,Lac Y and Lac A and a regulatory element(promoter and operator)
• Lac I gene is present outside the Lac operon,act as regulatory gene and control the expression of Lac operon genes
• Lac Z codes for β galactosidase which converts lactose into allolactose
• Lac Y codes for Lac permease which act as a membrane transporter and transport lactose into the cells
• Lac A codes for transacetylase which neutralize the toxic effects of lactose

Lac operon in absence of lactose

• In absence of lactose Lac operon exists in switch off state
• Lac I gene produces Lac repressor which binds to operator region and prevent binding of RNA Polymerase thus blocks transcription

Lac operon in presence of lactose

• In presence of lactose Lac operon exists in switch on state
• In presence of lactose few molecules of Lac operon enzyme catalyse conversion of lactose to allolactose
• Allolactose act as an inducer,binds to the Lac repressor and induce conformational changes causing dissociation of Lac repressor from operator
• In absence of Lac repressor RNA Polymerase binds to the promoter and starts transcription of genes which catabolize lactose

Lac promoter is a weak promoter and direct transcription of Lac operon genes in very low level in presence of lactose

• For higher level expression of Lac operon genes,operon system require conversion of weak promoter to strong promoter which is always mediated by catabolite protein-cAMP complex(CAP-cAMP complex)
• CAP cAMP complex binds to the upstream of promoter called CAP binding site and stimulate expression of Lac operon genes by facilitating binding of RNA Polymerase
• Catabolite activator protein(CAP) or cAMP receptor protein(CRP) activate when cAMP binds
• cAMP is a secondary messenger synthesized from ATP and act as co-activator of CRP
• With respect to CAP cAMP complex Lac operon system is positively controlled
• In prokaryotic cell cAMP is very low when glucose concentration is relatively high and vice versa
• At high concentration of glucose the growth rate is maximum and lactose catabolism is repressed called catabolite repression