Question

9. Complete the table to describe the role of each regulatory region or molecule in the operation of the lac operon. What It Does Regulatory Region or Molecule Repressor protein Operator RNA polymerase Lactose

Answer 1

Repressor protein - Repressor protein otherwise called as lac repressor that represses or inhibits transcription of the lac operon.when lactose or substrate is not available ,repressor protein binds tightly to the operator preventing transcription by RNA polymerase.But,when lactose is present,the repressor protein loses its ability to bind DNA.The represser binds to the operator gene and prevent it from initiating the synthesis of the protein.The presence or absence of certain repressor molecules determines whether the operon is turn off or on.

Operator - Operator is the site where the repressor protein binds and blocks RNA polymerase from binding to the promoter and transcibing the operon.The operator is the negative regulatory site bound by the lac repressor protein,whereas promoter is the binding site of RNA polymerase,the enzyme that performs transcription.In the absence of lactose,the repressor protein encoded by the I gene binds to the lac operator and prevents transcription.Binding of allolactose to the repressor causes it to leave the operator.This enables RNA polymerase to transcribe the three genes of the operon.

RNA polymerase - RNA polymerase is an enzyme which synthesizes RNA by following a strand of DNA.RNAP is an classic example of how the basic function of this enzyme,ie their ability to recognize or bind promoters,melt the DNA and initiate RNA synthesis.This process is positively regulated by transcription activators such as cyclic AMP-receptor protein,CRP,and negatively regulated by lac repressor.

Lactose - When lactose is present,the lac genes are expressed because allolactose binds to the lac repressor protein and keeps it from binding to the lac operator.RNA polymerase can then bind to the promoter and transcribe the lac genes.The actual inducer in the lac operon is allolactose.If lactose is present in the medium,then small amount of lactose will be converted to b-galactosidase that are present in the cell.Lac operon genes will only be expressed when lactose is present,Two regulators turn the operon on and off in response to the lactose and glucose levels.When lactose is present,the lac protein does not bind to the operator site and transcription can occur.