Question

Difference between inducible and repressible operon

Answer 1

• ✏When E.coli cultured in a medium devoid of lactose the enzymes for lactose breakdown are not required.

• ✏So the three structural genes are not expressed.

• ✏In such a case the lac operon remains switched off or repressed.

• ✏Under such a condition the repressor protein coded by regulator gene (i gene) is produced.

• ✏The regulator gene is a gene which suppress the activity of structural genes.

• ✏The repressor protein bind to the operator and blocks RNA polymerase movement.

• ✏The operator gene thus directly control the structural gene as a result structural genes do not synthesis the mRNA and thus the enzymes are not synthesised this is called repression.

• ✏When E.coli is cultured in medium containing lactose, E.coli needs enzymes for the breakdown of lactose, so the lac operon is switched on or derepressed.

• ✏Repressor cannot bind to the operator gene.

• ✏Therefore the operator gene is free it induces the RNA polymerase to bind with promoter gene.