Ncert Exercises & Solutions

Q. 28. In E.coli, the lac operon gets switched on when

(a) lactose is present and it binds to the repressor

(b) repressor binds to operator

(d) Lactose is present and it binds to RNA polymerase

(a)

In case of lactose presence

(i) Lactose acts as an inducer which binds to the repressor and formes an inactive repressor.

(ii) The repressor fails to bind to the operator region.

(iii) The RNA polymerase binds to the operator and transcript lac mRNA

(iv) lac mRNA is polycistronic, i.e., produces all three enzymes, $β$ -galactosidase, permease and transacetylase.

(v) The lac operon is switched on.

In the case of lactose absence

(i) When lactose is absent, i gene regulates and produces repressor mRNA which translates repression.

(ii) The repressor protein binds to the operator region of the operon and as a result prevents RNA polymerase to bind to the operon.

(iii) The operon is switched off.