(a) Describe how the lac operon operates both in the presence and absence of an inducer in E. coli.

Presence of Inducer `

-Repressor protein is synthesised from the i gene

-Repressor protein is inactivated by interaction with the inducer molecule ie lactose molecule and thus it does not bind to operator.

-Operon consist of three structural genes z,y and a

-RNA polymerase binds to promoter and initiates transcription ,

the genes (z, y, a) thus helping in metabolism of lactose.

Absence of Inducer

The regulatory gene i of lac operon produces a repressor protein and this protein binds to operator region of operon, and prevents RNA polymerase from transcribing the structural genes (z, y, a) not transcribed and thus enzymes are not produced.

• Phenylketonuria

• Cause –Mutation in the gene that codes for the enzymes phenyl alanine hydroxylase (single gene mutations )

• One symptoms -Mental retardation, Hair loss, Skin pigmentation

• Harmful Compounds Formed

Phenylpyruvic acid and other derivatives.