Uppose that you are studying an inducible operon that contains two genes required for the metabolism of compound xyz. gene a encodes enzyme a, and gene b encodes enzyme b. when compound xyz is absent, the operon is repressed, but when compound xyz is present, the operon is induced.

The exercise tells you to complete the table showing different cases of mutation in different parts of the operon.

First, what is an operon?
An operon is a genetic unit of coordinated expression. It comprises a group of adjacent structural genes encoding the enzymes of the same metabolic pathway, as well as the regulatory sequences affecting the transcription of these structural genes (like operon lactose or tryptophan). It is composed by:Structural genes/cistrons: code for the enzymes participating in the same metabolic pathway, they are co-transcribed into a single polycistronic mRNA.• Promoter common to all structural genes: this is the binding site of RNA polymerase.• The operator: it is a regulatory sequence, it is the site of binding of the protein of the regulation, it controls the transcription of the gene downstream.• The regulatory gene: located upstream of the operon promoter, it encodes regulatory proteins capable of binding on the operator. He owns his own promoter.

*If a mutation touch one of the genes that the operon code for, this gene will be not functional
*If a mutation touches the operator none of the enzymes will be synthesized
*If a mutation touches a repressor (a molecule which inhibits the expression of the operon genes by binding specifically to the DNA at the operator level).

Based on this, the right answers are:
Mutant 1 ==> gene A
Mutant 2 ==> repressor
Mutant 3 ==> gene B
Mutant 4 ==> promoter