Final answer:
Mutations in the I gene of the lac operon can lead to continuous expression of the lac genes since the repressor protein cannot bind to the operator sequence to inhibit transcription. This results in the continual production of enzymes for lactose metabolism, even in the absence of lactose as an inducer.
Step-by-step explanation:
Certain mutations in the regulator gene (I-) of the lac system result in maximal synthesis of β-galactosidase, permease, and transacetylase even in the absence of the inducer lactose. This happens because the I gene normally encodes a repressor protein that binds to the operator sequence to prevent transcription of the lac operon when lactose is not present. If the repressor protein is mutated such that it cannot bind to the operator, then the operon is constitutively expressed, meaning the genes are transcribed and the enzymes are produced regardless of the presence of lactose. This could potentially be due to a mutation in the repressor binding site or a change in the repressor protein structure preventing it from binding effectively to the operator sequence.
Experimental observations regarding different mutants within the lac operon have led to the understanding of how the system operates and what roles each gene plays in the metabolism of lactose. For example, one mutant may fail to make active β-galactosidase enzyme but still make permease, which indicates that the genes encoding these proteins are regulated separately and that the mutation affects only the structural gene for β-galactosidase. Variation among mutants helps researchers to dissect the operon's regulatory mechanisms and understand the impacts of different mutations.