Final answer:
The absence of the lac operon promoter, operator site, or lacA gene would lead to lack of gene expression, continuous gene expression regardless of lactose, or incomplete lactose metabolism, respectively.
Step-by-step explanation:
The lac operon is a set of genes involved in lactose metabolism in bacteria such as Escherichia coli and consists of three structural genes (lacZ, lacY, and lacA), an operator site, and a promoter. If the lac operon promoter were missing, RNA polymerase would be unable to bind and initiate transcription of the operon's genes, leading to no gene expression. In the absence of the operator site, the repressor would not be able to bind, resulting in continuous transcription of the operon regardless of the presence of lactose, which would be energetically wasteful. If the lacA gene was missing, the operon would still be able to express lacZ and lacY, but the cell would lack the full set of enzymes necessary for efficient lactose metabolism.
To summarize, a mutation or deletion in any part of the operon can significantly disrupt the normal function and regulation of the lac operon, affecting the cell's ability to utilize lactose as an energy source.