Question

How does lactose affect the functioning of the lac operon?

Answer 1

Hint: The lac operon is an operon with a single promoter, or group of genes (transcribed as a single mRNA). The genes in the operon encode proteins that allow lactose to be used as an energy source by bacteria.

Complete answer:
Lactose can be broken down by E. coli bacteria, but it's not their favorite fuel. If there is glucose around, they'd much prefer to use it. Glucose takes less steps and less energy to decompose than lactose. However, if lactose is the only available sugar, the E. Coli would go straight ahead, using it as a source of energy. In order to use lactose, bacteria have to express the lac operon genes, which encode important enzymes for lactose uptake and metabolism.
To be as successful as practicable, E. Lac operon coli can only be expressed when two conditions are met:
There is lactose at your disposal, and
Not required for glucose
By shutting off the repressor, lactose permits the transcription of the genes in the lac operon. A repressor binds to the operator site of the lac-operon in the absence of lactose. Consequently, the RNA-polymerase is unable to bind and lac gene transcription does not take place. This is called control of the negative.
An isomer called 'allolactose' is formed when lactose is present in the cell. Allolactose binds to the repressor, triggering an alteration of the conformation. As a consequence, and will be released, the repressor can no longer bind to the operator field. RNA-polymerase will now bind the lac genes and transcribe them. This makes sense since the genes code for enzymes metabolizing (allo) lactose in the lac operon. Only when lactose is present are these enzymes required. Eventually, the breakdown of (allo) lactose can lead to the release of the repressor that prevents the lac enzymes from synthesizing.

Note:
In prokaryotes, the absence of a nuclear membrane gives ribosomes direct access to mRNA transcripts, enabling their immediate translation into polypeptides. This makes transcription the rate-limiting step in the expression of prokaryotic genes and, hence, a major regulation stage. The classic example of controlling prokaryotic genes is that of the lac operon.