The lac operon:

The lac operon: Lec operon is a transcriptionally regulated system. A polycistronic structural gene is regulated by promoter and a regulatory gene.It is common in bacteria.Here “lac” stands for lactose,which act as an inducer. Lactose act as an inducer of operon and therefore,  controls the switching off and on of lac operon.

1) In the absence of inducer, repressor binds with the operator region and prevents  the RNA polymerase from transcribing the lac operon.

2) In presence of lactose, lactose binds with repressor and makes it inactive. Now repressor cannot binds with operator region and RNA polymerase can transcribe the operon.

3) When lactose become minimum, then it cannot prevent repressor to enter the operator region and repressor. Therefore, repressor switch off lac operon.

4) This control of operon by repressor is called negative feedback. For example; lac operon.Some examples of positive feedback are; His, Wal, Ara, etc.