Question

Describe the structure and function of Lac Operon.

Answer 1

Hint: A functional unit of DNA containing a cluster of genes regulated by a single promoter is known as an operon. The genes are transcribed as a single strand of mRNA and either translated together in the cytoplasm or spliced into monocistronic mRNAs that are translated separately, i.e. multiple strands of mRNA that each encode a single gene element.

Complete answer:
Lactose metabolism genes are found in the lac operon of E. coli. When lactose is present but glucose is not, it is expressed. The lac repressor and catabolite activator protein are two regulators that transform the operon "on" and "off" in response to lactose and glucose levels (CAP). Lactose is sensed by the lac repressor. When lactose is present, it stops acting as a repressor and prevents transcription of the operon. Lactose is sensed indirectly by the lac repressor through its isomer allolactose. CAP, or catabolite activator protein, is a glucose sensor. When glucose levels are low, it stimulates transcription of the operon. Via the "hunger signal" molecule cAMP, CAP detects glucose indirectly. This type of growth is known as diauxic growth.
One regulatory gene (the ‘I’ gene - the term I come from the word inhibitor) and three structural genes make up the lac operon (‘z’, ‘y’, and ‘a’).
Functions include:-
The lac operon repressor is encoded by the ‘I’ gene.
The ‘z’ gene codes for beta-galactosidase (-gal), a hydrolase enzyme that breaks down the disaccharide lactose into its monomeric groups, galactose, and glucose.
The ‘y’ gene produces permease, which increases the cell's permeability to p-galactosidase.
A transacetylase is encoded by the ‘a’ gene.

Note:
The mechanism was elucidated by Francois Jacob and Jacque Monad. In absence of lactose, the repressor protein binds the operator and does not allow the transcription of the lac genes.