Question

$Escherichia$ $coli$ growing on a medium having glucose is transferred to a lactose-containing medium. Which change occurs?

• A. $lac$ operon is induced
• B. $lac$ operon is suppressed
• C. All operons are induced
• D. The bacterium stops dividing

Answer 1

The correct option is A $lac$ operon is induced

Operon
An operon is a coordinated group of genes that function together to regulate gene expression in prokaryotes. Here, a polycistronic structural gene is regulated by a common promoter and regulatory genes. The lac operon is responsible for the transport and metabolism of lactose in bacteria like $E$. $coli$.
$lac$ operon
The $lac$ operon is an inducible operon that remains usually in a repressed condition and gets activated only when the substance to be metabolised (lactose) is present. This is because the inducer binds to the repressor protein and prevents it from binding to the operator region of DNA. This allows the binding of RNA polymerase to the promoter properly and the transcription of the downstream structural genes.
Hence, transfer of $E$. $coli$ from a medium containing glucose to that containing lactose results in activation of $lac$ operon.
Final answer
(A) lac operon is induced