What does '$4$' represent in the given figure?

Why can glucose and galactose not act as inducers for the $lac$ operon?

Wild type $E. coli$ cells are growing in a normal medium with glucose. They are transferred to a medium containing only lactose as the sugar. Which one of the following changes takes place?

In $Escherichia \ coli$,$lac$ operon is induced by

In which of the following are monocistronic genes found?

$E. coli$ cells with a mutated $Z$ gene of the $lac$ operon cannot grow in a medium containing only lactose as the source of energy because: