Question

The inducer for switching 'on' the lac operon in bacteria is

• A. Presence of lactose
• B. Number of bacteria
• C. Presence of structural genes in the bacteria
• D. Presence of sucrose
• E. Presence of RNA polymerase

Answer 1

Answer: (A)

Presence of lactose

The lac operon is an operon responsible for transport and metabolism of lactose in Escherichia coli and other bacteria, and is not switched on in the presence of sucrose. The gene that codes for any RNA / protein product which does not serve as regulatory protein is known as structural genes. The lac z, lac y and lac genes in lac operon are the structural genes; they are always present as part of an operon. RNA polymerase is an enzyme of transcription whose binding to the promoter is inhibited by repressor; the enzyme can not switch on the operon. In the absence of lactose sugar, the repressor protein binds to the operator and thereby does not allow the transcription of three lac genes. In the presence of lactose, allolactose; an isomer of lactose, binds to the repressor and causes an allosteric shift. The altered repressor can not bind to the operator, allowing RNA polymerase to transcribe the lac genes. The correct answer is A.