Arrange the given steps in correct sequence of their occurrence when lactose is provided in the growth medium of bacteria:
(A) Operator got free, RNA polymerase binds to promoter
(B) Lactose is hydrolysed to glucose and galactose
(C) Lactose binds to repressor protein
(D) Gene z, y, a are transcribed
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C), (A), (D), (B)
Lactose (the inducer) binds the repressor (C), which frees the operator, and RNA polymerase binds the promoter (A); genes z, y and a are transcribed (D); β-galactosidase then hydrolyses lactose to glucose and galactose (B).
NCERT Extract:
The Lac operon – “… The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon. In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds (Figure 5.14).”
The Lac operon – “… The z gene codes for beta-galactosidase (β-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose.”