When lactose is provided in the growth medium of E. coli in the absence of glucose, the lac operon is switched on. Arrange the following events in the correct sequence:
A. Lactose is transported into the cell through the action of permease.
B. The repressor is inactivated by interaction with the inducer.
C. RNA polymerase gets access to the promoter and transcription of the operon proceeds.
D. The lac mRNA is translated into beta-galactosidase, permease and transacetylase.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B, C, D
Lactose is the inducer of the lac operon. When glucose, the preferred carbon source, is absent and lactose is supplied, the operon is switched on step by step:
A. Lactose is transported into the cell through the action of permease. A very low level of the lac operon is always expressed, so a little permease is present to let lactose in.
B. Inside the cell, lactose acts as the inducer and inactivates the repressor by interacting with it.
C. With the repressor no longer on the operator, RNA polymerase gets access to the promoter and transcribes the operon into lac mRNA.
D. The lac mRNA is translated into beta-galactosidase, permease and transacetylase.
The correct sequence is A, B, C, D. Lactose cannot inactivate the repressor before it enters the cell, and the enzymes cannot be made before the mRNA is transcribed.
NCERT Extract:
The Lac Operon – “… In the absence of a preferred carbon source such as glucose, if lactose is provided in the growth medium of the bacteria, the lactose is transported into the cells through the action of permease (Remember, a very low level of expression of lac operon has to be present in the cell all the time, otherwise lactose cannot enter the cells). The lactose then induces the operon in the following manner. … 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).”
Figure 5.14 The lac Operon