Question

QUESTION 18

Briefly describe why E. coli wants to express different amounts of the lac operon genes in relation to the presence/absence of glucose and lactose and the molecular mechanism by which it does so for each of the four conditions with respect to glucose and lactose.

Answer 1

Ans. In presence of glucose there will not be expression of structural genes of lac operon i.e.  lacZ, lacY and lacA genes because the enzymes produced by these genes i.e. beta-galactosidase, galactoside permease and transacetylase respectively are required for lactose metabolism. When glucose is available there is no need for metabolism of lactose as source of energy because glucose is the preferred source of energy for bacteria. When glucose is absent there will be expression of these lacZ, lacY and lacA genes to metabolize lactose to use as source of energy. When only lactose is available in the medium i.e. glucose is absent, there will be expression of lacZ, lacY and lacZ to metabolise lactose and use as source of energy. Beta-galactosidase is responsible for breaking down of lactose into glucose and lactose. Galactoside permease is responsible for taking up of lactose inside the cell. Therefore, these genes will express when lactose is available as the only source of energy in the absence of glucose. When lactose is absent these genes will not express becasue these enzymes are required for lactose metabolism and hence in the absence of lactose these genes do not express.