Question

Predict the level of genetic activity of the lac operon as well as the status of the lac repressor and the CAP protein, and RNA polymerase, under the cellular conditions listed in the table

	Lactose	Glucose
(a)	-	-
(b)	+	-
(c)	-	+
(d)	+	+

Answer 1

Lac repressor bind to the operator region of operon and prevents RNA polymerase from transcribing the operon.

RNA polymerase binds to the promoter region and transcribes the downstream genes.

CAP is catabolic activator protein. It forms complexes with cyclic AMP, called as CAP cAMP complex. Whenever the concentration of glucose is high in the medium, adenyl cyclase enzyme is inhibited and the levels of cyclic AMP are low in the medium. In this case there is no formation of CAP cAMP complex and therefore no expression of lac operon. The CAP cAMP complex is involved in the recruitment of RNA polymerase to the promoter.

A. The cell will die in the absence of sugar because sugar the source of energy by which cell can generate ATP.

	Expression of operon	Lac repressor	CAP
B	the operon will be expressed	the repressor will be degraded by lactose and this will make operator free of repressor	it will form complexes with cyclic AMP and binds to CAP binding site and help in the recruitment of RNA polymerase to the promoter
C	operon will not be expressed	there is no lactose molecule to bind to repressor and therefore repressor will be associated with operator	levels of cyclic AMP will be less and there will be no formation of CAP cAMP complex
D	SAME as C

D. When both glucose and lactose will be present in the medium, the cell will use glucose as the source of energy. It is because it will not waste its energy in switching on the lac operon.

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