Question

1. In the presence of glucose the lac operon is:

	1.	On
	2.	Off
	3.	Could be either on or off

2.In the presence of lactose the lac operon is:

	a.	On
	b.	Off
	c.	Could be either on or off

3. How do tetrahymena feed?

	a.	Flagellar action
	b.	Phagocytosis
	c.	Absorption
	d.	Exocytosis

Answer 1

1st Answer)

Let's see what happens when the glucose is absent and present in the medium and how it decide the fate of the operon :

case 1 in the absence of glucose :

1. In the absence of glucose cytoplasmic cyclic AMP concentration will rise in a cell.

2.cAMP or cyclic AMP binds to Catabolite activator protein (CAP).

3. The combined cAMP-CAP complex sits on lac operon promoter. This, in turn, regulates the active transcription of genes under lac operon.

case 2 in the presence of glucose :

1. In the presence of glucose cytoplasmic cyclic AMP concentration will fall.

2. Further cAMP or cyclic AMP cannot bind to Catabolite activator protein (CAP).

3. The combined cAMP-CAP complex cannot sit on lac operon promoter. This, in turn, switches off transcription of the lac operon.

Therefore option 2 is the correct answer.

2nd answer)

Even here there are two cases:

case 1 in presence of lactose :

1. In the presence of lactose in the medium, it binds to a repressor of the lac operon.

2. This repressor usually places itself on the operator region of the lac operon and does not allow the transcription of lac genes.

3. In the presence of lactose, lactose binds to the repressor. This binding releases repressor from the operator of the lac operon.

4. This, in turn, leads to active transcription of genes under the operon.

case 2 in absence of lactose :

1. Repressor will be free to bind on to operator of the lac operon, thereby repressing the transcription of genes under the operon. In turn, it leads to shutting down of the operon.

Therefore option a is correct in the presence of lactose lac operon will be on.

3rd answer)

Tetrahymena phagocytoses the particles with the help of the cell membrane. Phagocytosed particles will one visible as food particles under the microscope.

Therefore option B is the correct answer.