1. (3 pt) Assume you have a culture of E. coli cells growing in medium B, which
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Question
1. (3 pt) Assume you have a culture of E. coli cells growing in medium B, which contains both lactose and glucose. At time t, 33% of the cells are transferred to medium L, which contains lactose but not glucose, 33% are transferred to medium G, which contains glucose but not lactose; and the remaining cells are left in medium B. For each of the following statements, indicate with an ‘L' if it is true of the cells transferred to medium L, a 'G' if it is true of the cells transferred to medium G, a 'B' if it is true of the cells left in medium B, and an 'N' if it is true of none of the cells. In some cases, more than one letter may apply. Assume that no amount of sugar is transferred with the E. coli and that after the E. coli is transferred, none of the sugar(s) in the new culture are depleted over the course of the experiment. Answer A. B. C. D. E. F. The levels of b-galactosidase decreases after time t. The intracellular cAMP level is greater after time t than before. The lac operator site has lac repressor bound to it after time t. Most of the lac repressor proteins exist as repressor-glucose complexes after time t. The rate of transcription of the lac operon is greater after time t than before. The lac operon has both CRP and the lac repressor protein bound to it after time t.Explanation / Answer
Lac operon contains three genes for synthesis of three enzymes and most important enzyme for lactose metabolism is beta galactosidase. The lac operon is activated only when lactose is the only source of substrate and when glucose is present in the medium, lac operon is inactivated by lac repressor protein. The transcription of lac operon is enhanced by CRP (cAMP-bound catabolite activator protein - CAP, also known as the cAMP receptor protein).
Medium B : Contains both glucose and lactose.
Medium L : Contains lactose
Medium G : Contains glucose
A. Answer : G
The levels of beta galactosidase decreases after time t. Because, previously the E.Coli was in medium L and later when transferred to medium G, the level of galactosidase decreases as there is no lactose.
B. Answer : L
The intracellular cAMP level is greater after time t than before. Because, previously the E.Coli was in medium B and later when transferred to medium L, it needs more of beta galactosidase. Therefore, cAMP and CAP protein binds to lac operon to enhance the rate of transcription.
C. Answer : G
The lac operator site has lac repressor bound to it after time t. Because, previously the E.Coli was in medium L and later when transferred to medium G, there is no need for lac operon activation. Hence, it is repressed by repressor protein.
D. Answer : N
Lac repressor - glucose complex is not possible.
E. Answer : L
Because, previously the E.Coli was in medium B and later when transferred to medium L, it needs more of beta galactosidase. Therefore, cAMP and CAP protein binds to lac operon to enhance the rate of transcription.
F. Answer : N
Because, it is a possible case. Repressor and CRP cannot bind to the lac operon at the same time. Repressor binds when lactose is not present or when glucose is available. CRP binds when lactose is present in order to enhance transcription.
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