Answer the following \"what if\" questions about the lac operon. Your answers ar
ID: 87899 • Letter: A
Question
Answer the following "what if" questions about the lac operon. Your answers are +/- "essentially no transcription" (tiny amounts of transcription like a leaking faucet), ++ "basal transcription" (which might be described some transcription), ++++ "activated transcription" (a level of transcription expected for a normal E. coli promoter). (I = lac repressor gene: Z, Y, .A = lac operon structural genes (cistrons): P = lac promoter: O = lac operator) A) As above, describe the transcription level of the lac operon if E. coli cells were grown in the presence of the following sugars: (a) glucose only, (b) glucose and lactose, (c) lactose only. B) Same question, but now the lacI gene has been deleted from the operon. C) Same question, but the lacl gene has been mutated so that it no longer binds allolactose. D) same question, but the lacZ cistron has been deleted from the polycistron lacZYA, so only lacYA remains to be transcribed. E) Same question, but now the CAP gene has been mutated, and now the CAP protein acts like cAMP is present even when it is not. F) Same question, but now the operator (lacO) has been deleted.Explanation / Answer
Ans1. (a) glucose only +/- essentially no transcription. Lac operon genes are expressed at high level only when lactose is available and glucose –the preferred energy source is not.
(b) ++ basal transcription. If glucose is present, the lac operon is repressed even in the presence of lactose. This repression is maintained until the glucose supply is exhausted.
(C)++++ Activated level of transcription will take place in the presence of lactose. This is because Lac repressor can bind DNA and repress transcription only in the absence of glucose. In the presence of lactose, repressor is inactive and gene is expressed.
Ans 2. (a) +/- i.e. essentially no transcription. Lac I gene mediates the lactose signal. When Lac I gene has been mutated and bacteria is grown in the presence of glucose only, no transcription take place.
(b) ++++ i.e. activated transcription in the presence of both glucose & lactose. Mutations can occur in the repressor genes such that the protein loses affinity for the operator, and causes the continual production of the inducible enzymes regardless of if lactose was present or not. Therefore, the lac operon becomes constitutive.
(c) ++++ i.e. activated transcription in the presence of lactose.
Ans 3. Answer is same as B.
Ans 4.(a) +/- i.e. essentially no transcription. The lac Z gene encodes the enzyme Beta-galactosidase which cleaves the sugar lactose into gluocose and galacotse. There is no effect of Lac Z deletion on glucose metabolism.
Abs (b). +/- i.e. essentially no transcription. The lac Z gene encodes the enzyme Beta-galactosidase which cleaves the sugar lactose into gluocose and galacotse.
Ans (C) +/- i.e. essentially no transcription. The lac Z gene encodes the enzyme Beta-galactosidase which cleaves the sugar lactose into gluocose and galacotse. Mutations in the structural genes are called polar mutations and will result in little or no production of the enzymes even in the presence of lactose.
Ans 5. (a) +/- i.e. No transcription. Because the CAP-cAMP complex is needed for transcription, the complex exerts a positive control over the expression of the lac operon.
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Ans (b). +/- i.e. No transcription. Because the CAP-cAMP complex is needed for transcription, the complex exerts a positive control over the expression of the lac operon.
Ans (C) +/- No transcription
Ans 6 (a) ++++ Higher transcription. A mutation in the operator site, make impossible for any molecule to bind to the operator. This also results in the lac operon becoming constitutive.
(b) ++++ Higher transcription. A mutation in the operator site, make impossible for any molecule to bind to the operator. This also results in the lac operon becoming constitutive.
(c) ++++ Higher transcription. A mutation in the operator site, make impossible for any molecule to bind to the operator. This also results in the lac operon becoming constitutive.