Please help 22-26 22. The lac repressor inhibits the transcription of the lac op
ID: 262613 • Letter: P
Question
Please help 22-26
22. The lac repressor inhibits the transcription of the lac operon mostly by ________.
A. allostery
B. steric hindrance
C. target protein modification
D. cooperative binding of DNA
23. When there is plenty of glucose but no lactose inside the cell, _________.
A. the DNA of the gene will loop and the bound lac repressors will interact with each other
B. the gene will completely shut down and no mRNA will be transcribed from the operon
C. the lac repressor will bind allolactose and two or more of the operators
D. two of these events will take place
E. all of these events will take place
24. Which of the following is the inducer(s) of the lac operon?
A. lactose
B. allolactose
C. cyclic AMP
D. lactose and allolactose
E. allolactose and cAMP
25. When E. coli grows in the presence of both glucose and lactose, all the following will take place except________.
A. the promoter will be de-repressed
B. the operon will be transcribed at the basal level
C. CRP will bind the UAS and activate the operon
D. lac repressor will bind allaolactose and not the operators
E. RNAP-sigma factor will be able to bind the promoter and transcribe the gene
26. Suppose: The number of mRNAs transcribed from the lac operon is (i) 0.01 unit when the operon is repressed, (ii) 2.0 units when the operon is de-repressed (or at the basal level), and (iii) 100 units when the operon is activated by CRP. Which of the following explains 50-fold increase (over the basal level) of transcription by CRP?
A. The CRP-cAMP complex binds the UAS and frequently recruits RNAP/sigma to the promoter
B. The lac promoter is weak thus RNAP/sigma bind the promoter with low affinity
C. CRP-cAMP binds and induces conformational changes in the RNAP/sigma
D. Two of these explain the degree of activation
E. All these explain the degree of activation
Explanation / Answer
22. A. allostery; lac repressor is an allosteric protein i.e. has two forms where in without inducer it binds DNA and inhibits transcription and in the presence of inducer takes up a form that doesnt bind DNA, allowing transcription.
23. B. the gene will completely shut down and no mRNA will be transcribed from the operon; this is because as glucose concentration is increased, the cAMP levels are decreased and cAMP-CAP complex is decreased. This complex binds to the promoter and helps RNA pol inititate transcription
24. allolactose; allolactose is produced from lactose from Beta galactosidase inside the cell.
25. A. the promoter will be de-repressed; in the presence of glucose, the inducer will not be taken up by the cell because of catabolic repression and the promoter will be repressed
26. A. The CRP-cAMP complex binds the UAS and frequently recruits RNAP/sigma to the promoter; The promoter has two binding sites, one from RNA pol and one of CRP-cAMP