Ch. 16 HW Instructor: Dr.Sanghamitra Mohanty MCQ: Genetic regulation in eukaryotes can take place at a variety of levels from transcriptional to post-translational. At what level is genetic regulation considered most likely in prokaryotes? transcriptional capping polyadenylation at the 3? end of the mRNA intron processing exon processing What term would be applied to a regulatory condition when a protein greatly reduces transcription on binding to a particular section of DNA? positive control negative control activation inhibition stimulation In the lac operon, the product of structural gene lacZ is capable of non-autonomous replication forming lactose from two glucose molecule replacing hexokinase in the early steps of glycolysis splitting the ?-linkage of lactose forming ATP from pyruvate Which of the following terms best characterizes catabolite repression associated with the lac operon in E.coli? inducible system repressible system negative control positive control constitutive expression When referring to attenuation in regulation of the tryptophan operon, it would be safe to say that when there are high levels of tryptophan available to the organism, the tryptophan operon is being transcribed at relatively high levels translational termination is likely transcriptional termination is likely tryptophan is inactivating the repressor protein ribosomes are stalling during translation of the attenuator region In E.coli, if tryptophan is present in the medium and available to the bacterium, the repressor is inactive and the tryptophan operon is ?off? the repressor is inactive and the tryptophan operon is ?on? the repressor is bound to the operator, thus blocking transcription the five structural genes associated with tryptophan synthesis are being transcribed at ?induced? levels positive control is occurring Under the system of genetic control of the tryptophan operon, when there are high levels of tryptophan in the medium, transcription of the trp operon occurs at high level when there is no tryptophan in the medium, transcription of the trp operon occurs at high level when there are high levels of tryptophan in the medium, ribosomes stall and reduce the levels of tryptophan synthesized no transcription occurs under any nutritional circumstance because negative controls inhibit transcription when there are high levels of tryptophan in the medium, transcription of CAP occurs at high level True/ False: CAP-cAMP binds to the CAP-binding site on the promoter and blocks the binding of RNA polymerase to transcribe the lac genes. T / F IPTG, like allolactose can bind to the lac repressor and induce transcription of the lac operon. T/ F In an inducible operon, transcription is normally turned on and has to be repressed to shut off gene expression. T / F Like the lac repressor, the trp repressor has two binding sites; one that binds to the DNA at the operator site and another that binds to the corepressor, tryptophan. T / F If tryptophan is plenty, the ?antiterminator hairpin? is formed. T / F Ara operon is the repressible arabinose operon in E.coli. T / F Clusters of genes along with their regulatory elements are called operon, which is a single transcription unit. T / F Short Questions: Briefly describe the lac operon and how it controls the metabolism of lactose. What is attenuation? What are the mechanisms by which the attenuator forms when tryptophan levels are high and the antiterminator forms when tryptophan levels are low? Briefly describe the structure of TRAP and how its association with the leader sequence decides the fate of transcription, to be continued or terminated. Which characteristic of the ara operon makes it unique? How does it regulate gene expression? Describe briefly. Problems: A mutation prevents the CAP (catabolite activator protein) from binding to the promoter in the lac operon. What will be the effect of this mutation on the transcription of the operon? The ?mmm operon?, which has sequences A, B, C and D; encodes enzymes 1 and 2. Mutations in the sequences A, B, C and D have the following effects, where a plus sign (+) indicates that the enzyme is synthesized and a minus sign (-) indicates that the enzyme is not synthesized. Mutation Mmm absent Mmm present Sequence Enz 1 Enz 2 Enz 1 Enz 2 No mutation + + - - A - + - - B + + + + C + - - - D - - - - Is the ?mmm operon? inducible or repressible? Indicate which sequence (A, B, C, D) is part of the following components of the operon? regulator gene promoter structural gene for enzyme I structural gene for enzyme 2
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