Chapter 16.2. Animation Activity: Attenuation

2

Correct. In prokaryotes transcription and translation are coupled. This makes attenuation possible. In eukaryotes, transcription takes place in the nucleus and most translation takes place in the cytoplasm.

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Incorrect. In prokaryotes transcription and translation are coupled. This makes attenuation possible. In eukaryotes, transcription takes place in the nucleus and most translation takes place in the cytoplasm.

2

Correct. In the absence of the Shine-Dalgarno sequence prior to the leader sequence, the leader sequence will not be translated. Therefore, the ribosome will never prevent the binding of region 2 with region 3 and the terminator loop no matter what the level of tryptophan is.

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Incorrect. In the absence of the Shine-Dalgarno sequence prior to the leader sequence, the leader sequence will not be translated. Therefore, the ribosome will never prevent the binding of region 2 with region 3 and the terminator loop no matter what the level of tryptophan is.

2

Correct. If region 3 is deleted, the terminator loop consisting of region 3 and region 4 will not form no matter what the level of tryptophan is. This assumes that region 2 could not pair with region 4 to create an alternate termination loop.

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Incorrect. If region 3 is deleted, the terminator loop consisting of region 3 and region 4 will not form no matter what the level of tryptophan is. This assumes that region 2 could not pair with region 4 to create an alternate termination loop.

2

Correct. If the trp codons were removed, there would be no stalling of the ribosome, so region 2 will always be blocked from pairing with region 3, causing constitutive formation of the termination loop consisting region 3 paired with region 4. Therefore, the structural genes of the operon will never be transcribed no matter what the intracellular levels of tryptophan are.

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Incorrect. If the trp codons were removed, there would be no stalling of the ribosome, so region 2 will always be blocked from pairing with region 3, causing constitutive formation of the termination loop consisting region 3 paired with region 4. Therefore, the structural genes of the operon will never be transcribed no matter what the intracellular levels of tryptophan are.

2

Correct. If the trp codons were replaced by isoleucine codons, the ribosomes would stall on the leader sequence only when isoleucine levels were limited. Therefore the transcription of the structural genes will depend on the intracellular level of leucine rather than tryptophan.

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Incorrect. If the trp codons were replaced by isoleucine codons, the ribosomes would stall on the leader sequence only when isoleucine levels were limited. Therefore the transcription of the structural genes will depend on the intracellular level of leucine rather than tryptophan.

2

Correct. If the T:A base pairs are removed following region 4, the structure will no longer resemble a rho-independent terminator (review Chapter 13), and termination will not follow the formation of the terminator loop consisting of the pairing of regions 3 and 4. This would allow transcription to continue through the structural genes of the operon.

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Incorrect. If the T:A base pairs are removed following region 4, the structure will no longer resemble a rho-independent terminator (review Chapter 13), and termination will not follow the formation of the terminator loop consisting of the pairing of regions 3 and 4. This would allow transcription to continue through the structural genes of the operon.

2

Correct. If the Shine-Dalgarno sequence of the trpD gene were mutated, the trpD gene would not be translated. Therefore, the bacterial cell could not produce tryptophan from chorismate and tryptophan could not be synthesized by the bacterial cell. This would cause levels of tryptophan within the cell to fall and would lead to constitutive formation of the antiterminator (pairing of regions 2 and 3) unless abundant tryptophan could be obtained from the environment.

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Incorrect. If the Shine-Dalgarno sequence of the trpD gene were mutated, the trpD gene would not be translated. Therefore, the bacterial cell could not produce tryptophan from chorismate and tryptophan could not be synthesized by the bacterial cell. This would cause levels of tryptophan within the cell to fall and would lead to constitutive formation of the antiterminator (pairing of regions 2 and 3) unless abundant tryptophan could be obtained from the environment.

2

Correct. If the bacterial trp operon were given a eukaryotic promoter and transcription termination sequence, the operon will be transcribed by RNA polymerase II in eukaryotic cells, but it would be very unlikely that any of the structural genes would be translated. Eukaryotic ribosomes do not recognize the Shine-Dalgarno sequence but scan for appropriate AUG codons from the 5’ end of the transcript; therefore while the leader peptide might initiate translation, none of the structural genes would be expected to initiate translation. In any case, because transcription and translation are physically separated in eukaryotic cells, and transcription termination in eukaryotes differs from that in prokaryotes, attenuation will not occur no matter what the levels of tryptophan in the cell.

You have one more chance to answer the question correctly. Try again.

Incorrect. If the bacterial trp operon were given a eukaryotic promoter and transcription termination sequence, the operon will be transcribed by RNA polymerase II in eukaryotic cells, but it would be very unlikely that any of the structural genes would be translated. Eukaryotic ribosomes do not recognize the Shine-Dalgarno sequence but scan for appropriate AUG codons from the 5’ end of the transcript; therefore while the leader peptide might initiate translation, none of the structural genes would be expected to initiate translation. In any case, because transcription and translation are physically separated in eukaryotic cells, and transcription termination in eukaryotes differs from that in prokaryotes, attenuation will not occur no matter what the levels of tryptophan in the cell.