S14 - S1.Researchers have identified mutations in the...

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S1.Researchers have identified mutations in the promoter region of the lacI gene that make it more difficult for the lac operon to be induced. These are called lacI Q mutants, because a greater Q uantity of lac repressor protein is made. Explain why an increased transcription of the lacI gene makes it more difficult to induce the lac operon. Answer: An increase in the amount of lac repressor protein makes it easier to repress the lac operon. When the cells become exposed to lactose, allolactose levels slowly rise. Some of the allolactose binds to the lac repressor protein and causes it to be released from the operator. If there are many more lac repressor proteins found within the cell, it takes more allolactose to ensure that there are no unoccupied repressor proteins that can repress the operon. S2. Explain how the pausing of the ribosome in the presence or absence of tryptophan affects the formation of a terminator stem-loop. Answer: The key issue is the location where the ribosome stalls. In the absence of tryptophan, it stalls over the trp codons in the trpL mRNA. Stalling at this site shields region 1 in the attenuator region. Since region 1 is unavailable to hydrogen bond with region 2, region 2 hydrogen bonds with region 3. Therefore, region 3 cannot form a terminator stem-loop with region 4. Alternatively, if there is sufficient tryptophan in the cell, the ribosome pauses over the stop codon in trpL. In this case, the ribosome shields region 2. Therefore, region 3 and 4 hydrogen bond with each other to form a terminator stem-loop. This abruptly halts the continued transcription of the trp operon. S3. With regard to the key proteins that affect the choice between the lytic and lysogenic cycles, which one(s) may be degraded by cellular proteases? Answer:
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S14 - S1.Researchers have identified mutations in the...

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