SCIENCE
VIRUS AND BACTERIA
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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permanently turned on.
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turned on when tryptophan is present in the growth medium.
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turned off only when glucose is present in the growth medium.
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turned off when tryptophan is added to the growth medium.
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Detailed explanation-1: -The trp operon is expressed (turned “on") when tryptophan levels are low and repressed (turned “off") when they are high. The trp operon is regulated by the trp repressor. When bound to tryptophan, the trp repressor blocks expression of the operon.
Detailed explanation-2: -The trp operon responds to a repressor protein that binds to two molecules of tryptophan. When the tryptophan is plentiful, this repressor-tryptophan complex binds to the trp operator. This binding prevents the binding of RNA polymerase, so the operon is not transcribed (Fig.
Detailed explanation-3: -When tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed. A DNA sequence that codes for proteins is referred to as the coding region. The five coding regions for the tryptophan biosynthesis enzymes are arranged sequentially on the chromosome in the operon.
Detailed explanation-4: -When tryptophan (an amino acid) is present in the external medium, the E. coli bacterium brings in tryptophan molecules and does not need to make them.