GENETICS AND EVOLUTION
MOLECULAR BASIS OF INHERITANCE
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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turned on only when glucose is present in the growth medium.
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turned off whenever tryptophan is added to the growth medium.
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turned off only when glucose is present in the growth medium.
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permanently turned on.
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Detailed explanation-1: -This operon is always turned off unless an inducer-lactose-is available from the environment; lactose triggers the expression of genes in this operon. The trp operon is a repressible system; this operon is always expressed unless tryptophan, the corepressor, becomes available in the cell.
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: -The trp operon, found in E. coli bacteria, is a group of genes that encode biosynthetic enzymes for the amino acid tryptophan. The trp operon is expressed (turned “on") when tryptophan levels are low and repressed (turned “off") when they are high.
Detailed explanation-4: -Why is the tryptophan operon turned off in the presence of tryptophan? Tryptophan binds to and activates the repressor proteins; the repressor proteins, in turn, bind to the operator, preventing transcription.