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Question

The end product of a metabolic pathway may bind a repressor to make the latter active enough to bind to operator. In this case the end-product is called as

A
Inducer
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B
Apo-repressor
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C
Co-repressor
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D
Regulator
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Solution

The correct option is C Co-repressor
  • A substance that inhibits the expression of genes not by direct binding to DNA but instead indirectly regulates gene expression by binding to repressors is known as co-repressor.
  • For example, in trp operon, two tryptophan molecules serve as co-repressor and bind to the trp repressor. This binding activates the inactive repressor to bind to the trp operator.
  • Binding of the tryptophan repressor complex at the operator prevents the RNA polymerase from binding the downstream genes and thus presents their transcription.
  • An inactive repressor that needs the binding of a corepressor for activation is termed as an apo-repressor or inactive repressor.
  • When tryptophan is not present in the cell, the repressor is inactive and can not bind to the operator and is called apo-repressor.
  • A molecule, that regulates gene expression by binding to repressors or activators to prevent or start the transcription, respectively, is inducer.
  • Allolactose serves as an inducer in the lac operon and binds to the repressor and decreases the repressor's affinity for the operator site; thereby facilitating transcription.
  • A regulator gene controls the expression of one/more genes by its encoded repressor proteins.

The correct option is C.

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BIOLOGY
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