Sketch the main elements of an inducible operon, such as the lactose operon, and explain the functions of the operator
and promoter regions. What will be an ideal response?
Concepts to Consider: An operon is a set of functionally related structural genes and the
DNA sequences that control them. The operator sequence overlaps part of the promoter
region and is upstream from the structural genes. In the absence of lactose, the lactose
repressor binds to this region, blocking RNA polymerase from binding to the promoter site so
that the genes are not transcribed. When lactose is present, some molecules of lactose are
converted into allolactose, which binds to an allosteric site on the repressor such that it can
no longer bind to the operator sequence. This allows the RNA polymerase to bind to the
unblocked promoter region and the structural genes of the operon to be transcribed. As a
result of this control sequence, the gene products of the lactose operon are produced only
when needed.
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