Genetic Regulatory Mechanisms. Inducible Genes - Operon Model. Definition: Genes whose expression is turned on by the presence of some substance Lactose induces expression of the lac genes Catabolic pathways. Absence of lactose. z. y. a. i. p. o. Active. No lac mRNA.
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can bindand transcribe
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.
Figure 18.23aPositive Gene Regulation- CAP
Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
available, a small amount of it
is taken up and converted to
allolactose by β-galactosidase.
The allolactose binds to the
repressor, causing it to fall off
the operator site.Induction of Lac Operon
2. lac operon proteins
are synthesized. This
promotes the efficient
metabolism of lactose.
4. Most proteins involved
with lactose utilization
3. The lactose is depleted.
Allolactose levels decrease.
Allolactose is released from
the repressor, allowing it to
bind to the operator site.