Green = lac repressor

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Red = DNA, operator region

Blue = part of the operator (not a hairpin loop, its double stranded DNA)

Repressor = a dimer and when it binds to the operator, it creates a loop to make it impossible for polymerase to get through

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In the lac operon, the beta galatoside bond is hydrolyzed by galactosidease either into glucose and galactose or by flipping the bond it is converted into allolactose

***Inducible (normal –> off until turned on)

  • lacI – makes repressor; inducible
  • lacZ – always off; no activity
  • lacY – permease (transports lactose into cells)

NEGATIVE control à b/c binding that protein reduces expression (negative has nothing to do with lactose or enzyme expression, just what happens when protein binds to DNA)

Epigenetics – that DNA can learn from its environment, genome can respond to its environment

CAP binding

Binding site is upstream of the promoter – diamer

  • Binding of CAP bends helix in a way that makes promoter more accessible (and stable) to RNA polymerase –> therefore expression goes UP
  • Only happens with CAP/cAMP complex
Major Hormones: Origin, Target, Function

Metabolism of glucose results in decreasing cAMP (as metabolism of glucose increases, concentration of cAMP decreases – CAP binding drops – lac operon expression drops)

  • Many operons are also under regulation of CAP


CAP binds = expression goes up

Doesn’t bind (presence of glucose) = expression goes down

REGULON –> many genes control by same binding proteins

  • Lactose present; glucose low/absent
  • Lactose present; glucose present


When glucose is added, nothing happens

When lactose is added, galactosidease expression increases


Assumption that mutations are bad and cause decrease in function – Not TRUE, mutations can make things better

Mutation causes loss of function in CAP – mutation might be in CAP binding site, promoter, gene, etc (could be several mutations, but how can you tell the difference?): operon remains inducible, expression rises in presence of lactose but not repressed in presence of glucose cause CAP cannot bind

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