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

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

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

DNA Mutations

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

Cite this article as: William Anderson (Schoolworkhelper Editorial Team), "Lac Operon: Function, CAP Binding, Mutations," in SchoolWorkHelper, 2019,
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