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Lecture 13

Biology 1002B Lecture Notes - Lecture 13: Mutation, Lac Operon, Operon

Course Code
BIOL 1002B
Tom Haffie

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Lecture 13 Outcomes
1. Mechanism of positive vs. negative regulation of lac operon
Negative control: if gene expression goes down
Positive control: if gene expression goes up
In lac operon:
oNegative: lac repressor binding to operator, not allowing polymerase to get through
lac repressor functions as a dimer distorts/changes conformation of the helix,
preventing transcription
oPositive: CAP activator protein that binds DNA upstream of the promoter CAP
binds to operon and bends the DNA, making it more attractive = increasing rate of
2. Possible location of mutations, given mutant phenotype
Mutation in operator:
oRepressor protein can no longer bind to the DNA no stopping transcription
oSystem is already full of lactose, it will have no effect
oExpression of lac genes remain constant
Mutation in promoter:
oDecrease in expression of lac genes can’t be transcribed
Mutation in lacY gene:
oActivity of lacZ decreases because lactose can’t get into the cell
oNot enough lactose to take the repressor off
Mutation in lacI gene (repressor):
oCan no longer be repressed
oAny repression going on would be relieved
oSlight increase in gene expression
Mutation in lacI promoter:
oSlight increase
3. Likely phenotype, given location of mutation
Location of Mutation Likely Phenotype
Operator No effect
Promoter Decrease
LacY gene Decrease
LacI gene Slight increase
LacI promoter Slight increase
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