Post on 21-Dec-2015
The lac Operon Laboratory
Purpose: Learning to analyze an example of gene expression
Example: lac operonType of control: negative inducible
The Laboratory Worksheet asks:
How do mutations affect gene expression of the lac operon?
Review: Lac Operon:
Mutations and gene expressio of lac operon
Oc mutation: a cis acting mutation
I- mutation a trans acting mutation
Is mutation a trans acting mutation
Lab Worksheet: Haploid and Heterozygous Diploid Mutants
Predict if you expect to obtain: transcription/translation of the lacZ and lacY genes
when lactose is absent or present for the E.coli.
Try out your ability to reason through the questions.
Lactose No lactose Lactose
I+P+O+Z+Y+ - +I-P+O+Z+Y+
I+P-O+Z+Y+
I+P+OcZ+Y+
I+P+O+Z-Y-/ I-P+O+Z+Y+
I+P+OcZ+Y-/ I+P+O+Z-Y+
I+P+OcZ-Y+/ I-P+O+Z+Y-
ISP+O+Z+Y-/ I-P+O+Z-Y+
I+P-OcZ+Y+/ I+P+O+Z-Y-
Β-galactosidase Permease
No Lactose
1
2
3
4
5
6
7
8
9
- +
The End.
Laboratory Exercise
Try out your ability to reason through the questions.
Lactose No lactose Lactose
I+P+O+Z+Y+ - +I-P+O+Z+Y+
I+P-O+Z+Y+
I+P+OcZ+Y+
I+P+OcZ+Y-/ I+P+O+Z-Y+
I+P+OcZ-Y+/ I-P+O+Z+Y-
ISP+O+Z+Y-/ I-P+O+Z-Y+
I+P-OcZ+Y+/ I+P+O+Z-Y-
PROTEIN
PRODUCT Β-galactosidase Permease
No Lactose
- +
I+P+O+Z-Y-/ I-P+O+Z+Y+
The next slides are a “key” to the mutations.
Experimental tests on the repressor: I+ cells synthesize full levels of the lac enzymes only in the
presence of an inducer
I- mutations NO DNA BINDING ACTIVITY, TRANSCRIPTION OCCURS EVEN IN ABSENCE OF INDUCER!
Is mutation NO LACTOSE BINDING ACTIVITY but has DNA BINDING ACTIVITY. NO TRANSCRIPTION EVEN WITH INDUCER!
Experimental mutations to the repressor
Lactose binding site
DNA binding site
I - = DNA binding site mutated, prevents binding, allows transcription
I s=inducer binding region mutatedInhibits transcription, even with inducer