The lac Operon
Laboratory
Purpose: Learning to analyze an example of gene expression
Example: lac operon
Type 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
c
O mutation:
a cis acting mutation
I
mutation a trans acting mutation
s
I
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.
Β-galactosidase
No Lactose
1
2
3
4
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+
5
I+P+OcZ+Y-/ I+P+O+Z-Y+
6
I+P+OcZ-Y+/ I-P+O+Z+Y-
7
ISP+O+Z+Y-/ I-P+O+Z-Y+
8
I+P-OcZ+Y+/ I+P+O+Z-Y-
9
-
Lactose
+
Permease
No lactose
-
Lactose
+
The End.
Laboratory Exercise
Try out your ability to reason
through the questions.
PROTEIN
PRODUCT
Β-galactosidase
No 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+YISP+O+Z+Y-/ I-P+O+Z-Y+
I+P-OcZ+Y+/ I+P+O+Z-Y-
-
Lactose
+
Permease
No lactose
-
Lactose
+
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
I s=inducer binding region mutated
Inhibits transcription, even with inducer
Lactose
binding site
DNA binding site
I - = DNA binding site mutated,
prevents binding, allows transcription