1. G. GENE CONTROL MECHANISMS
1. Genes ON or OFF?
a. each cell in a multicellular organism has the same genetic information
b. yet each cell makes a different set of proteins and these may change over time………HOW?
c. most genes in most cells are turned off most of the time

2. 2. Types of control pg 255 Table 1
watch short clip on lac operon

3. 3. Transcriptional control parts or OPERONS
a. promoter – a sequence of DNA that is rich in A & T to which RNA polymerase binds this is located ‘upstream’ from the TAC ‘start’ signal on the DNA
b. operator – a sequence of DNA beside the promoter to which a repressor protein binds. If the repressor protein does bind to the operator, the RNA polymerase can not bind to the promoter and so no transcription occurs

4. repressor protein – binds to the operator unless it first binds
repressor protein – binds to the operator unless it first binds to the inducer.
Binding to the inducer changes the shape of the repressor and now it no longer binds to the operator.
Therefore the RNA polymerase can bind to the promoter and transcription can occur
d. inducer – a molecule that binds to the repressor and prevents it from stopping transcription

5. 4. trp operon in bacteria
trp is an amino acid that is rare in the environment
and cells need it so normally the gene to make trp is ON
b. if trp is present, it binds to the repressor protein and changes its shape so that the protein does bind to the promoter and blocks the RNA polymerase from binding which turns the gene OFF
c. because it helps bind, trp is called a co-repressor
d. see pg 257

6. 5. lac operon in bacteria
a. lactose is a rare sugar [food source] in the environment so the lac genes are usually OFF
b. if lactose is present, it is the INDUCER that binds to the repressor protein which now does NOT bind to the operator
c. when lactose is present, the gene is ON

7. eg. of how transcriptional control works
structural genes
lacZ
lacY
lacA
PROMOTER
OPERATOR
repressor
protein
genes are NOT transcribed
RNA
polymerase

8. lactose / inducer structural genes lacZ lacY lacA RNA polymerase
PROMOTER
structural genes
lacZ
lacY
lacA
OPERATOR
RNA
polymerase
genes are transcribed
modified
repressor
protein
repressor protein
lactose / inducer

9. H. MUTATIONS
read pg 259 – 263
answer pg 263 # 1, 2, 3, 4, 6 and pg 274 # 6