1. Controlling Gene Expression

2. Control Mechanisms
Determine when to make more proteins and when to stop making more
Cell has mechanisms to control transcription and translation
Housekeeping genes are genes that are always needed, and are constantly synthesizing proteins (switched on)

3. There are four levels of control:
transcriptional (controls transcription from DNA to mRNA)
posttranscriptional (controls the removal of introns)
translational (controls rate that mRNA is activated through ribosomes)
posttranslational (affects the rate proteins can leave the cell)

4. The lac Operon An example of control mechanism is the lac operon
Operon: a cluster of genes under the control of one promoter and one operator
Operator: regulatory sequence of DNA to which a repressor protein binds

5. The lac Operon in action!
dI1zvk
Lactose:
* disaccharide found in milk, broken down by E. coli
B-galactosidase: enzyme responsible for the degradation of lactose
The lac operon is a cluster of three genes that code for the metabolism of lactose:
lacZ codes for B-galactosidase
lacY codes for B-galactosidase permease which lets lactose cross the cell membrane
lacA codes for transacetylase; unknown function.

6. Repressed State

7. Repressed State
LacI protein (blue block) is a repressor protein that blocks the transcription of lacZ; binds to the lactose operator and inhibits RNA polymerase by covering the promoter site (switch is of when there is no lactose in the system).

8. Repressed State
If there is no lactose present, lac operon genes are not transcribed or translated. Lactose is an effector/inducer; if it is present, it induces the removal of the repressor and allows the lac genes to be transcribed and translated. (switch is on if there is lactose in the system)

9. Induced State
When lactose binds to LacI protein, it changes and the new complex cannot bind to the operator of the lac operon. This results in RNA polymerase being able to bind to the DNA and start protein synthesis.

10. Induced State

11. Page 258 #1-5