Major Points
1. Gene expression must be regulated
2. Regulation can occur at several levels
3. The lac operon as a model of gene control
The lac Operon
Jacob & Monod studied induction of the enzyme beta-galactosidase by
lactose
Adding lactose increases activity about 500-fold
This is termed "induction"
Activity of three enzymes increases coordinately: beta-galactosidase, lac
permease, and transacetylase
The lac Operon
The genes encoding the three lac enzymes are clustered
They are expressed on a single mRNA
It is the synthesis of the mRNA which is "induced"
The amount of enzyme correlates to the amount of mRNA
Controlling transcription controls gene expression
Other Forms of Gene Control
Genome rearrangements
Over-replication
Increased gene copy causes increased expression
DNA deletion
Segments of antibody encoding genes are removed to generate
antibodies with various specificities
Other Forms of Gene Control
Post-Transcriptional Processing
Splicing: genes can be spliced in multiple ways
Multiple forms of the muscle protein troponin
Sex-determination in Drosophila
Other Forms of Gene Control
Translational Control
Stability of mRNAs
Availability of mRNAs for translation
Early in embryogenesis mRNAs are stored for later use
Translational repression
A protein binds to the mRNA and blocks translation
Other Forms of Gene Control
Post-Translational Modification
Processing during export of proteins
Occurs in the endoplasmic reticulum and Golgi body
Some proteins are selectively degraded