1. Controlling Gene Expression
SBI4U Biology

2. Gene Expression Follows the Central Dogma: DNA  RNA  Protein
Most eukaryotes are diploid; they have two copies of every gene in each cell.
So, if that’s the case, why aren’t skin cells making blood proteins? Or brain cells making digestive enzymes?
It’s a question of regulatory control: not every gene gets transcribed…

3. Gene Structure A gene has a specific locus
Preceded by promoter, ended by STOP sequence
Gene includes introns & exons

4. Levels of Control
Four levels: transcriptional, post-transcriptional, transport, translational.

5. Regulators, Operators, Promoters
RNA polymerase binds at a Promoter
Often have a Regulator gene, influencing binding
The regulator gene’s product binds at an Operator sequence, blocks promoter.

6. The LAC operon An operon = gene control system in prokaryotes.
Genes “Off” when lactose is absent

7. The LAC operon
Genes “On” when lactose is present:

8. Transcriptional Control
In nucleus
What gets transcribed into mRNA, and what doesn’t
Some genes are inactivated & never transcribed; ‘junk’ DNA (SINES, LINES)
Post-Transcriptional Control
In nucleus
Editing of Introns by snRNA spliceosomes
Some mRNA’s are degraded at this point!

9. Translational Control
Transport Control
In nucleus & cytoplasm
What goes into the cyto, what doesn’t
Non-shuttling proteins block transport
Some products stay in nucleus, i.e. histones, rRNA, RNA polymerase…
Translational Control
In cytoplasm; Golgi, ER, free ribosome…
Inhibitor molecules bind to some mRNA, inactivating it; no translation.
Often by a feedback mechanism, i.e. ferritin protein = only when Fe3+ high

10. Translation & Transport Control

11. Post-Translational Modification
The translated polypeptide may need some ‘tweaking’ to make it functional:
Enzymatic activation: i.e. insulin is made as a single chain, which is cleaved into 2, held together by disulphide bonds
Environmental factors: i.e., pepsinogen is converted to pepsin by HCl.
Structural modification: creating the 2o, 3o or 4o structures of proteins; i.e. hemoglobin = 4 chains, held together by Fe3+
Glycosylation: Adding sugars to protein; i.e., glycoproteins aid in cell identity.
Methylation: activation by adding CH3