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Controlling Gene ExpressionSBI4U Biology
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…
Gene Structure• A gene has a specific locus• Preceded by promoter, ended by
STOP sequence• Gene includes introns & exons
Levels of Control• Four levels: transcriptional, post-
transcriptional, transport, translational.
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.
The LAC operon• An operon = gene control system in
prokaryotes.• Genes “Off” when lactose is absent
The LAC operon• Genes “On” when lactose is present:
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!
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
Translation & Transport Control
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