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