Controlling Gene Expression

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Presentation transcript:

Controlling Gene Expression SBI4U 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!

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

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