Gene Control

1. Gene Control Prokaryotes vs. Eukaryotes

2. Gene regulation • Two types of genes: • Structural genes – code specific proteins • Regulatory genes – control activity of structural genes (gene expression)

3. OPERON Regulator gene Structural gene A Promoter Operator RNA polymerase binding site Repressor binding site Produces the repressor Gene regulation in prokaryotes • Operonscontrol rate of transcription. • group of genes working together code for enzymes regulating specific metabolic pathway. Structural gene B

4. The Operon Model These 3 make up an operon! • Operon – group of genes with related functions • Genes– code for specific proteins • Promoter – RNA pol binding site • controls transcription of ALL genes in operon • Single mRNA produced • Operator – binding site of repressor protein (turns off gene)

5. Quick Review in Metabolic regulation Allosteric inhibitors – provide feedback inhibition (enzyme regulators) Product of pathwaysignals continuation ofpath to STOP Making all these enzymes is wasteful

6. Induction: Ex-The Lac Operon Repressor protein **NO lactose present** RNA Polymerase Codesfor Regulator gene Promoter Operator 1 2 3 *Presence or absence of lactose regulates production of enzymes from structural genes of the lac operon*

7. **Lactose present** Lactose molecules Repressor protein Codesfor Regulator gene Promoter Operator 1 2 3 • Lactose is an inducer *Why do cells have these “on/off” switches?*Animation

8. trp RNA polymerase RNA polymerase repressor repressor repressor enzyme1 enzyme2 enzyme3 enzyme4 1 2 3 4 promoter repressor protein operator tryptophan trp trp trp trp trp trp trp trp trp tryptophan – repressor protein complex Repression: Ex – Tryp operon • Excess tryptophan present, binds to tryp repressor proteintriggering repressor to bind to DNA • blocks (represses) transcription • tend to be anabolic pathways gene1 gene2 gene3 gene4 DNA TATA mRNA Tryp is an effector- activates repressor trp trp

9. Gene regulation in prokaryotes - summary • Genes for metabolic pathways linked together in operons with a common switch mechanism (operator). • No introns – no RNA processing • Structural genes undergo transcription & translation simultaneously. • Regulation occurs by switching all genes of a pathway on or off.

10. Eukaryotes often multicellular • Must maintain homeostasis • Coordinate body as a whole • Differentiated & specialized cells Battle changing environment What about Eukaryotes? How are they different? What might the process have to accommodate for? How might it do this?

11. When does gene control occur??? Packing/unpacking DNA Transcription mRNA processing Translation Protein processing Protein degradation

12. 1. DNA Packing from DNA double helix to condensed chromosome If all 46 of your chromosomes were lined up in a row, your DNA would be over 3 feet long. How can your cells contain this large amount of material when cells are microscopic? • Coils & Folds • Double helix • Nucleosomes • Chromatin fiber • Looped domains • Chromosomes

13. Degree of DNA packing regulates transcription • Tightly packed = no transcription = genes OFF • “Dark” DNA = tight • “Light” DNA = loose

14. Repressors - (ex: adding -CH3’s) block transcription factors  no transcription  genes OFF! Activators -(ex: adding –COCH3’s) unwind DNA  coils loosen  transcription  genes ON!

15. 2. Transcription Initiation • Control regions on DNA • Promoter • nearby control sequence – “standard” rate • bind RNA pol • bind transcription factors • Enhancer • distant control sequence – “enhanced” rate • bind activator proteins

16. Transcription complex… Activator Proteins • regulatory proteins bind to DNA at distant enhancer sites • increase the rate of transcription Enhancer Sites regulatory sites on DNA distant from gene Activator Activator Activator Coactivator E F B RNA polymerase II H TFIID A Coding region T A T A Core promoter and initiation complex Initiation Complex- transcrip activated when “hairpin loop” brings TF’s on enhancer sequence (activators) to TF’s bound to RNA pol on promoter  protein-protein interactions KEY!

17. 3. Post-transcriptional control • Alternate splicing pattern • ↑ variation in protein family

18. 4. Regulation of mRNA degradation • Lifespan of mRNA controls amt of protein synthesized • mRNA can last from hrs to weeks!

19. What if degradation is interfered with??? • RNAi • Small interfering RNAs (siRNA) • short RNA (21-28 bases) • bind to mRNA • create sections of double-stranded mRNA • “death” tag for mRNA • triggers degradation • gene “silencing” • post-transcriptional control • turns off gene = no protein

20. 5. Control of Translation • Block initiation of translation • Regulatory proteins attach to 5’ end • Prevent attachment of ribosome & initiator tRNA • Synthesis turned OFF

21. 6/7. Protein Processing & Degradation • Protein processing • Folding, cleaving, adding sugar groups, targeting for transport • Protein degradation • “death tags” (ubiquitin -76 aa’s) • Proteasomes – degradation machinery

22. Regulation Prokaryotes Eukaryotes Pathways separated, no operons. Genes switched on separately. Introns on genes removed in RNA processing. Transcription & translation do not occur simultaneously. Large number of control elements. • Genes for metabolic pathways linked together in operons w/common switch mechanism (operator). 2. No introns - no RNA processing 3. Structural genes undergo transcription & translation simultaneously. 4. Regulation occurs by switching all genes in pathway on or off.