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Lecture

Control of Prokaryotic Gene Expression Notes

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Department
Biological Sciences
Course Code
BIOSC 0160
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All

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2/18 Chapter 17: Prokaryotic Gene  Expression Overview (Both Pro­ and Eukaryotic) • Binding to promoter o For transcription to occur, RNA polymerase needs to bind efficiently to a promoter  Promoter is a short, protein-binding DNA sequence just upstream of the gene to be transcribed o Some proteins block access to the promoter (down regulation) o Some proteins enhance the affinity of RNA polymerase for the promoter (up regulation) • Prokaryotes o Genes are rapidly turned on, off, and on again o This occurs in association with rapid response to changing environmental conditions • Eukaryotes o Gene regulation is more complex o Regulation occurs throughout development o Different tissues express different genes o In many tissues, genes are activated briefly and then shut down permanently o In many tissues, many genes are never expressed Viral Life Cycles • Lytic Cycle (Bacteriophages) 1. Phage attaches to host 2. Injects its genome into the host 3. Degrades the host chromosome 2/18 4. Uses host’s metabolism to make phage genomes and capsid proteins 5. Phage particles form 6. Host cell lyses • Lysogenic Cycle 1. Phage attaches to host 2. Injects its genome into the host 3. Phage genome inserts into the host chromosome as a prophage 4. Phage spreads in host population (some never leave) 5. Under environmental stress, the prophage excises and enters the lytic cycle Prokaryotic Gene Expression with Operons • Structure and Function o Make-up:  Cluster of metabolically-related genes (all transcribed at once)  Upstream of genes, there is a single promoter  An operator (part of the promoter) acts as an on/off switch o Function  Regulates synthesis of enzymes  Changes in response to environmental conditions • The presence of a substance to be degraded (like lactose) • The absence of a substance to be synthesized (like tryptophan) • Repressible Operons o Associated with anabolic pathways o Transcription inhibited by a molecule that would otherwise need to be synthesized o Molecule activates a repressor protein 2/18 o TRP Operon  Regulatory gene makes an inactive repressor  If TRP is present: • TRP binds to and activates the repressor • Repressor binds to the operator • Upstream, RNA polymerase is present (but not working) • The activated repressor bound to the operator blocks polymerase, thus blocking transcription 
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