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Control of Gene Expression
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Much Diversity due to Gene Expression
Each tissue in our body is very different despite having the same DNA Even identical twins have many differences due to gene expression
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Operons Simple gene expression mechanisms in prokaryotes
Not thought to be present in eukaryotes
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Components of the Operon
Operator, sequence next to the promoter Promoter (where RNA polymerase binds) Genes that work together
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Basic Function of Operons
Binding of regulatory molecules to the operator controls the transcription of the gene
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Types of Operons Some genes are normally “off” and can be turned “on”
Presence of a molecule induces gene expression or enhances gene expression Some genes are normally “on” and can be turned “off” Presence of a molecule represses gene expression
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Inducing Gene Expression
Bacteria only need lactase when lactose is present Regulatory gene codes for repressor protein Binds to operator, blocking promoter
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Inducing Gene Expression (+ control)
Presence of lactose disrupts the regulatory molecule, allowing gene expression
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Inducible Operon Regulatory mRNA Lactase RNA Transcription Blocked
Allolactose (inducer) RNA Polymerase
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Enhancing Gene Expression
Instead of a repressor molecule, an enhancer molecule binds to the operator Helps facilitate binding of RNA polymerase and increases gene expression
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Repressible Operon The repressor is normally inactive
A molecule (sometimes even the gene products) activates the repressor, shutting off gene expression
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Repressing Gene Expression (- control)
Repressor is normally inactive, but can be activated Active Repressor Inactive Repressor No More Transcription RNA Polymerase
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More Complex in Humans Pre-transcriptional control
Post-transcriptional control Post-translational control
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Regulatory Sequences Stretches of DNA that interact with regulatory proteins like transcription factors (non-coding) I.e. the promoter Enhancers are sequences that increase transcription
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Code for proteins or mRNAs which affect gene expression e.g. microRNAs
Regulatory Genes Code for proteins or mRNAs which affect gene expression e.g. microRNAs Repressor proteins Transcription factors/ enhancers
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Access to the DNA Is wrapped up in histone proteins forming nucleosomes
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Wrapped DNA… Does not tend to get expressed
Condensing the DNA and causing more of it to be wrapped shuts off DNA expression Loosening up DNA increases gene expression
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Gene Activation
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Gene Acetylation (activation long-term)
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Genes can also be Deactivated
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DNA Methylation
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The Epigenome Epi = above, or in addition to
Lots of research now into how epigenome is passed on and changed Your actions affect your epigenome Your epigenome is passed on (sometimes)
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Post-Transcriptional Modification
mRNA gets modified or broken down Example microRNAs bind to mRNA and prevent translation from occurring
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Post-Translational Enzymes can modify or alter protein products
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Millions of Proteins from 20k genes
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The Main Point Genes can be turned on and off
These on/off switches can be short term or long term and might be passed down Environment plays a huge role in the epigenome
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Animation In pairs, animate out: Repressible operon Inducible operon
DNA methylation Histone Acetylation
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