Control of Gene Expression

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

Operons Simple gene expression mechanisms in prokaryotes Not thought to be present in eukaryotes

Components of the Operon Operator, sequence next to the promoter Promoter (where RNA polymerase binds) Genes that work together

Basic Function of Operons Binding of regulatory molecules to the operator controls the transcription of the gene

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

Inducing Gene Expression Bacteria only need lactase when lactose is present Regulatory gene codes for repressor protein Binds to operator, blocking promoter

Inducing Gene Expression (+ control) Presence of lactose disrupts the regulatory molecule, allowing gene expression

Inducible Operon Regulatory mRNA Lactase RNA Transcription Blocked Allolactose (inducer) RNA Polymerase

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

Repressible Operon The repressor is normally inactive A molecule (sometimes even the gene products) activates the repressor, shutting off gene expression

Repressing Gene Expression (- control) Repressor is normally inactive, but can be activated Active Repressor Inactive Repressor No More Transcription RNA Polymerase

More Complex in Humans Pre-transcriptional control Post-transcriptional control Post-translational control

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

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

Access to the DNA Is wrapped up in histone proteins forming nucleosomes

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

Gene Activation

Gene Acetylation (activation long-term)

Genes can also be Deactivated

DNA Methylation

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)

Post-Transcriptional Modification mRNA gets modified or broken down Example microRNAs bind to mRNA and prevent translation from occurring

Post-Translational Enzymes can modify or alter protein products

Millions of Proteins from 20k genes

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

Animation In pairs, animate out: Repressible operon Inducible operon DNA methylation Histone Acetylation