Eukaryotic Gene Expression

Introduction Every cell in a multi-cellular eukaryote does not express all its genes, all the time (usually only 3-5%) –Long-term control of gene expression in tissue = differentiation How to prevent expression? –Regulation at transcription –Regulation after transcription

Chromatin Regulation Chromatin remodeling allows transcription –Chromatin = DNA + proteins –Chromatin coiled around histones = nucleosomes –Allows DNA to be packed into nucleus, but also physically regulates expression by making regions ‘available’ or not

Chromatin Regulation Con’t Chromatin regulation can be small-scale (gene) or large scale (chromosome) –Non-expressed = heterochromatin (condensed) –Expressed = euchromatin (relaxed) Example of whole-chromosome regulation: Barr bodies

Gene Amplification To increase gene expression, make temporary copies of a gene, aka gene amplification Can happen in certain tissues or stages of development –Embryos require massive volumes of rRNA (to make ribosomes); in early development there are a million+ extra rRNA genes (not able to replicate) present in nucleus

Transcription Regulation What we know from prokaryotes: –Several related genes can be transcribed together (ie. lac operon) –Need RNA Polymerase to recognize a promoter region Why eukaryotes are different: –Genes are nearly always transcribed individually –3 RNA Polymerases occur, requiring multiple proteins to initiate transcription

Transcription Regulation Con’t Typical prokaryotic promoter: recognition sequence + TATA box -> RNA Polymerase attachment -> transcription Typical eukaryotic promoter: recognition sequence + TATA box + transcription factors -> RNA Polymerase II attachment -> transcription

Transcription Regulation Con’t RNA polymerase interacts w/promoter, regulator sequences, & enhancer sequences to begin transcription –Regulator proteins bind to regulator sequences to activate transcription Found prior to promoter –Enhancer sequences bind activator proteins Typically far from the gene Silencer sequences stop transcription if they bind with repressor proteins

Transcription Regulation Con’t If eukaryotic genes are typically ‘alone’, how to regulate expression of several? Conserve regulatory sequences!

Now, Can You: Explain why gene expression control is necessary in a eukaryotic cell? Describe how expression is regulated in before & during transcription? Tell me what differentiation is? Euchromatin? A silencer sequence? Explain how gene expression regulation is different in eukaryotes/prokaryotes?

Post-Transcription Regulation Have mRNA variation –Alternative splicing: shuffling exons –Allows various proteins to be produced in different tissues from the same gene Change the lifespan of mRNA –Produce micro RNA that will damage mRNA, preventing translation Edit RNA & change the polypeptide produced –Insert or alter the genetic code

Translation Regulation mRNA present in cytosol does not necessarily get translated into proteins –Control the rate of translation to regulate gene expression How? –Modify the 5’ cap –Feedback regulation (build up of products = less translation)

Translation Regulation Con’t Modify the lifespan of proteins: –Attach ubiquitin = target for breakdown via proteasome (woodchipper)

So… What are the ways that a cell can regulate gene expression AFTER transcription? How can the process of RNA splicing allow one pre-mRNA to produce 5 different proteins in 5 different tissues? And…

Can you accurately fill in this table?