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?