• Day 2!

Chapter 15Eukaryotic Gene

Regulation

Almost all the cells in an organism are genetically identical.

Differences between cell types result from differential gene expression, the expression of different genes by cells with the same genome.

Stages in gene expression that can be regulated in eukaryotic cells

Signal

NUCLEUSChromatin

Chromatin modification:DNA unpacking involvinghistone acetylation and

DNA demethylationDNA

Gene

RNA Exon

Gene availablefor transcription

Transcription

Primary transcript

IntronRNA processing

TailmRNA in nucleus

Transport to cytoplasm

CYTOPLASM

mRNA in cytoplasm

TranslationDegradationof mRNA

Polypeptide

Cap

Protein processing, suchas cleavage and

chemical modification

Active protein

Transport to cellulardestination

Degradationof protein

Cellular function(such as enzymaticactivity, structural support)

Euchromatin

Heterochromatin

Methylation

Acetylation

You Must Know

• The control of gene expression in eukaryotes

• An activator is a protein that binds to an enhancer and stimulates transcription of a gene.

• Activators have two domains, one that binds DNA and a second that activates transcription.

DNAUpstream

Enhancer(distal control

elements)

Proximalcontrol

elements

Exon Intron Exon

Transcriptionstart site

Promoter

Intron Exon

Poly-A signalsequence

Transcriptiontermination

region

Down-stream

Activators

Figure 14.9

TATA box

Promoter Nontemplate strand

Start point

DNA353

5 TA T A A A A

A T A T T T

Transcription factors

353

5

T

Transcription initiation complex

RNA transcript

35

5 3 35

Transcription factors

RNA polymerase II

Templatestrand

In eukaryotes, high levels of transcription of particular genes depend on interaction between control elements and specific transcription factors.

Figure 15.10-3

DNA

EnhancerDistal controlelement

Activators PromoterGene

DNA-bendingprotein

Group of mediator proteins

General transcriptionfactors

TATA box

RNApolymerase II

RNApolymerase II

RNA synthesisTranscriptioninitiation complex

• A particular combination of control elements can activate transcription only when the appropriate activator proteins are present.

Combinatorial Control of Gene Activation

Availableactivators

(a) LIVER CELL NUCLEUS

Crystallin genenot expressed

Albumin geneexpressed

Availableactivators

(a) LIVER CELL NUCLEUS

Figure 15.11b

(b) LENS CELL NUCLEUS

Availableactivators

(b) LENS CELL NUCLEUS

Availableactivators

Albumin genenot expressed

Crystallin geneexpressed

IacI

Allolactose(inducer)

IacZ IacY IacAIacI

DNA lac operon

Permease Transacetylase-Galactosidase

mRNA

Protein

RNA polymerase

mRNA 53

5

Prokaryotes coordinate gene expression with operons.

• Eukaryotes coordinated gene expression with activators.

• The activators recognize specific control elements and promote simultaneous transcription of the genes, even when the genes are on different chromosomes.

Activators

Chromatin modification

Transcription

RNA processing

TranslationmRNAdegradation

Proteinprocessing

and degradation

Alternative RNA splicing

DNA

PrimaryRNAtranscript

mRNA or

Exons

Troponin T gene

RNA splicing

1 2 3 4 5

1 2 3 5 1 2 4 5

1 2 3 4 5

Chromatin modification

Transcription

RNA processing

mRNAdegradation

mRNA

Coding segment

3

5 5 3Cap UTRStart

codonStop

codon UTR Poly-Atail

G P P P AAAAAA

Chromatin modification

Transcription

RNA processing

TranslationmRNAdegradation

Proteinprocessing

and degradation

The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNA.

Chromatin modification

Transcription

RNA processing

TranslationmRNAdegradation

Proteinprocessing

and degradation

Translation of all mRNAs in a cell may be regulated simultaneously. For example, translation initiation factors are simultaneously activated in an egg following fertilization.

Chromatin modification

Transcription

RNA processing

TranslationmRNAdegradation

Proteinprocessing

and degradation