Eukaryotic Gene Expression

Transduction

Transformation

Conjugation

Transposition

Differential Gene Expression

If all cells have the same genome, how do cells become differentiated in a multicellular organism?

only ~20% genes expressed in a typical cell

addition of methyl groups to bases of DNA

can condense chromatin and lead to reduced transcription

histone acetylation vs DNA methylation

acetyl groups are attached to histone tails

loosens chromatin structure, promoting transcription

Figure 15.7

Nucleosome

Unacetylated histonesAcetylated histones

Histonetails

Epigenetics

inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance

Epigenetic modifications can be reversed, unlike mutations in DNA sequence

Can be passed to future generations

Regulation of Transcription

provide initial control of gene expression by making a region of DNA either more or less able to be transcribed

Figure 15.6aSignal

NUCLEUSChromatin

Chromatin modification:DNA unpacking involvinghistone acetylation and

DNA demethylationDNA

Gene

RNA Exon

Gene availablefor transcription

Transcription

Primary transcriptIntron

RNA processingTail

mRNA in nucleus

Transport to cytoplasm

Cap

CYTOPLASM

Organization of a Eukaryotic Gene

control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription

Figure 15.8

DNAUpstream

Enhancer(distal control

elements)

Proximalcontrol

elementsTranscription

start siteExon Intron Exon

Promoter

Intron Exon

Poly-A signalsequence

Transcriptiontermination

region

Down-stream

Transcription

Exon Intron IntronExon Exon

Poly-Asignal

Primary RNAtranscript(pre-mRNA)

5 Cleaved 3end ofprimarytranscriptIntron RNA

mRNA

RNA processing

Coding segment

3

5 5 3Cap UTRStart

codonStop

codon UTR Poly-Atail

G P P P AAAAAA

Proximal control elements are located close to the promoterDistal control elements, or enhancers, may be far away from a gene

Transcription FactorsFigure 15.10-3

DNAEnhancer Distal control

element

Activators PromoterGene

TATA box

DNA-bendingprotein Group of mediator proteins

General transcriptionfactors

RNApolymerase II

RNApolymerase II

RNA synthesisTranscriptioninitiation complex

• To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors

• control elements must interact with specific transcription factors

Transcription FactorsFigure 15.10-3

DNAEnhancer Distal control

element

Activators PromoterGene

TATA box

DNA-bendingprotein Group of mediator proteins

General transcriptionfactors

RNApolymerase II

RNApolymerase II

RNA synthesisTranscriptioninitiation complex

activator is a protein that binds to an enhancer and stimulates transcription

Bound activators are brought into contact with a group of mediator proteins through DNA bending

The mediator proteins in turn interact with proteins at the promoter forming transcription initiation complex

Transcription Factors

Combinatorial Control of Gene Activation

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

Figure 15.11Albumin gene

Crystallin gene

Promoter

Promoter

(b) LENS CELL NUCLEUS

Availableactivators

Albumin genenot expressed

Crystallin geneexpressed

Crystallin genenot expressed

Albumin geneexpressed

Availableactivators

(a) LIVER CELL NUCLEUS

Controlelements Enhancer

Enhancer

Coordinately Controlled Genes in Eukaryotes

Unlike the genes of a prokaryotic operon, each of the co-expressed eukaryotic genes has a promoter and control elements

These genes can be scattered over different chromosomes, but each has the same combination of control elements

Activators recognize specific control elements and promote simultaneous transcription of the genes

Post-Transcriptional Regulation alternative RNA splicing, different mRNA molecules are

produced from the same primary transcriptFigure 15.12

DNA

PrimaryRNAtranscript

mRNA or

Exons

RNA splicing

1 2 3 4 5

1 2 3 5 1 2 4 5

1 2 3 4 5

RNA interference (RNAi) and MicroRNAs

MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to complementary mRNA sequences

These can degrade the mRNA or block its translation

Figure 15.13miRNA

miRNA-proteincomplex

Translation blockedmRNA degraded

The miRNA bindsto a target mRNA.

1

If bases are completely complementary, mRNA is degraded.If match is less than complete, translation is blocked.

2

bli-1 worm: Notice the large clear area on the side of the worm (a blister in the cuticle)

dpy-10 adult: Dumpy worms are shorter and wider than wild-type

rol-6 adult: Roller worms have twisted

bodies and roll in circles.

unc-22 worms: unc-22 worms tend to lie still, are often

outstretched (not S-shaped), and twitch.

Wild-type adult:. Wild-type worms are very active and move sinusoidally

 

 

Summary of mutant phenotypes

http://www.pbs.org/wgbh/nova/body/rnai.html

RNAi Nova Video

How do you think this technology can

help with the treatment of Huntington's

disease?

Hint: Huntington's disease is caused by a single autosomal dominant mutant gene. The gene produces a protein that causes brain abnormalities, which interfere with coordination, speech, and metal abilities.

The discovery of RNAi has made it possible for researchers to switch genes on and off at will, simply by inserting double-stranded RNA into cells. It also holds the promise of allowing medical scientists to turn off the expression of genes from viruses and cancer cells, and it may provide new ways to treat and perhaps even cure diseases.

Regulatin’ Genes video

http://www.youtube.com/watch?v=9k_oKK4Teco