eukaryotic gene expression
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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
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