©2000 Timothy G. Standish
Transkripsi
©2000 Timothy G. Standish
All Genes Can’t be Expressed At The Same Time
Some gene products are needed by all cells all the time. These constitutive genes are expressed by all cells.
Other genes are only needed by certain cells or at specific times, expression of these inducible genes is tightly controlled in most cells.
For example, pancreatic b cells make insulin by expressing the insulin gene. If neurons expressed insulin, problems would result.
©2000 Timothy G. Standish
3’
5’
5’
3’
Transcription And Translation In Prokaryotes
Ribosome
Ribosome5’
mRNA
RNAPol.
©2000 Timothy G. Standish
The mRNA Sequence Can Fold In Two Ways
4
1 23
Terminatorharipin
4
1 2
3
©2000 Timothy G. Standish
Expression Control In Eukaryotes Some of the general methods used to control
expression in prokaryotes are used in eukaryotes, but nothing resembling operons is known
Eukaryotic genes are controlled individually and each gene has specific control sequences preceding the transcription start site
In addition to controlling transcription, there are additional ways in which expression can be controlled in eukaryotes
©2000 Timothy G. Standish
Eukaryotes Have Large Complex Geneomes
The human genome is about 3 x 109 base pairs or ≈ 1 m of DNA
Because humans are diploid, each nucleus contains 6 x 109 base pairs or ≈ 2 m of DNA
Some gene families are located close to one another on the same chromosome
Genes with related functions appear to be distributed almost at random throughout the the genome
©2000 Timothy G. Standish
Highly Packaged DNA Cannot be Expressed
Because of its size, eukaryotic DNA must be packaged
Heterochromatin, the most highly packaged form of DNA, cannot be transcribed, therefore expression of genes is prevented
Chromosome puffs on some insect chomosomes illustrate areas of active gene expression
©2000 Timothy G. Standish
Only a Subset of Genes is Expressed at any Given Time
It takes lots of energy to express genes Thus it would be wasteful to express all
genes all the time By differential expression of genes, cells
can respond to changes in the environment Differential expression, allows cells to
specialize in multicelled organisms. Differential expression also allows
organisms to develop over time.
©2000 Timothy G. Standish
DNA
Cytoplasm
NucleusG AAAAAA
Export
Degradation etc.G AAAAAA
Control of Gene Expression
G AAAAAA
RNAProcessing
mRNA
RNA
Transcription
Nuclear pores
Ribosome
Translation
Packaging
Modification
Transportation
Degradation
©2000 Timothy G. Standish
Logical Expression Control Points DNA packaging Transcription RNA processing mRNA Export mRNA masking/unmasking
and/or modification mRNA degradation Translation Protein modification Protein transport Protein degradation
Increasing cost
Increasing cost
The logical place to control expression is before
the gene is
transcribed
The logical place to control expression is before
the gene is
transcribed
©2000 Timothy G. Standish
Three Eukaryotic RNA Polymerases
1RNA Polymerase I - Produces rRNA in the nucleolus, accounts for 50 - 70 % of transcription
2RNA Polymerase II - Produces mRNA in the nucleoplasm - 20 - 40 % of transcription
3RNA Polymerase III - Produces tRNA in the nucleoplasm - 10 % of transcription
©2000 Timothy G. Standish
A “Simple” Eukaryotic Gene
Terminator Sequence
Promoter/Control Region
Transcription Start Site5’ Untranslated Region
3’ Untranslated Region
Exons
Introns
3’5’ Exon 2 Exon 3Int. 2Exon 1Int. 1
RNA Transcript
©2000 Timothy G. Standish
5’DNA
3’
Enhancers
Enhancer Transcribed Region
3’5’ TF TFTF
3’5’ TF TFTF
5’RNA
RNAPol.
RNAPol.
Many bases
Promoter
©2000 Timothy G. Standish
Eukaryotic RNA Polymerase II RNA polymerase is a very fancy enzyme that does many tasks in conjunction with other proteins
RNA polymerase II is a protein complex of over 500 kD with more than 10 subunits:
©2000 Timothy G. Standish
Eukaryotic RNA Polymerase II Promoters
Several sequence elements spread over about 200 bp upstream from the transcription start site make up RNA Pol II promoters
Enhancers, in addition to promoters, influence the expression of genes
Eukaryotic expression control involves many more factors than control in prokaryotes
This allows much finer control of gene expression
©2000 Timothy G. Standish
RNA Pol. II
Initiation
T. F.
RNA Pol. II
5’mRNA
Promoter
T. F.
T. F.
©2000 Timothy G. Standish
Eukaryotic Promoters
5’ Exon 1Promoter
Sequence elements
~200 bp
TATA
~-25
InitiatorInitiator“TATA Box”“TATA Box”
Transcription start site
(Template strand) -1+1SSTATAAAASSSSSNNNNNNNNNNNNNNNNNYYCAYYYYYNN
S = C or G Y = C or T N = A, T, G or C
©2000 Timothy G. Standish
InitiationTFIID Binding
-1+1
Transcription start site
TFIID
“TATA Box”
TBP Associated Factors (TAFs)
TATA Binding Protein (TBP)
©2000 Timothy G. Standish
InitiationTFIID Binding
TFIID
80o Bend
-1+1
Transcription start site
©2000 Timothy G. Standish
InitiationTFIIA and B Binding
TFIID
TFIIA
-1+1
Transcription start site
TFIIB
©2000 Timothy G. Standish
InitiationTFIIF and RNA Polymerase Binding
TFIID
TFIIA
-1+1
Transcription start site
TFIIB
RNA PolymeraseTFIIF
©2000 Timothy G. Standish
InitiationTFIIE Binding
TFIID
TFIIA
-1+1
Transcription start siteRNA Polymerase
TFIIBTFIIFTFIIE
TFIIE has some helicase activity and may by involved in unwinding DNA so that transcription can start
©2000 Timothy G. Standish
InitiationTFIIH and TFIIJ Binding
TFIID
TFIIA
-1+1
Transcription start siteRNA Polymerase
TFIIBTFIIFTFIIE
TFIIH has some helicase activity and may by involved in unwinding DNA so that transcription can start
TFIIH
P PP
TFIIJ
©2000 Timothy G. Standish
InitiationTFIIH and TFIIJ Binding
TFIID
TFIIA
-1+1
Transcription start siteRNA Polymerase
TFIIBTFIIFTFIIE
TFIIH
P PP
TFIIJ
©2000 Timothy G. Standish
InitiationTFIIH and TFIIJ Binding
-1+1
Transcription start site RNA Polymerase
P PP
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