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Levels at which eukaryotic gene expression is controlled
• Initiating or inhibiting Transcription [majority]– Transcriptional activators, coactivators, repressors
• Promoters• DNA response elements, enhancers, silencers
– DNA packaging and chromatin structure• Histone acetylation, histone variation• DNA methylation
• Initiating or inhibiting Translation– mRNA processing, alternative splicing– RNA stability– RNA silencing
• Initiating or inhibiting protein activity (Post-translational modification)
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Found in the mature mRNA from all cell types
Not found in all mature mRNAs
Alternatively spliced versions vary in function to meet the needs of the different cell types
Brooker, Figure 17.17
1 654 10987 1413121132
1 654 1098 142
1 654 1098 12113
Intron Exon α-tropomyosin pre-mRNA
5′ 3′
5′ 3′
Constitutive exons
Alternativesplicing
or
Alternative exons
Smooth muscle cells
5′ 3′ Striated muscle cells
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Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Brooker Figure 15.17a
1 432
1 432
1 432
1 43
5′
5′ 3′ 5′ 3′ 5′ 3′
3′
5′ 3′
The spliceosomerecognizes all thesplice junctions.
All 4 exons are containedwithin the mRNA.
Splicing repressors
Splicejunctions
5′
5′ 3′ 5′ 3′ 5′ 3′
3′
5′ 3′
A splicing repressor prevents therecognition of a 3′-splice junction.The next 3′-splice junction thatprecedes exon 3 will be chosen.
Exon 2 is skipped and notincluded in the mRNA.
Splicejunctions
Splicingrepressor
Alternativesplicing
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Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Figure 17.18b
1 432 1 432
1 42 1 432
5′
5′ 3′ 5′ 3′ 5′ 3′
3′
5′ 3′
The spliceosome onlyrecognizes 4 of the6 splice junctions.
Exon 3 is included in the mRNA.
Splicing enhancers
Splicejunctions
5′
5′ 3′ 5′ 3′ 5′ 3′
3′
Splicejunctions
Alternativesplicing
The binding of splicing enhancerspromotes the recognition ofpoorly recognized junctions. All 6junctions are recognized.
5′ 3′
Exon 3 is not included in the mRNA.
These 2 splice junctionsare not readily recognizedby the spliceosome.
Splicingenhancer
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Homeotic Transcription
Factors: “Master Genes” turn on
cascade of other genes (which
developmental pathway?)
WT
Homeotic Mutant
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(a) Homeotic gene
(b) Homeotic protein bound to DNA
DNA-binding domain(shown in orange),the Homeodomain
DNA
Homeobox (180 bp)
Transcriptionalactivation domain
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Brooker, Fig 24 - 14
Homeotic Genes often identify transcription
factor proteins with DNA-binding Homeodomains
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Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Drosophila development occurs in specific segments
Fig from iGenetics, Russell
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Embryo(10 hours)
Flychromosome
Adult
Antennapediacomplex
bithoraxcomplex
lab pb Dfd Scr Antp Ubx abd-A Abd-B
Brooker, Fig 24.12
Co-linearity of Homeotic genes
the order of gene expression, from
anterior to posterior, parallels the order of
genes on the chromosome
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Did Drosophila evolution include a homeotic mutation in T3?
Dragonflies and butterflies are evolutionarily older than Drosophila
WT dragonflies and butterflies have four sets of wings
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Specific development of each region and segment of body determined by a homeotic gene
Mouse homeobox genes
Chromosome 6,
Chromosome 11,
Chromosome 15,
Chromosome 2,
•Drosophila have one set of Homeobox genes (“HOM-C genes”)
•Mammals have four sets of Homeobox genes (Mammalian homeobox genes = “Hox genes”)
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Expression of Mouse Homeobox Genes (Hox) coincides with region of
tissue specification
Anterior Posterior
CaudalSacralLumbarThoracicOccipital Cervical
Ver
teb
rae
Expression patternHox A1
Hox B1
Hox A3
Hox D4
Hox A4
Hox A5
Hox B4
Hox B5
Hox A6
Hox A7
Hox D8
Hox B7
Hox C9
Hox D8
Hox D9
Hox D10
Hox D11
Hox D13
Hox D12
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Hox10 and Hox11 Genes AreRequired to Globally Pattern the
Mammalian Skeleton
Deneen M. Wellik and Mario R. Capecchi*Science, Vol 301, p 363. July 2003
(An example of “reverse genetics”, where a known gene sequence is knocked out in order to determine the
phenotype.)
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Axial Skeleton Pattern in Mouse (WT)
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Triple mutant in three Hox 10 genes: Loss of Hox-A10, Hox-C10 and Hox-D10 shows extra ribs (thoracic vertebrae) and
no lumbar or sacral
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Triple mutant in three Hox 11 genes: Loss of Hox-A11, Hox-C11 and Hox-D11 shows extra lumbar vertebrae, no
sacral
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Brooker, Fig 24 - 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Vertebrae:
Vertebrae:
Vertebrae:
Vertebrae: No "neck," no forelimb
Mouse
Chicken
Snake
Goose
Hundreds of vertebrae
44 total vertebrae
36 total vertebrae
36 total vertebrae
Neck vertebrae
HoxC-6
HoxC-6
HoxC-6
HoxC-6
Anterior boundary of HoxC-6 gene expression