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Psalm 102:24

Psalm 102:24 25 In the beginning you laid the foundations of the earth, and the heavens are the work of your hands. Initiation of Transcription: In Eukaryotes. Timothy G. Standish, Ph. D. Expression Control In Eukaryotes.

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Psalm 102:24

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  1. Psalm 102:24 25 In the beginning you laid the foundations of the earth, and the heavens are the work of your hands.

  2. Initiation of Transcription:In Eukaryotes Timothy G. Standish, Ph. D.

  3. 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

  4. 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

  5. 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

  6. 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.

  7. Control of Gene Expression Cytoplasm Nuclear pores Degradation AAAAAA AAAAAA DNA Transcription Modification RNA RNA Processing G G Degradation etc. Ribosome mRNA G AAAAAA Export Translation Nucleus Packaging Transportation

  8. Logical Expression Control Points Increasing cost • DNA packaging • Transcription • RNA processing • mRNA Export • mRNA masking/unmasking and/or modification • mRNA degradation • Translation • Protein modification • Protein transport • Protein degradation The logical place to control expression is before the gene is transcribed

  9. Three Eukaryotic RNA Polymerases • RNA Polymerase I - Produces rRNA in the nucleolus, accounts for 50 - 70 % of transcription • RNA Polymerase II - Produces mRNA in the nucleoplasm - 20 - 40 % of transcription • RNA Polymerase III - Produces tRNA in the nucleoplasm - 10 % of transcription

  10. A “Simple” Eukaryotic Gene Transcription Start Site 3’ Untranslated Region 5’ Untranslated Region Introns 5’ 3’ Int. 1 Int. 2 Exon 1 Exon 2 Exon 3 Promoter/ Control Region Terminator Sequence Exons RNA Transcript

  11. Enhancers DNA 5’ 3’ Enhancer Promoter Transcribed Region 3’ 5’ TF 3’ 5’ TF TF RNA Pol. RNA Pol. RNA 5’ Many bases TF TF TF

  12. 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:

  13. 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

  14. Initiation Promoter T. F. RNA Pol. II T. F. RNA Pol. II mRNA 5’ T. F.

  15. Eukaryotic Promoters Exon 1 Promoter 5’ Sequence elements TATA ~200 bp Transcription start site “TATA Box” Initiator SSTATAAAASSSSSNNNNNNNNNNNNNNNNNYYCAYYYYYNN -1+1 S = C or G Y = C or T N = A, T, G or C (Template strand) ~-25

  16. InitiationTFIID Binding TFIID TBP Associated Factors (TAFs) Transcription start site “TATA Box” -1+1 TATA Binding Protein (TBP)

  17. InitiationTFIID Binding 80o Bend Transcription start site TFIID -1+1

  18. InitiationTFIIA and B Binding TFIIB Transcription start site TFIID -1+1 TFIIA

  19. InitiationTFIIF and RNA Polymerase Binding TFIIB Transcription start site TFIID -1+1 TFIIA RNA Polymerase TFIIF

  20. InitiationTFIIE Binding TFIIB Transcription start site TFIIE TFIID RNA Polymerase TFIIF -1+1 TFIIA TFIIE has some helicase activity and may by involved in unwinding DNA so that transcription can start

  21. InitiationTFIIH and TFIIJ Binding TFIIH TFIIB P P P Transcription start site TFIIJ TFIIE TFIID RNA Polymerase TFIIF -1+1 TFIIA TFIIH has some helicase activity and may by involved in unwinding DNA so that transcription can start

  22. InitiationTFIIH and TFIIJ Binding TFIIH TFIIB P P P Transcription start site TFIIJ TFIIE TFIID TFIIF RNA Polymerase -1+1 TFIIA

  23. InitiationTFIIH and TFIIJ Binding P P P Transcription start site RNA Polymerase -1+1

  24. The End

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