Transkripsi

1. Transkripsi

2. 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.

3. 5’ 3’ 3’ 5’ RNA Pol. Ribosome mRNA Ribosome 5’ Transcription And Translation In Prokaryotes

4. 1 2 1 2 3 3 4 4 The mRNA Sequence Can Fold In Two Ways Terminator haripin

5. 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

6. 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

7. 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

8. 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.

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

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

11. 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

12. 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 A “Simple” Eukaryotic Gene

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

14. 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:

15. 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

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

17. 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 Eukaryotic Promoters (Template strand) ~-25

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

19. 80o Bend Transcription start site InitiationTFIID Binding TFIID -1+1

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

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

22. TFIIB Transcription start site InitiationTFIIE Binding 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

23. TFIIH TFIIB P P P Transcription start site InitiationTFIIH and TFIIJ Binding 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

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

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