Transcriptional control in Eukaryotes: (chapter 13 pp276) Chromatin structure affects gene expression. Chromatin Array of nuc

Transcription

1 Transcriptional control in Eukaryotes: (chapter 13 pp276) Chromatin structure affects gene expression Chromatin Array of nuc 1

2 Transcriptional control in Eukaryotes: Chromatin undergoes structural changes Condensed structure Open structure How does this happen? Inaccessible to txn machinary Transcription inhibited Accessible to txn machinary Transcription Epigenetic gene control : Controlling gene expression by causing changes in chromatin structure 2

3 Transcriptional control in Eukaryotes: Chromatin structure affects gene expression 3

4 Transcriptional control in Eukaryotes: Nucleosome : Basic unit of chromatin :DNA+ 8 Histones Histone tail Short stretch of amino acid at the N- terminal end of histone Protrude out from Core nucleosome 4

5 Transcriptional control in Eukaryotes: Site of covalent modifications e.g., acetylation, phosphorylation, methylation, ubiquitination Histone tail 5

6 Pattern of modifications at histone tails affect degree of chromatin compaction and transcription Histone code 6

7 Example of histone modifications Histone acetyltransferase Ac Ac Ac Ac Ac Ac Ac Activator binds to regulatory element Recruit proteins that acetylate histone tails Transcriptional machinery 7

8 DNA methylation DNA methylation Attaches methyl groups to cytosine Chromatin compaction & TXN inhibition 8

9 The epigenetics of identical twins 9

10 The epigenetics of identical twins 10

11 Gene regulation in post-transcription level DNA > RNA > Transcription RNA processing >Protein 1) 5 capping 2) 3 Poly A tails 11

12 1) 5 capping RNA capping and addition of poly A tail 7-methyl guanosine cap Attachment of modified Guanosine at the 5 end of RNA 1) Required for proper exit of RNA to the cytoplasm 2) promote Ribosome binding Fig

13 RNA capping and addition of poly A tail 2) 3 poly A Poly Adenine tail Make mrna more stable Aid exit to cytosol Fig

14 RNA capping and addition of poly A tail 14

15 Gene regulation in post-transcription level DNA > RNA > Transcription RNA processing >Protein 1) 5 capping 2) 3 Poly A tails splicing 15

16 RNA Splicing DNA Translated into protein Premature RNA Exon 1 Exon 2 Exon 3 Intron 1 Intron 2 RNA splicing Intervening seq between exons Mature RNA How does a cell know where to cut? 16

17 RNA Splicing 5 splicing site 3 splicing site Branch site : important role in splicing 5 splicing site and branch site are recognized by 2snRNPs U1 snrnp snrnp U2 snrnp (small nuclear Ribonucleoprotein) =containing RNA and protein 17

18 RNA Splicing 5 splicing site 3 splicing site 5 site is cut Branch site : important role in splicing 5 splicing site and branch site are recognized by 2snRNPs U1 snrnp U2 snrnp 3 site is cut Additional snrnp Spliceosome Creation of a loop Mature RNA Fig

19 RNA Splicing RNA processing 19

20 Fig pre mrna Gene regulation at the level of RNA processing Cell type A Cell type B Mature mrna Produce proteins with Protein A significant differences Protein A Alternative splicing 20

21 An Example of alternative splicing SXL gene: involved in expression of female specific genes pre mrna Exon1 Exon2 Exon3 Exon4 Male Mature mrna Exon1 Exon2 Exon3 Exon4 Female Exon1 Exon2 Exon4 Stop codon Functional Sxl protein Non-functional protein Expression of female Specific genes 21

22 Gene regulation at the various levels DNA > Transcription RNA > Alternative splicing >Protein Advantage: Different type of cells require functions of different types of proteins to keep their identity One gene One gene Single protein Multiple proteins Minimize genome size 22

23 Gene regulation at the various levels DNA > Transcription RNA > Alternative splicing >Protein RNA interference 23

24 RNA interference (RNAi) Silencing of mrna by small non-coding RNA molecules e.g., micro RNA Widely found in animals and plants 24

25 Pre-miRNA 1) Transcription 2) Export to cytoplasm 3) Processing RISC 3) Associate with RISC 4) Bind to its target mrna and silence (mrna degradation or inhibition of translation) 25 Similar to Fig 13.21

26 Discovery of RNA interference (RNAi) 26

27 The use of RNA interference Gene X with unknown function What does it do? How? Introduce double stranded small RNA Sequence complementary to Gene X mrna Gene X Gene X mrna mrna silencing 27

28 The use of RNA interference Q> What are the genes involved in cell survival? sirna library: Collection of small double stranded RNA that target different genes Each well contains same number of cells Add different sirna to different well Examine which well has decreased cell number? A B C RNAi screening 28

29 Gene expression DNA > RNA > Transcription RNA processing >Protein Translation 1) 5 capping 2) 3 Poly A tails splicing 29