Eukaryotic Gene Regulation

1. Eukaryotic Gene Regulation Chapter 18

2. Overview • Eukaryotes can regulate gene expression at multiple stages from gene to functional protein • Regulation of chromatin structure • DNA methylation • Transcription initiation factors • Alternative RNA processing • Protein degradation

3. -- blue = DNA -- orange = RNA -- purple = protein --Each of these is a possible site for regulation, but not all are used in any instance or cell

4. How do we get different cell types? • Red blood cells, muscle cells, neurons… • Every cell has the same genes • Different cells express only a fraction of their genes • 20% of cell’s genes are expressed

5. Histone Acetylation -- DNA level of regulation -- Histone proteins have protruding “tails” -- Acetyl groups can be added to these tails -- Acetylated histones lose their + charge, and are unable to bind to other nucleosomes -- Acetylated histones = transcription more likely

6. Histone Code Hypothesis • Histone tails can be • Acetylated, methylated, or phosphorylated • Methylation = condensation of chromatin • Phosphorylation = separation of histones • So which determines the proteins produced: acetylation or the specific combination of these modifications?

7. DNA Methylation • DNA itself can be methylated as well • Actually methyl groups are attached to the nitrogenous bases of nucleotides • Specifically cytosine • Methylated bases are not able to be expressed • Remember methylation from Inactivated X chromosomes? • Interfere with normal methylation = weird results

8. Important Difference… • Histone acetylation = INCREASED transcription • DNA methylation = DECREASED transcription

9. Why are identical twins different? • They have the same genome, so WTF? • Base-pair mutations are one way to get genetic diversity • Different DNA sequences may be methylated, this results in certain sequences being turned off • So same DNA but phenotypic variation • Identical twins, but one has schizophrenia while the other does not • Called epigenetic inheritance (traits that are NOT contained on nucleotide sequences)

10. Transcriptional Modification • Most important area of regulation or control of gene expression • Was this true in prokaryotes? • Involves Enhancer regions on the DNA • Activator proteins bind to mediator proteins • The complex is called transcription initiation complex • Transcription of the downstream regions is enhanced

12. -- Activator proteins bind to the enhancer region of DNA -- Activator proteins also bind to Mediator proteins + Transcription factors -- Forms transcription initiation complex -- Almost guarantees that the gene will be expressed

13. -- Activator proteins bind to enhancer DNA region -- Different activator proteins = different gene transcribed & expressed -- Activator proteins = directors of transcription in eukaryotes

14. Alternative RNA Splicing -- Spliceosomes can splice the primary RNA transcript differently -- Creates different proteins -- Fruit fly gene = 38,000 different combinations of proteins -- Yet again, is phenotypic variation due to genetic sequences?

15. siRNA Cure for Ebola? • 1.5% of genome codes for proteins • Even smaller amount codes for RNAs (tRNA, mRNA, rRNA) • So is any part of the 98% ever transcribed?

17. miRNA • microRNAs are capable of binding complementary sequences in mRNA molecules • Usually degrades the mRNA it binds OR blocks translation of the mRNA • 1/3 of all genes regulated via miRNAs

18. RNA Interference (RNAi) • Inject dsRNA molecules into a cell • This turns off gene expression of those genes with same sequence as the dsRNA • Small Interfering RNA (siRNA) were the dsRNA responsible for the interference • How did this lead to a treatment for Ebola? • Ebola is an RNA based virus • What about HIV? Hepatitis A or C? common cold? • Dengue fever? influenza? H1N1, H5N1?

19. Skip 18.4

20. Cancer Genes • Oncogenes = cancer-causing genes • Proto-oncogenes = genes that codes for proteins that promote normal cell growth • Proto-oncogenes can become oncogenes • Leads to an increase in protein production • OR an increase in the activity of normal protein production • Either leads to TOO MUCH mitosis

21. Tumor-Suppressor Genes • The produced proteins inhibit cell division • If a mutation decreases production of these products, cell division will accelerate • 2 ways to get neoplastic growths (cancer): • Mutation which alters proto-oncogenes into oncogenes • Over-produces protein OR hyperactive protein production • This interferes with usual mechanism of cell cycle regulation • Mutation interferes with tumor-suppressor genes • Insufficient production leads to mitotic hyperactivity

22. Again…

23. Cell Cycle Stimulator Pathway Mutation in ras? -- Activity even though no growth factor has been received by the RTK -- Outcome = Excessive Mitosis

24. p53 gene -- Commonly called the “guardian angel of the genome” -- Halts cell cycle by binding CdK proteins -- Allows time for DNA repair --p53 is also directly involved in DNA repair --p53 initiates apoptosis if DNA damage is beyond repair

25. MultiStep Model of the Development of Colorectal Cancer