Control of Gene Expression

1. Control of Gene Expression

2. DNA Transcription RNA Translation Protein The Central DogmaFrom DNA to Proteins Genotype Phenotype

3. Review of Replication, Transcription and Translation • DNA Replication • Ch9 Q3 • Note primase and Okazaki fragments • Transcription + Translation • Protein Synthesis

4. Control of Gene Expression • Every somatic cell has the same DNA • Cells are very different because each cell makes certain proteins and not others • How does a cell know which genes to transcribe and which not to?

5. Transcription Factors • Proteins which control the expression of other genes • Link the genome with the environment • Activated by signals from outside the cell (e.g. hormones, sugar, etc.) • Allow RNA polymerase to bind to the promoter so that transcription can begin • Gene must also be exposed –DNA must unwind in that area.

7. RNA Processing • The average total size of a gene is 27,000 bases but the average size of the coding portion is only 1,340 bases! • mRNA transcripts are modified before use as a template for translation: • Addition of capping nucleotide at the 5’ end • Addition of polyA tail to 3’ end • Important for moving transcript out of nucleus and for regulating when translation occurs

8. RNA Processing -Splicing • Splicing occurs, removing internal sequences • Introns are sequences removed • Introns =Intervening sequences • Used to be called Junk DNA! • Exons are sequences remaining • There is alternate splicing of mRNA in different tissues • The # of proteins (~200,000) far outnumbers the # of genes (~20,000) • An intron in one context may be an exon in another context

9. RNA Processing Figure 10.10

10. Translation: Multiple Copies of a Protein Are Made Simultaneously Figure 10.16

11. Protein Conformation • Primary (1) structure • Sequence of amino acids • Secondary (2) structure • Folding of the protein into -helices and -pleated sheets • Tertiary (3) structure • 3D shape • Quaternary (4) structure • Complex with other polypeptides (same or different proteins)

13. The Proteasome • Misfolded proteins have ubiquitin molecules attached to them • Ubiquitinated proteins are sent to the proteasome to be degraded Listen up for a possible BONUS test question

14. Prions • Sole difference between normal and disease protein is conformation • All mammals have PrP but nearby proteins and polysaccharides keep it correctly folded Infectious PrPSc conformation with -sheets Normal PrPC conformation with many -helices

15. Spread of Prion Disease PrPC PrPSc

16. Spongiform Encephalopathies

17. Gene Expression Can Change Over Time • Example: Globin chain switching • Hemoglobin molecules • Transport oxygen molecules in the blood • Composed of 4 globular proteins

18. Globin Chain Switching Figure 11.2

19. Proteomics • Looking at all of the proteins made in a particular cell (or tissue, organ, etc.) • i.e. the proteome

20. Chromatin Remodeling • DNA is wrapped around histones to form nucleosomes • Chromosome packaging • Acetylation =acetyl groups are added to histones • Exposes the primer so RNA polymerase can bind and transcription can begin • Deacetylation =acetyl groups are removed from histones

21. RNA Interference • Occasionally, both DNA strands are transcribed • Complementary strands bind to one another • Gene sequence may allow formation of a “hairpin loop” • RNA strand binds to itself • Segments of dsRNA attract RNA-induced silencing complexes (RISCs) • Can be used experimentally (clinically?)

22. RNA Interference