REGULATION of GENE EXPRESSION

1. REGULATION of GENE EXPRESSION

2. GENE EXPRESSION • all cells in one organism contain same DNA • every cell has same genotype • phenotypes differ • skin cells have different structure & function from muscle cells

3. GENE EXPRESSION • differences -due to differences in gene expression • some genes are turned on • others are turned off in different cells • functionally eliminates particular cell from doing certain functions • cell cannot make proteins needed to do certain functions

4. GENE EXPRESSION • expression of most genes is controlled at transcription • some genes are actively transcribed • others remain quiescent • some function at all times • 30,000 are expressed in nearly all cell types • housekeeping genes • carry out basic metabolic processes • called constitutive • other genes are regulated • turned on or off as needed

5. Transcription Factors • proteins which bind to promoter & enhancerregions of DNA to turn on (or off) genes • ability to be turned on is inducible • ability to be turned off is repressible • genes are most often regulated as a group • located next to one another on a chromosome • these genes along with their regulatory sequences of DNA are called an operon

6. The Lac Operon • E. coli cells • use different sugars for energy • glucose & lactose • ability to use lactose requires special enzymes • transacetylase • lactose permease • beta-galactosidase • genes for these enzymes are found on a single unit-operon

7. The Lac Operon • tells cell machinery to make or not to make enzymes • Consists of genes that make enzymes , promoter & operator-control sequences • promoter region • transcription enzyme-RNA polymerase attaches • begins transcription • operator • functions as switch • determines if RNA polymerase can attach to promoter region

8. Lac Operon • transcription of 3 enzymes is repressed-turned off by repressor protein • binds to operator • blocks attachment of RNA polymerase • regulatory gene located outside operon codes for repressor • regulatory gene is expressed all the time • if regulatory gene is always being transcribed • there is always repressor protein to stop transcription of enzymes needed to use lactose • How is lacoperon turned on? • lactosein environment

9. Lac Operon • lactose binds to repressor protein changes its shape. • new shape means it cannot bind to active site of operatorsite is turned on • RNA polymerase attaches • transcription of enzymes needed to metabolize lactose begins • genes that code for enzymes that lets cell use lactose are made only when lactose is present • induction • presence of a small molecule causes enzymes to be made

12. trpoperon • bacteria • repressor-inactive alone • to be active combines with specific small molecule • that small molecule is amino acid- tryptophan • E. coli can make tryptophan using enzymes in trp operon but if tryptophan do not make their own • tryptophan binds to repressor • activates repressor • turns off operon • when tryptophan is not present repressor is not active operon is turned ontryptophan is made

13. Repressor Operon • arginine is an essential amino acid • when plentifule. coli cellsuse it • arginine not presente. coli must make it • requires enzymes • mechanism allows e. coli cells to save cellular resources by shutting genes off for particular substance when substance is available

14. Gene Regulation in Eukaryotes • cells differ in appearance & function • inherit same, complete set of genetic information • differences in appearance & function is not due to different genes • differences due to genes being turned on or off • cells performing particular functions are termed specialized • during development cells differentiate & stay differentiated • terminally differentiated

15. Gene Expression-Eukaryotes • begins at chromosome level • DNA in one chromosome is about 4 cm long • entire amount can fit into nucleus because of way it is packaged

16. DNA PACKAGING • DNA helix is wound around small proteins- histones • DNA-histone complex looks like beads on a string • each bead-nucleosome • segment of DNA wound around 8 histones • short DNA segments-linkers make up string part between nucleosomes

17. DNA PACKAGING • beaded strings are wrapped into tight helical fibers • which in turn are coiled into supercoils • looping & folding further compacts DNA

18. DNA PACKAGING • extreme packaging is important in gene regulation • prevents gene expression by preventing transcription proteins from contacting DNA • some regions-heterochromatin • so condensed-never transcribed • 10% of genome • remainder of complex- euchromatin • less condensed • can be transcribed • 10% is active at any given time

19. Fine Control of Transcription in Eukaryotic Cells • fine tuning is done with control of RNA synthesis-transcription • most important way of regulating gene expression

20. Control of Transcription in Eukaryotic Cells • regulatory proteins bind to DNA to turn transcription of genes on & off • each eukaryotic gene has its own promoter & other control sequences • Activator proteins are more important in eukaryotic cells than in prokaryotic cells • in most eukaryotic organisms genes are turned off • small percentage of genes must be turned on for any one particular cell to make proteins required to carry out its particular job

21. Control of Transcription in Eukaryotic Cells • regulatory proteins in eukaryotic cells are transcription factors • required for RNA polymerase to transcribe DNA

22. Control of Transcription in Eukaryotic Cells • first step in gene transcription is binding of transcription factors to DNA sequences-enhancers • usually far away from genes they regulate • binding of activators to enhancers causes DNA to change shape • it bends • with bending bound activators can interact with transcription factor proteins which act as a complex at promoter area of gene • this complex promotes attachment of RNA polymerase to promoter transcription begins • there are also repressor proteins- silencers • inhibit transcription

23. Splicing & Regulation • transcription of DNA  mRNA • used to make a specific protein by translation • mRNA can be regulated by splicing

24. Splicing & Regulation • during splicing certain segments of RNA are eliminated • the way a piece of mRNA is spliced giving rise to different types of mRNA • gives rise to different proteins

25. Regulation of Translation • after mRNA has been fully processed and is in the cytoplasm other regulatory processes may occur • mRNA breakdown • initiation of translation • protein activation • protein breakdown

26. mRNA Breakdown • mRNA molecules do not stay intact forever • broken down by enzymes • time of breakdown is important • regulates amount of protein that is made • longer living mRNAs can make more protein

27. Initiation of Translation • many proteins control initiation of translation of RNA • in red blood cells, translation does not occur unless heme is present

28. Protein Activation • after translation is complete proteins often need altering to become functional • many made as proenzyme • Inactive • cleaving part of protein makes it functional

29. Protein Breakdown • proteins can be broken down after a short or after a long time • broken down after short timehave limited time to carry out functions • may be important in short term regulatory activity in cells