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Regulation of Gene Expression Chapter 18

Regulation of Gene Expression Chapter 18. Gene expression. Flow of genetic information Genotype to phenotype Genes to proteins Proteins not made at random Specific purposes Appropriate times. Control of gene expression. Selective expression of genes

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Regulation of Gene Expression Chapter 18

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  1. Regulation of Gene ExpressionChapter 18

  2. Gene expression • Flow of genetic information • Genotype to phenotype • Genes to proteins • Proteins not made at random • Specific purposes • Appropriate times

  3. Control of gene expression • Selective expression of genes • All genes are not expressed at the same time • Expressed at different times

  4. Prokaryote regulation

  5. Control of gene expression • Regulate at transcription • Gene expression responds to • Environmental conditions • Type of nutrients • Amounts of nutrients • Rapid turn over of proteins

  6. Precursor Feedback inhibition Fig. 18-2 trpE gene Enzyme 1 trpD gene Regulation of gene expression trpC gene Enzyme 2 trpB gene Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production

  7. Prokaryote • Anabolism: • Building up of a substance • Catabolism: • Breaking apart a substance

  8. Prokaryote • Operon • Section of DNA • Enzyme-coding genes • Promoter • Operator • Sequence of nucleotides • Overlaps promoter site • Controls RNA polymerase access to the promoter

  9. Prokaryote • Multiple genes are expressed in a single gene expression • trp operon • Trytophan • Synthesis • Lac operon • Lactose • Degradation

  10. Prokaryote • trp Operon: • Control system to make tryptophan • Several genes that make tryptophan • Regulatory region

  11. Fig. 18-3a trp operon Promoter Genes of operon trpD trpE trpC trpB trpA Operator Stop codon Start codon mRNA 5 RNA polymerase mRNA 5 B A D C E Polypeptide subunits that make up enzymes for tryptophan synthesis

  12. Prokaryote • ⇧tryptophan present • Bacteria will not make tryptophan • Genes are not transcribed • Enzymes will not be made • Repression

  13. Prokaryote • Repressors • Proteins • Bind regulatory sites (operator) • Prevent RNA polymerase attaching to promoter • Prevent or decrease the initiation of transcription

  14. Prokaryote • Repressors • Allosteric proteins • Changes shape • Active or inactive

  15. Prokaryote • ⇧tryptophan • Tryptophan binds the trp repressor • Repressor changes shape • Active shape • Repressor fits DNA better • Stops transcription • Tryptophan is a corepressor

  16. Fig. 18-3b-2 DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

  17. Prokaryote • ⇩tryptophan • Nothing binds the repressor • Inactive shape • RNA polymerase can transcribe

  18. Fig. 18-3a trp operon Promoter Promoter Genes of operon DNA trpD trpR trpE trpC trpB trpA Operator Regulatory gene Stop codon Start codon 3 mRNA 5 RNA polymerase mRNA 5 B A D C E Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on

  19. Prokaryote • Lactose • Sugar used for energy • Enzymes needed to break it down • Lactose present • Enzymes are synthesized • Induced

  20. Prokaryote • lac Operon • Promoter • Operator • Genes to code for enzymes • Metabolize (break down) lactose

  21. Prokaryote • Lactose is present • Repressor released • Genes expressed • Lactose absent • Repressor binds DNA • Stops transcription

  22. Prokaryote • Allolactose: • Binds repressor • Repressor releases from DNA • Inducer • Transcription begins • Lactose levels fall • Allolactose released from repressor • Repressor binds DNA blocks transcription

  23. Fig. 18-4b lac operon lacY DNA lacI lacZ lacA RNA polymerase 3 mRNA mRNA 5 5 Permease Transacetylase -Galactosidase Protein Inactive repressor Allolactose (inducer) (b) Lactose present, repressor inactive, operon on

  24. Fig. 18-4a Regulatory gene Promoter Operator lacI lacZ DNA No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off

  25. Prokaryote • Lactose & tryptophan metabolism • Adjustment by bacteria • Regulates protein synthesis • Response to environment • Negative control of genes • Operons turned off by active repressors • Tryptophan repressible operon • Lactose inducible operon

  26. Prokaryote

  27. Prokaryote • Activators: • Bind DNA • Stimulate transcription • Involved in glucose metabolism • lac operon

  28. Prokaryote • Activator: • Catabolite activator protein(CAP) • Stimulates transcription of operons • Code for enzymes to metabolize sugars • cAMP helps CAP • cAMP binds CAP to activate it • CAP binds to DNA (lac Operon)

  29. Prokaryote • Glucose elevated cAMP low • cAMP not available to bind CAP • Does not stimulate transcription • Bacteria use glucose • Preferred sugar over others.

  30. Prokaryote • lac operon • Regulated by positive & negative control • Low lactose • Repressor blocks transcription • High lactose • Allolactose binds repressor • Transcription happens

  31. Prokaryote • lac operon • Glucose also present • CAP unable to bind • Transcription will proceed slowly • Glucose absent • CAP binds promoter • Transcription goes quickly

  32. Eukaryote gene expression • All cells in an organism have the same genes • Some genes turned on • Others remain off • Leads to development of specialized cells • Cellular differentiation

  33. Eukaryote gene expression • Gene expression assists in regulating development • Homeostasis • Changes in gene expression in one cell helps entire organism

  34. Control of gene expression • Chromosome structure • Transcriptional control • Posttranscriptional control

  35. Signal NUCLEUS Fig. 18-6 Chromatin Chromatin modification DNA Gene available for transcription Gene Transcription RNA Exon Primary transcript Intron RNA processing Tail mRNA in nucleus Cap Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Translation Degradation of mRNA Polypeptide Protein processing Active protein Degradation of protein Transport to cellular destination Cellular function

  36. Eukaryotes • 1. DNA is organized into chromatin • 2. Transcription occurs in nucleus • 3. Each gene has its own promoter

  37. Chromatin structure • DNA is tightly packaged • Heterochromatin: • Tightly packed • Euchromatin: • Less tightly packed • Influences gene expression • Promoter location • Modification of histones

  38. Chromatin structure • Histone acetylation • Acetyl groups (-COCH3) • Attach to Lysines in histone tails • Loosen packing • Histone methylation • Methyl groups (-CH3) • Tightens packing

  39. Fig. 18-7 Histone tails Amino acids available for chemical modification DNA double helix (a) Histone tails protrude outward from a nucleosome Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription

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