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Both prokaryotes and eukaryotes

Both prokaryotes and eukaryotes. Must alter their patterns of gene expression in response to changes in environmental conditions How do prokaryotes do this??? Now, let’s learn how eukaryotes do it!!!. Regulation of enzyme production. Regulation of enzyme activity. Precursor. Feedback

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Both prokaryotes and eukaryotes

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  1. Both prokaryotes and eukaryotes • Must alter their patterns of gene expression in response to changes in environmental conditions • How do prokaryotes do this??? • Now, let’s learn how eukaryotes do it!!!

  2. Regulation of enzyme production Regulation of enzyme activity Precursor Feedback inhibition Enzyme 1 Gene 1 LE 18-20 Gene 2 Enzyme 2 Regulation of gene expression Gene 3 Enzyme 3 Enzyme 4 Gene 4 Gene 5 Enzyme 5 Tryptophan

  3. Regulation of Gene Expression • During development  cells of a multicellular organism undergo a process of specialization in form and function called cell differentiation • Most gene expression is regulated at the level of TRANSCRIPTION. • Each cell of a multicellular eukaryote has SAME DNA • BUT…Expresses only a fraction of its genes

  4. PRE- TRANSCRIPTIONAL MODIFICATION POST- TRANSCRIPTIONAL MODIFICATION

  5. POST- TRANSLATIONAL MODIFICATION aa’s

  6. Pre Transcriptional Regulation (BEFORE mRNA is made)

  7. Chromatin changes Transcription RNA processing Translation mRNA degradation Protein processing and degradation Histone tails DNA double helix HistoneModification • Chemically modifying histones • affects the configuration of chromatin  affects gene expression

  8. Acetylated histones Unacetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription 1. Histone acetylation (add an acetyl group, –COCH3) • Acetylated histones causes chromatin to have a looser structure/configuration • Enhances transcription b/c transcription proteins and enzymes can gain access to genes

  9. 2. DNA Methylation • Addition of methyl groups (-CH3) to certain bases after DNA synthesis • reduces transcription • Ex: genes not expressed (therefore not transcribed) are heavily methylated

  10. Enhancer (distal control elements) Poly-A signal sequence Termination region Proximal control elements Exon Intron Intron Exon Exon DNA Downstream Upstream Promoter Transcription Poly-A signal Exon Exon Intron Intron Exon Cleared 3 end of primary transport Primary RNA transcript (pre-mRNA) 5 Chromatin changes RNA processing: Cap and tail added; introns excised and exons spliced together Transcription Intron RNA RNA processing Coding segment mRNA degradation Translation mRNA P Protein processing and degradation G P P Start codon Poly-A tail Stop codon 3 UTR (untranslated region) 5 Cap 5 UTR (untranslated region) Organization of a Typical Eukaryotic Gene • Associated with most eukaryotic genes are multiple control elements • Segments of noncoding DNA that help regulate transcription by binding certain proteins Figure 19.5

  11. Regulation of Transcription Initiation • A cluster of proteins called the transcription-initiation complex assembles on promoter. Consists ofproteins called transcription factors and RNA polymerase. * Transcription factors: regulatory proteins that bind to DNA (specifically @ promoter) & stimulates transcription of a gene. Regulate whether RNA polymerase can bind to promoter.

  12. Activators and Repressors • Some transcription factors (protein) are repressors • inhibit expression of particular gene • An activator is a protein that binds to a part of DNA called the enhancer & stimulates transcription of a gene • Some activators and repressors • Act indirectly by influencing chromatin structure (activators cause acetylation, repressors cause de-acetylation/“silencing”)

  13. Eukaryotic Gene Expression • http://wps.prenhall.com/wps/media/objects/487/499125/CDA15_1/CDA15_1b/CDA15_1b.htm http://wps.prenhall.com/wps/media/objects/487/499125/CDA15_1/CDA15_1a/CDA15_1a.htm • http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html# • Animation #3: Enhancers and repressors

  14. Review Questions • 1. Where does the transcription initiation complex form? • At the promoter • 2. What is an activator? • A regulatory protein/transcription factor that speeds up/stimulates transcription • 3. Where do activators bind? • To sequences of DNA called enhancers • 4. What is a repressor? • A regulatory protein/transcription factor that inhibits/slows down transcription

  15. An activatorIs a protein that binds to an enhancer and stimulates transcription of a gene Activator proteins bind to the enhancer DNA bending proteins brings activators close to promoter. Activators bind to transcription factors to form initiation complex on promoter

  16. The combination of transcription factors (repressor & activators) binding to regulatory regions (ie. enhancers) at any one time determines how much, if any, of the gene product (ie. albumin) is produced.

  17. Good Animation over the topics we just covered • http://wps.prenhall.com/wps/media/objects/487/499125/CDA15_1/CDA15_1a/CDA15_1a.htm 1) Is most chromatin loose or tightly coiled? Tightly coiled 2) What are the names of the proteins which chromatin wraps around? Histone proteins 3) Why is DNA attracted to histone proteins? DNA is negatively charged, while histone proteins are positively charged

  18. 4) What is a [transcription] activator? A regulatory protein, which can alter chromatin structure and stimulates transcription/gene expression 5) What is one way that chromatin can be chemically modified? By the addition of acetyl groups to histone proteins 6) Does this increase or decrease transcription/gene expression? Increase 7) What do transcription factors do? Attract RNA polymerase to the start of the gene (the promoter)

  19. Post Transcriptional Regulation (AFTER mRNA is made)

  20. 1. Alternative RNA splicing Different mRNA molecules are made from same pre-mRNA, depending on which RNA segments are treated as exons and which as introns

  21. The bound miRNA can base-pair with any target mRNA that contains complementary sequence. The miRNA-protein complex prevents gene expression either by degrading the target mRNA or by blocking its translation. The micro- RNA (miRNA) precursor folds back on itself, held together by hydrogen bonds. One strand of each short double- stranded RNA is degraded; the other strand (miRNA) then associates with a complex of proteins. An enzyme (Dicer) moves along double- stranded RNA, cutting it into segments. 3 2 Chromatin changes Transcription RNA processing mRNA degradation Translation Protein complex Protein processing and degradation Dicer Degradation of mRNA OR miRNA Target mRNA Hydrogen bond Blockage of translation 2. RNA interference by single-stranded microRNAs (miRNAs) • Can lead to degradation of an mRNA or block its translation 5

  22. RNAi Video (15 min) • http://www.pbs.org/wgbh/nova/body/rnai.html Other Animations • http://highered.mcgraw-hill.com/sites/0072835125/student_view0/animations.html# • http://www.hippocampus.org/Biology

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