Division Ave. High School AP Biology

Transcription

1 Control of Eukaryotic Genes The BIG Questions n How are genes turned on & off in eukaryotes? n How do cells with the same genes differentiate to perform completely different, specialized functions? Evolution of gene regulation n Prokaryotes u single-celled u evolved to grow & divide rapidly u must respond quickly to changes in external environment n exploit transient resources n Gene regulation u turn genes on & off rapidly n flexibility & reversibility u adjust levels of enzymes for synthesis & digestion 1

2 Evolution of gene regulation n Eukaryotes u multicellular u evolved to maintain constant internal conditions while facing changing external conditions n homeostasis u regulate body as a whole n growth & development w long term processes n specialization w turn on & off large number of genes n must coordinate the body as a whole rather than serve the needs of individual cells Points of control nthe control of gene expression can occur at any step in the pathway from gene to functional protein 1. packing/unpacking DNA 2. transcription 3. mrna processing 4. mrna transport 5. translation 6. protein processing 7. protein degradation 1. DNA packing How do you fit all that DNA into nucleus? u DNA coiling & folding n double helix n nucleosomes n chromatin fiber n looped domains n chromosome from DNA double helix to condensed chromosome 2

3 Nucleosomes n Beads on a string u 1 st level of DNA packing u histone proteins n 8 protein molecules n positively charged amino acids n bind tightly to negatively charged DNA 8 histone molecules DNA packing movie DNA packing as gene control n Degree of packing of DNA regulates transcription u tightly wrapped around histones n no transcription n genes turned off heterochromatin darker DNA (H) = tightly packed euchromatin lighter DNA (E) = loosely packed H E DNA methylation n Methylation of DNA blocks transcription factors u no transcription genes turned off u attachment of methyl groups ( CH 3 ) to cytosine n C = cytosine u nearly permanent inactivation of genes n ex. inactivated mammalian X chromosome = Barr body 3

4 Histone acetylation n Acetylation of histones unwinds DNA u loosely wrapped around histones n enables transcription n genes turned on u attachment of acetyl groups ( COCH 3 ) to histones n conformational change in histone proteins n transcription factors have easier access to genes 2. Transcription initiation n Control regions on DNA u promoter n nearby control sequence on DNA n binding of RNA polymerase & transcription factors n base rate of transcription u enhancer n distant control sequences on DNA n binding of activator proteins n enhanced rate (high level) of transcription Model for Enhancer action n Enhancer DNA sequences u distant control sequences n Activator proteins u bind to enhancer sequence & stimulates transcription n Silencer proteins u bind to enhancer sequence & block gene transcription Turning on Gene movie 4

5 Transcription complex Activator Proteins regulatory proteins bind to DNA at distant enhancer sites increase the rate of transcription Enhancer Sites regulatory sites on DNA distant from gene Enhancer Activator Activator Coactivator B F E A TFIID H Activator RNA polymerase II Core promoter and initiation complex Initiation Complex at Promoter Site binding site of RNA polymerase 3. Post-transcriptional control n Alternative RNA splicing u variable processing of exons creates a family of proteins 4. Regulation of mrna degradation n Life span of mrna determines amount of protein synthesis u mrna can last from hours to weeks RNA processing movie 5

6 Division Ave. High School RNA interference n Small u interfering RNAs (sirna) short segments of RNA (21-28 bases) bind to mrna create sections of double-stranded mrna n death tag for mrna n n w triggers degradation of mrna u cause gene silencing n n post-transcriptional control turns off gene = no protein produced sirna Action of sirna dicer enzyme mrna for translation sirna double-stranded mirna + sirna breakdown enzyme (RISC) mrna degraded RNA interference functionally turns gene off 1990s 2006 for their discovery of RNA interference gene silencing by double-stranded RNA Andrew Fire AP Biology Stanford Craig Mello U Mass 6

7 5. Control of translation n Block initiation of translation stage u regulatory proteins attach to 5' end of mrna n prevent attachment of ribosomal subunits & initiator trna n block translation of mrna to protein Control of translation movie 6-7. Protein processing & degradation n Protein processing u folding, cleaving, adding sugar groups, targeting for transport n Protein degradation u ubiquitin tagging u proteasome degradation Protein processing movie Ubiquitin n Death tag u mark unwanted proteins with a label u 76 amino acid polypeptide, ubiquitin u labeled proteins are broken down rapidly in "waste disposers" n proteasomes 1980s 2004 Aaron Ciechanover Israel Avram Hershko Israel Irwin Rose UC Riverside 7

8 Proteasome n Protein-degrading machine u cell s waste disposer u breaks down any proteins into 7-9 amino acid fragments n cellular recycling play Nobel animation 8