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06 transcription-stacy

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  • 1.  
  • 2. Central Dogma Video
    • http://www.wiley.com/legacy/college/boyer/0470003790/animations/central_dogma/central_dogma.htm
  • 3. Prokaryotic versus Eukaryotic protein synthesis Fig. 17.2ab
  • 4. Four Major Steps
    • Initiation
    • Elongation
    • Termination
    • Posttranscriptional Modification
  • 5. Parts of a Gene
    • promoter - DNA sequences which indicate the location of a gene
    • promoters are located upstream from the DNA region that contains the information to be transcribed into mRNA
    promoter transcription region termination sequence gene
  • 6. Orientation About a Gene
    • RNA polymerase – transcription enzyme (synthesizes mRNA in 5’  3’ direction)
    • uses upstream, promoter region to determine where to start mRNA transcription
    0 positive numbers negative numbers upstream downstream start transcription
  • 7. Initiation
    • promoter regions are often sequences of A’s and T’s
      • 2 H-bonds between A&T
      • easier to break than 3 H-bonds between G&C
      • prokaryotic genes have a TATA box
    • dsDNA (double stranded DNA) needs to be opened for mRNA to be made
    • RNA polymerase opens the dsDNA
  • 8. Fig. 17.6a
  • 9. Initiation
    • transcription factors - numerous protein factors are involved in starting transcription
    • some of these proteins help control how often genes are transcribed
  • 10. Elongation
    • RNA polymerase synthesizes mRNA in the 5’  3’ direction
      • no primer is necessary
    • template strand - only one strand of the DNA is transcribed
  • 11. Elongation Nomenclature
    • the template strand is the antisense strand
    5’ A T T A C G A T C T G C A C A A G A T C C T 3’ 5’ A U U A C G A U C U G C A C A A G A U C C U 3’ 3’ T A A T G C T A G A C G T G T T C T A G G A 5’ SENSE STRAND ANTISENSE STRAND DNA DNA mRNA
  • 12.  
  • 13. Termination
    • RNA polymerase stops transcribing once it reaches the termination sequence
      • Signal is actually the RNA sequence (transcribed terminator)
      • Eukaryote: AAUAAA
    • enzyme dissociates with DNA strand and binds to another promoter sequence
    Fig. 17.6a
  • 14. Termination
    • termination sequences differ between prokaryotes and eukaryotes
    • Prokaryote: transcription ends immediately at signal
    • Eukaryote: RNAP continues for hundreds of nucleotides past termination signal; at 10-35 nucleotide past signal pre-mRNA released
    Fig. 17.8
  • 15. Transcription
  • 16. Transcription Animation
    • http://www.youtube.com/watch?v=Jqx4Y0OjWW4
  • 17.  
  • 18. Posttranscriptional Modification
    • mRNA of eukaryotic cells need to be modified before moving into the cytoplasm
    • primary transcript – initial eukaryotic mRNA transcript, before modification
    • 5’ cap – 7-methylguanosine triphosphate
    • poly-A tail – approx. 200 adenine ribonucleotides are added at the end
    • modifications prevent cellular enzymes from breaking down mRNA before it is translated into protein
  • 19.
    • 5’ cap
      • Modified guanine (7-methylguanosine triphosphate) added to 5’ end
      • Protect mRNA from degradation
      • Signals ribosome attachment
    • PolyA tail
      • 50-250 adenine added to 3’ end
      • Same as 5’cap: inhibit degradation, help ribosome attach
      • Facilitate export of mRNA from nucleus
  • 20. 5’ 3’ 5’ cap added by capping enzyme complex primary transcript 3’ poly-A tail added by poly-A polymerase AAAAAAA mG mG
  • 21.  
  • 22. Modifications: Introns / Exons
    • eukaryotic genes are longer than prokaryotic genes
      • we carry extra “junk” DNA
      • most of this “junk” DNA signals when and how often genes should be transcribed  regulatory DNA
    • primary transcript is longer than necessary
      • exons – RNA sequences that will be ex pressed ; helps makes the protein
      • introns – in terfering RNA sequences; need to be removed before translation
  • 23. AAAAAAA mG exon exon exon intron intron intron intron introns removed by spliceosome proteins AAAAAAA mG exon exon exon mRNA transcript
  • 24.  
  • 25. Splicing Video
    • http://highered.mcgraw-hill.com/olc/dl/120077/bio30.swf
  • 26. Spliceosome Complex
    • Spliceosomes are a series of small nuclear ribonucleoproteins (snRNP) that work together to remove introns.
    • snRNPs recognize specific sequences on the introns
      • cuts out intron sequences
      • splices exon sequences together

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