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Transcription

transcription in prokaryotes - 

  • RNA polymerase - has 5 subunits
    • 2 a subunits bind regulatory proteins
    • b' subunit binds DNA template
    • b subunit binds RNA nucleotide subunits
    • s subunit starts synthesis, recognizes promoter
    • template strand (antisense strand) - strand of DNA that’s copied
    • coding strand (sense strand) - strand of DNA not copied, identical to RNA
  • promoter - sites where transcription starts
    • short sequence not transcribed by polymerase
    • -35 sequence - TTGACA, 35 nucleotides from where transcription starts
    • -10 sequence - TATAAT, 10 nucleotides from where transcription starts
  • initiation - binding of RNA polymerase to promoter
    • s subunit detects promoter (w/o unwinding DNA), attaches polymerase there
    • polymerase begins to unwind DNA helix (section about 17 base-pairs long)
  • elongation - uses ATP or GTP to add ribonucleotides
    • transcription bubble - area containing RNA polymerase, DNA, growing RNA
    • DNA rewinds after leaving bubble
    • RNA polymerase can’t proofread, makes more errors than replication
  • termination - “stop” sequences cause phosphodiester bonds to stop forming
    • RNA-DNA dissociates
    • RNA polymerase releases DNA
    • DNA rewinds
    • GC hairpin - causes polymerase to pause, eventually let go of DNA

transcription in eukaryotes - 

  • multiple RNA polymerases - 3 different ones used
    • RNA polymerase I - transcribes rRNA, recognizes promoters
    • RNA polymerase II - transcribes mRNA, small nuclear RNA
    • RNA polymerase III - transcribes tRNA, small RNA
  • promoters - different 1 for each polymerase
    • specific for each species
    • pol II promoters - most complex out of the 3
    • TATA box - resembles -10 sequence; found in “core promoters”
    • pol III promoters - internal to gene itself
  • initiation - more complex than prokaryotic initiation
    • initiation complex - forms at promoter/pol II by general transcription factors
  • posttranscriptional modifications - mRNA packaged differently
    • 5’ cap - GTP added to 5’ phosphate group, protects mRNA from degradation
    • 3’ poly-A tail - adenine residues added by poly-A polymerase to end of mRNA, stabilizes mRNA
Subject: 
Biology
Subject X2: 
Biology

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