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