DNA transcription

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DNA makes RNA

  • a small portion of the DNA double helix unwinds to expose the bases on each strand
  • one of the two strands of the DNA then acts as a template
  • the transcription initiation complex is assembled
  • the RNA transcript is elongated one nucleotide at a time and is complementary to the sense strand
  • as the RNA chain is made, the DNA helix reforms behind it, and the RNA is released as a single strand
  • RNA molecules are quite small compared to DNA, usually a few thousand bases long
  • RNA is synthesised 5' to 3'

The product of transcription is RNA

  • transcription is the synthesis of RNA from a double stranded DNA template
  • RNA is usually single stranded and can exist in a variety of 3D shapes (secondary structure)
  • U instead of T

Transcription in bacteria

  • prokaryotic transcription is carried out by an RNA polymerase; the RNA polymerase resembles a crab claw and is made of several subunits
  • the RNA polymerase uses dNTOS (A, C, and U) to produce a complementary copy of the template DNA by adding nucleotides to the 3' end of the growung RNA chain
  • in bacteria a polygenic mRNA that contains information for more than one peptide is produced
  • the mRNA starts to be translated as soon as it is made
  • initiation:
    • the core RNA polymerase bind at a promoter sequence
    • a closed complex is formed and then a transition to an open complex takes place
    • promoter escape - the polymerase moves off from the promoter
  • elongation:
    • the RNA polymerase unwinds the DNA in front and a transcription bubble is produced; this bubble moves with the polymerase
    • RNA is made and proofread by the polymerase from the template strand forming a DNA/RNA hybrid
    • the RNA dissociates and the DNA rewinds behind the enzyme
  • translation:
    • the RNA polymerase dissociates from template DNA
    • this takes place at specific sequences at the end of the gene (terminator)
    • some terminators need a protein called 'rhe'

Eukaryotic transcription

  • more complex than in bacteria
  • RNA is transcribed in the nucleus
  • mRNA is processed (capped, spliced, polyadenylated)

RNA polymerases

  • RNA pol I synthesises rRNA
  • RNA pol II synthesises mRNA
  • RNA pol III synthesises tRNA, 5S rRNA, small nuclear RNAs


  • the entire gene, including the introns as well as the exons, are copied…


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