DNA transcription

Transcription

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

Pre-mRNA

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