Transcription

• What is transcription?

  Makes a molecule of mRNA using the triplet codes on the gene on DNA (Transcription is the process of converting DNA into RNA.)
• What are the stages of Transcription?

  • RNA polymerase binds to the start of the gene to be transcribed
  • RNA polymerase moves along the DNA template strand (DNA helicase breaks H bonds to expose the bases on the template strand)
  • RNA nucleotides bind to exposed bases by specific comp base pairing
  • RNA polymerase joins the (m)RNA nucleotides together by forming phosphodiester bonds to form a sugar phosphate backbone
  • STOP codon reached, RNA polymerase detaches and DNA forms hydrogen bonds again
• Splicing in mRNA (this happens during or immediately after transcription)

  • In eukaryotes, introns and exons are both copied into mRNA, this is pre-mRNA
  • Splicing then occurs, to remove introns (non-coding regions)and joins the exons together (coding regions)
  • This forms mature mRNA which happens in the nucleus
  • Prokaryotes don’t need this step because their DNA doesn’t contain introns
• Why doesn’t prokaryotes need to carry out splicing during transcription?

  Their DNA doesn’t contain introns
• Summary of transcription

  1. Hydrogen bonds between complementary DNA bases break
  2. One DNA strand acts as a template
  3. Free RNA nucleotides complementary base pair
  4. (In RNA) Uracil base pairs with adenine (on DNA)
  5. RNA polymerase joins adjacent RNA nucleotides
  6. By phosphodiester bonds in eukaryotic cells
  7. Pre mRNA is spliced/ introns removed (to form mRNA)