Transcription

• In the nucleus; RNA-polymerase attaches to a region of DNA’s double helix; at a codon for methionine (UAG, the start signal), a length of the DNA molecule, corresponding to one gene, unwinds, catalysed by DNA helicase
• DNA strands separate by the breaking of the hydrogen bonds between the bases; one (coding) strand acting as a template for mRNA, ribonucleotides line up against their complementary bases on the DNA template
• Ribonucleotides are joined up by phosphodiester bonds from condensation reactions catalysed by RNA polymerase; the mRNA molecule detaches from the DNA as it forms and leaves through a nuclear pore into the cytoplasm
• A three-base sequence on the mRNA is called a codon, codes for one amino acid, before leaving the nucleus, 1 guanine is added to the 5’ end of the chain called the cap (promotes translation once ribosome is reached)
• Also to the 3’ end of the chain, about 100 adenines are added to give a poly-A tail; thought to act as a signal for the export of mRNA from the nucleus and as protection from enzyme action in the cytoplasm
• Eukaryotic cells contain non-coding regions called introns (”Interruptions”) which are removed from the mRNA by the enzyme action - after the cap and tail have been added but before it leaves the nucleus
• Transcription Diagram

  A) DNA Helicase

  B) hydrogen

  C) nucleotide

  D) mRNA

  E) mRNA

  F) reference

  G) RNA

  H) polymerase

  I) mRNA

  J) mRNA

  K) Phosphodiester

  L) backbone bonds