Protein Synthesis

• Transcription

  • RNA-polymerase attaches to DNA region at start codon (methionine, UAG)
  • Length, for 1 gene, unwinds, catalysed by DNA helicase
  • Strands separate, h bonds break
  • One (coding) strand act as mRNA template
  • Ribonucleotides line up to complementary bases on template, joined by phosphodiester bonds catalysed by RNA polymerase
  • mRNA detaches and leaves through nuclear pore into cytoplasm
  • Before leaving nucleus, 1 guanine is added to 5’ end called cap (promotes translation)
  • 100 adenines added to 3' end; poly-A tail (signal for mRNA export and protection from enzyme action)
  • Introns
  • ”Interruptions”
  • Non-coding regions in eukaryotic cells 
  • Removed from mRNA by enzyme action
  • After cap + tail have been added but before it leaves nucleus
• Translation

  • mRNA attach to ribosomes at binding site (groove)
  • 2 tRNA molecules held at once
  • Anticodons on tRNA bind to complementary codons on mRNA
  • Held by temporary H bonds
  • First codon on mRNA, start, is AUG (methionine)
  • Ribosomes move along mRNA
  • Read codons
  • Translate to AA sequence; 15/sec added to polypeptide chain
  • Energy from breaking bonds between tRNA + AA form peptide bonds between AAs
  • Translation stops when ribosome reach stop codon
  • No complementary tRNA
  • Polypeptide breaks away + folds

  Post-translation

  • Completed polypeptides pass to cisternae of RER
  • Budded into RER vesicles
  • Pass to Golgi; modified to form functional proteins, packaged to Golgi vesicles
  • Many ribosomes (polysome) attach to same mRNA moving along sequence to produce many of same polypeptide chain