transcription and translation

• DNA


  Genetic blueprint
• Transcription


  Synthesis of RNA from DNA template
• RNA


  Messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA)
• The central dogma states that information in nucleic acid can be perpetrated or transferred but the transfer of information to protein is irreversible
• RNA in prokaryotes


  • Single-stranded
  • Transcription and translation occur in cytoplasm
  • Transcription and translation are coupled
• RNA in eukaryotes


  • Transcription and RNA processing occur in nucleus
  • Translation occurs in cytoplasm
• Coding strand (5'-3')


  The non-template strand of DNA that is read by RNA polymerase to synthesize RNA
• Template strand (3'-5')


  The strand of DNA that is used as a template for RNA synthesis
• Transcription initiation


  1. RNA polymerase binds to promoter on DNA template

  2. Catalyzes synthesis of complementary RNA
• RNA polymerase


  • Synthesizes RNA from DNA, does not need a primer to begin
• Transcription elongation


  1. DNA unwinds, RNA polymerase and the template strand move together

  2. RNA is synthesized 5' to 3'
• Transcription termination


  1. RNA polymerase encounters termination sequence on DNA template

  2. Releases RNA transcript
• RNA processing in eukaryotes

  1. Pre-mRNA undergoes splicing to remove introns

  2. 5' end receives a cap

  3. 3' end receives a poly-A tail
• Exons


  Coding regions of DNA that are joined together to form mature mRNA
• Introns


  Non-coding regions of DNA that are removed during RNA processing
• Spliceosome


  A protein-RNA complex that recognizes 5' and 3' splice sites, excises introns, and ligates flanking exons
• Transcription regulation


  • Transcription factors bind to promoter and help form transcription initiation complex
  • Control which genes are turned on or off
• Prokaryotic transcription units


  • Can encode more than one polypeptide
  • Polycistronic - multiple genes are produced at once
• Eukaryotic transcription units


  • Monocistronic - code for a single polypeptide
• Translation


  Ribosome synthesizes polypeptide chain using mRNA as template and tRNA to bring in amino acids
• Codon


  Three-base sequence in mRNA that specifies a particular amino acid
• Start codon


  AUG - codes for methionine, signals the beginning of translation
• Stop codons


  UAA, UAG, UGA - do not code for amino acids, signal the end of translation
• Ribosome


  • Cytoplasmic organelle where protein synthesis occurs
  • Has 3 sites: A-site, P-site, E-site
• tRNA


  Carries specific amino acid to ribosome, has anticodon complementary to mRNA codon
• Translation initiation


  1. mRNA, initiator tRNA, and small ribosomal subunit bind together

  2. Ribosome scans for start codon
• Translation elongation


  Amino acids are added one by one to the growing polypeptide chain
• Translation termination


  1. Ribosome reaches stop codon, release factors promote hydrolysis of bond between polypeptide and tRNA

  2. Ribosomal subunits dissociate
• Polyribosomes


  • Clusters of multiple ribosomes translating the same mRNA simultaneously
• Post-translational modifications


  Changes made to proteins after translation to make them functional
• Sickle cell anemia is caused by a single amino acid change in hemoglobin that impacts oxygen carrying function