RNA Translation

• Synthesises proteins in condensation reactions:
  • Residues synthisised N->C
• Cytoplasm on ribsomes
• mRNA read in 5'->3' direction

• Enzymes
• Contain rRNA and protein
• Assembled in nucleolus and matured in cytoplasm
• Read mRNA, translate and assemble amino acids into linear polymer
• Eukaryotes - 80S
  • 60S - >49 proteins, 3 rRNA
  • 40S - >33 proteins, 1 rRNA
• Prokaryotes - 70S
  • 50S - 34 proteins, 2 rRNA
  • 30S - 21 proteins, 1 rRNA
• Catalysis due to rRNA components e.g. ribozyme, not protein
• 3 RNA types used in translation:
  • mRNA - codes for proteins
  • rRNA - ribosomal RNAs
  • tRNA - adaptor molecule that carries activated amino acids to ribosomes, 1 for each codon, has a folded structure (internal base pairing)
• Contain small and large joined subunits, creating 3 sites for tRNAs to occupy, mRNA slides through channel on small subunit

• Analysed genetic crosses resulting in addition/subtraction mutations:
  • Add 1 base = inactive protein
  • Add 2 bases = inactive protein
  • Add 3 bases = active protein
• Each codon must have 3 bases, giving 64 combinations
• Also suggested non-overlapping reading frame (discrete) and no punctuation

• In Vitro experiments with synthetic RNA polymers to determine diff. codons
• More codon types than types of amino acids = degenerate code
• Properties of the genetic code:
  • Triplet code
  • Degenerate
  • Non-overlapping
  • No punctuation
  • Leucine has 6 codons
  • Serine has 6 codons
  • Tryptophan and methionine have 1 each
  • 3 codons specify stop
  • Universal code w/ few exceptions e.g. human mitochondria
• Human mitochondria translation like prokaryotic cells:
  • 55S ribosomes
  • Different tRNA genes similar to chloroplasts 

• First amino acid always (eukaryotes) methionine (AUG) - added as formylmethionine
• May be cleaved off after synthesis
• In prokaryotic cells:
  • Ribosome binding site before AUG
• In Eukaryotic cells:
  • Translation starts at first AUG from 5' end
• Amino acids linked to tRNA (adaptor molecule)
  • Contain a anticodon triplet complementary to codon triplet on mRNA
  • Amino acid added is determinded by its attachment to correct tRNA
  • ATP + AMINO ACID ---> AMINOACYL ADENYLATE + PPi
  • AMINOACYL ADENYATE + tRNA ---> AMINOACYL-tRNA + AMP
• Aminoacyl-tRNA synthases responsible for fidelity of addition (few errors)
• Elongation - addition of futher amino acids 
• Translocation - movement of ribosome from codon to codon
• Ribsosomes have peptidyl-transferase activity - transfers next amino acid onto growing polypeptide
• mRNA has multiple ribosomes (polysomes) moving along strand
• Prokaryotes - transcription and translation at the same time
• Synthesis occurs untill a stop codon when the polypeptide is released and ribsosomal subunits dissociate

Antibiotics act on protein synthesis working on the prokaryotic 70s ribosome:

• Streptomycin - freezes initiation complex = misreading of mRNA
• Tetracycline - prevents binding of aminoacyl-tRNA
• Chloratriphericol - inhibits peptidyl transferase
• Erythromycin - binds to 50s and inhibits translocation
• Puromycin - mimics termius of aminoacyl-tRNA so is added on to chain = premature termination