11.4 Protein Synthesis (Translation)

The genetic code is like a universal language for life, translating DNA into proteins. It's nearly identical across organisms, with a few exceptions, hinting at a shared origin for all living things on Earth.

Protein synthesis is a complex process involving initiation, elongation, and termination. It's similar in prokaryotes and eukaryotes, but with key differences in ribosome structure, location, and antibiotic sensitivity.

The Genetic Code and Its Universality

Universality of genetic code

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• Set of rules that defines how information encoded in genetic material (DNA or mRNA sequences) is translated into proteins
  • Codons: Genetic code is read in groups of three nucleotides
  • Amino acids: Each codon specifies a particular amino acid or stop signal
• Nearly universal across most living organisms
  • Common evolutionary origin for all life on Earth
• Deviations from standard genetic code exist in some organisms and organelles
  • Mitochondria and some protozoans may have slight variations

Protein Synthesis (Translation) Process

Key steps in protein synthesis

1. Initiation
   • Small ribosomal subunit binds to mRNA at start codon (AUG) with help of initiation factors
   • Initiator tRNA (carrying methionine) binds to start codon
   • Large ribosomal subunit joins complex, forming complete ribosome
2. Elongation
   • Aminoacyl-tRNA synthetases attach appropriate amino acids to corresponding tRNAs
   • Ribosome moves along mRNA, reading codons and facilitating binding of appropriate aminoacyl-tRNAs
   • Peptide bonds form between amino acids carried by tRNAs, growing polypeptide chain
   • Ribosome translocates to next codon, process repeats
   • GTP hydrolysis provides energy for various steps in elongation
3. Termination
   • Ribosome reaches stop codon (UAA, UAG, or UGA), release factors bind to ribosome
   • Release factors trigger hydrolysis of bond between polypeptide chain and final tRNA
   • Completed polypeptide chain is released, ribosomal subunits dissociate from mRNA

Prokaryotic vs eukaryotic translation

• Initiation differences
  • Prokaryotes: Shine-Dalgarno sequences help recruit ribosome to mRNA
  • Eukaryotes: 5' cap and poly-A tail on mRNA help with ribosome recruitment and binding
• Ribosome structure varies
  • Prokaryotic ribosomes: 70S (30S and 50S subunits)
  • Eukaryotic ribosomes: 80S (40S and 60S subunits)
• Antibiotic targeting
  • Some antibiotics (streptomycin) specifically target prokaryotic ribosomes, exploiting differences
• Localization differs
  • Prokaryotes: Transcription and translation are coupled, occurring simultaneously in cytoplasm
  • Eukaryotes: Transcription occurs in nucleus, mRNA transported to cytoplasm for translation

Ribosome structure and function

• A site (aminoacyl-tRNA site): Accepts incoming aminoacyl-tRNA
• P site (peptidyl-tRNA site): Holds tRNA with growing peptide chain
• E site (exit site): Holds deacylated tRNA before it exits the ribosome
• tRNA anticodon base-pairs with mRNA codon in A site, ensuring correct amino acid incorporation