Translation and Protein Synthesis

Introduction

• Translation: Process of synthesizing proteins from mRNA.
• Key RNAs involved: mRNA, tRNA, rRNA.
• Genetic code: Set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins.

Genetic Code

• mRNA Structure:

  • Contains codons (triplets of nucleotides).
  • Nucleotides: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
  • Orientation: 5' cap, 3' poly-A tail.

• Codons:

  • 64 possible codons from 4 nucleotides raised to the third power.
  • 61 codons code for amino acids; 3 are stop codons.
  • Examples:
    • AUG codes for Methionine (start codon).
    • Stop codons: UGA, UAA, UAG.

• tRNA and Anticodons:

  • tRNA carries amino acids and contains anticodons complementary to mRNA codons.
  • Anticodon example: If mRNA codon is AUG, anticodon is UAC.
  • Enzyme: Aminoacyl-tRNA synthetase links amino acids to tRNA.

• Characteristics of Genetic Code:

  • Commaless, non-overlapping.
  • Redundant: Multiple codons can code for the same amino acid (except Methionine and Tryptophan).
  • Wobble Hypothesis: Flexibility in pairing between the 3rd base of a codon and 1st base of an anticodon.

Ribosome Structure

• Components:

  • Large and small subunits; made of rRNA and proteins.
  • Eukaryotes: 80S (60S + 40S).
  • Prokaryotes: 70S (50S + 30S).

• Antibiotic Targeting:

  • Some antibiotics target bacterial ribosomes to inhibit protein synthesis.

Phases of Translation

1. Initiation:

   • Small ribosomal subunit binds to mRNA.
   • Initiator tRNA recognizes start codon (AUG).
   • Large ribosomal subunit joins.

2. Elongation:

   • tRNA brings amino acids to the ribosome where codons are read.
   • Peptidyl transferase catalyzes peptide bonds between amino acids.
   • Translocation moves tRNA and mRNA through ribosome.

3. Termination:

   • Occurs when a stop codon enters the A site.
   • Release factor binds to stop codon, terminating translation.
   • Peptide chain is released.

Translation Location

• Free Ribosomes:

  • Synthesize proteins for use within the cytosol, nucleus, mitochondria, and peroxisomes.

• Rough Endoplasmic Reticulum (RER):

  • Synthesize proteins destined for secretion, cell membrane integration, or lysosomes.
  • Signal recognition particle (SRP) guides the ribosome to RER.

Protein Modification

• Types of Modifications:
  • Glycosylation: Addition of sugars.
  • Lipidation: Addition of lipids.
  • Phosphorylation: Addition of phosphates (regulation of protein activity).
  • Hydroxylation: Important for collagen synthesis.
  • Methylation/Acetylation: Modifies histones affecting transcription.
  • Trimming: Activation of pre-proteins (e.g., digestion enzymes).

Conclusion

• Translation is a critical process converting genetic information into functional proteins.
• Understanding of translation is essential for insights into cellular function and medical applications.