8.2 Translation and Protein Synthesis

Translation is the process of making proteins from mRNA instructions. Ribosomes and tRNAs work together to read the genetic code and build polypeptide chains. This crucial step in gene expression turns information into functional molecules.

The process involves three main stages: initiation, elongation, and termination. Each stage plays a vital role in accurately producing proteins from the genetic blueprint, ensuring cells have the tools they need to function properly.

Ribosome and tRNA

Ribosomal Structure and Function

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• Ribosomes are organelles composed of rRNA and proteins that serve as the site of protein synthesis in cells
• Consist of two subunits: a large subunit and a small subunit
• Ribosomes contain three binding sites for tRNA: the A site (aminoacyl-tRNA binding site), the P site (peptidyl-tRNA binding site), and the E site (exit site)
• Ribosomes catalyze the formation of peptide bonds between amino acids, linking them together to form polypeptide chains

tRNA and Anticodon-Codon Interactions

• tRNA (transfer RNA) is a type of RNA molecule that delivers amino acids to the ribosome during protein synthesis
• Each tRNA molecule has a specific anticodon, a sequence of three nucleotides that is complementary to the codon on the mRNA
• The anticodon of the tRNA binds to the complementary codon on the mRNA through base pairing (A pairs with U, G pairs with C)
• This anticodon-codon interaction ensures that the correct amino acid is added to the growing polypeptide chain
• Amino acids are attached to the appropriate tRNA molecules by aminoacyl-tRNA synthetases, which use ATP to form an amino acid-tRNA complex

Protein Synthesis Stages

Initiation of Translation

• Initiation is the first stage of protein synthesis, where the ribosome assembles on the mRNA and the first tRNA binds to the start codon
• The small ribosomal subunit binds to the 5' end of the mRNA and scans for the start codon (AUG), which codes for the amino acid methionine
• The initiator tRNA, carrying methionine, binds to the start codon in the P site of the ribosome
• The large ribosomal subunit then joins the small subunit, forming the complete initiation complex

Elongation and Polypeptide Formation

• Elongation is the stage where amino acids are sequentially added to the growing polypeptide chain
• The tRNA carrying the next amino acid enters the A site of the ribosome, and its anticodon base pairs with the corresponding codon on the mRNA
• The ribosome catalyzes the formation of a peptide bond between the amino acid in the P site and the amino acid in the A site, extending the polypeptide chain
• The tRNA in the P site is then released, and the ribosome shifts one codon along the mRNA (translocation), moving the tRNA in the A site to the P site and exposing the next codon in the A site
• This process repeats, with new tRNAs entering the A site and the polypeptide chain growing one amino acid at a time

Termination of Translation

• Termination occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA
• Stop codons do not code for any amino acids and instead signal the end of the polypeptide chain
• Release factors bind to the stop codon in the A site of the ribosome, triggering the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site
• The completed polypeptide chain is released from the ribosome, and the ribosomal subunits dissociate from the mRNA, ready to begin a new round of protein synthesis