1. mRNA Binding to the Ribosome
* Initiation: The small ribosomal subunit (40S in eukaryotes, 30S in prokaryotes) binds to the mRNA molecule. The ribosome scans the mRNA until it encounters the start codon (AUG).
* Recruitment of the Large Subunit: The large ribosomal subunit (60S in eukaryotes, 50S in prokaryotes) joins the complex, forming a functional ribosome.
2. tRNA Binding and Peptide Bond Formation
* Codon Recognition: A tRNA molecule carrying the matching anticodon for the start codon (AUG) binds to the ribosome's A site (aminoacyl site).
* Peptide Bond Formation: The amino acid attached to the tRNA in the A site is transferred to the amino acid attached to the tRNA in the P site (peptidyl site).
* Translocation: The ribosome moves one codon down the mRNA, shifting the tRNA in the A site to the P site, and the tRNA in the P site to the E site (exit site). The tRNA in the E site is ejected.
3. Elongation
* Repetition: The cycle of codon recognition, peptide bond formation, and translocation repeats as the ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.
* tRNA Cycling: New tRNAs enter the A site, bringing their corresponding amino acids, and are released from the E site after participating in peptide bond formation.
4. Termination
* Stop Codon Recognition: When the ribosome reaches a stop codon (UAA, UAG, or UGA), a release factor protein binds to the A site.
* Polypeptide Release: The release factor triggers the release of the completed polypeptide chain from the ribosome.
* Ribosome Disassembly: The ribosome detaches from the mRNA, and its subunits separate.
Key Components:
* mRNA (messenger RNA): Carries the genetic code from DNA to the ribosome.
* tRNA (transfer RNA): Transports amino acids to the ribosome, matching them to the codons in the mRNA.
* Ribosome: The cellular machinery that reads the mRNA and synthesizes the polypeptide chain.
* Amino Acids: The building blocks of proteins.
Visual Analogy: Imagine a train track (mRNA) with different colored flags (codons) on it. A train (ribosome) moves along the track, picking up and dropping off passengers (tRNAs) carrying luggage (amino acids) at each flag stop.