Translation is the fundamental cellular process where genetic information encoded in messenger RNA (mRNA) is used to synthesize proteins. This process decodes mRNA sequences into specific amino acid chains. Translation occurs in three primary stages: initiation, elongation, and termination. This discussion focuses on the elongation phase, detailing how the protein chain is progressively extended.
Setting the Stage: Ribosome Sites and Initial Positioning
The ribosome serves as the cellular machinery responsible for protein synthesis, a complex structure composed of a large and a small subunit. Within the ribosome, three distinct binding sites accommodate transfer RNA (tRNA) molecules: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. The A site receives new aminoacyl-tRNAs, the P site holds the tRNA attached to the growing polypeptide chain, and the E site is where deacylated tRNAs depart. As elongation begins, the tRNA carrying the nascent polypeptide chain is positioned in the P site, leaving the A site vacant for the next aminoacyl-tRNA.
Bringing in the New Building Block: Aminoacyl-tRNA Entry
The first step of each elongation cycle involves the entry of a new aminoacyl-tRNA into the A site of the ribosome. This delivery is facilitated by elongation factor Tu (EF-Tu) in prokaryotes, or its eukaryotic counterpart eEF1A, which forms a ternary complex with GTP and the aminoacyl-tRNA. The entry relies on accurate codon-anticodon pairing between the mRNA codon in the A site and the anticodon of the incoming aminoacyl-tRNA. Upon correct pairing, the ribosome stimulates the GTPase activity of EF-Tu, leading to the hydrolysis of GTP to GDP and inorganic phosphate. This process ensures the fidelity of amino acid selection and efficient delivery, as the conformational change in EF-Tu following GTP hydrolysis causes its dissociation, allowing the aminoacyl-tRNA to enter the A site.
Forming the Link: Peptide Bond Creation
Following the entry of the aminoacyl-tRNA into the A site, the ribosome catalyzes the formation of a peptide bond, linking the newly arrived amino acid to the growing polypeptide chain. This chemical reaction is mediated by the peptidyl transferase center (PTC), an integral component of the large ribosomal subunit. The PTC is a ribozyme, meaning its catalytic activity is primarily carried out by ribosomal RNA (rRNA) rather than proteins. During this reaction, the carboxyl group of the amino acid attached to the tRNA in the P site is transferred to the amino group of the amino acid on the tRNA in the A site. This results in the entire polypeptide chain being attached to the tRNA in the A site, while the tRNA in the P site becomes deacylated.
Advancing the Process: Translocation and tRNA Release
The final step in each elongation cycle is translocation, a coordinated movement that repositions the mRNA and tRNAs within the ribosome, preparing it for the next amino acid addition. This movement is catalyzed by elongation factor G (EF-G) in prokaryotes, or eEF2 in eukaryotes, which binds to the ribosome and utilizes the energy from GTP hydrolysis. As EF-G facilitates this process, the ribosome shifts three nucleotides along the mRNA in the 5′ to 3′ direction. This shift moves the tRNA carrying the elongated polypeptide chain from the A site to the P site, and simultaneously, the deacylated tRNA previously in the P site moves to the E site. Once in the E site, this tRNA is released from the ribosome, completing one full cycle of elongation.