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subunit). The mRNA is also loaded on, and positioned so that the initiation codon (usually AUG) is base paired with the anticodon of the initiator tRNA. The large subunit then binds to the small subunit. The resulting complex of ribosome, mRNA, and methionine-bearing initiator tRNA is called an initiation complex. Formation of this complex also requires a number of helper proteins called initiation factors.

The second stage is called chain elongation. During this stage, additional amino acids are progressively added. The methionine-bearing initiator tRNA sits on a site of the ribosome called the P (peptidyl) site. A new tRNA, bearing the next amino acid is base paired via its anticodon to the next codon of the mRNA, using a site called the A (acceptor) site. This new amino acid is then attached to the amino acid carried by the P site tRNA, forming a peptide bond. This enzymatic step is carried out by the ribosome, at a site called the peptidyl-transferase center.

The tRNA that has so far been attached to the amino acid in the P site is then released through the E (exit) site, and the new tRNA, now carrying both its own amino acid and the methionine moves into the P site. The mRNA also slides three bases to bring the next codon into position at the A site. A third tRNA, again carrying a specific amino acid and recognizing the third codon of the mRNA, moves into the A site, and the cycle is repeated. As these steps are continued, the mRNA slides along the ribo-

?some, three bases at a time, and the peptide (amino acid) chain continues to grow. As with initiation, elongation requires helper proteins, called elongation factors. Energy is also required for peptide bond formation.

The final stage of translation is termination. The signal to stop adding amino acids to the polypeptide is a stop codon (UAA, UAG, or UGA), for which there is no partner tRNA. Rather, special proteins called release factors bind to the A site of the ribosome and trigger an enzymatic reaction by the ribosome. This reaction causes the ribosome to release the polypep-tide and mRNA, ending the elongation process.

At a given time, more than one ribosome may be translating a single mRNA molecule. The resulting clusters of ribosomes, which resemble beads on a string, are called polysomes.

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