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|Chapter Six: DNA, RNA, and Protein Synthesis|
Translation is a term which refers to the actual synthesis of a protein molecule at the ribosomes based upon the codons in the mRNA. Once an mRNA molecule, created by transcription in the nucleus, reaches a ribosome, the small subunit of the ribosome attaches to the first codon (the one at the 5' end) on the mRNA molecule, which is always AUG. This codes for the amino acid methionine which subsequently becomes the first amino acid in the protein about to be produced.
After the small subunit has attached to the first codon, a tRNA molecule with the appropriate anticodon to match with AUG (the anticodon is UAC) bonds to the codon on the mRNA molecule. This tRNA molecule has attached to it a molecule of methionine. Next, the large ribosomal subunit attaches itself on top of the tRNA molecule. The large subunit has two sites at which tRNA molecules can be placed: the P site and the A site. When the large subunit attaches itself, the tRNA molecule with methionine is positioned in the P site.
The remaining steps of translation are repeated over and over again to create protein molecules. First, a tRNA molecule with the matching anticodon for the codon lined up with the A site attaches itself there. Then, a peptide bond forms between the two amino acids which are being held adjacent to one another. This bond formation requires energy from ATP. Once the bond is completed, the tRNA molecule in the P site releases its bond to its amino acid, and the tRNA in the A site moves to the P site. From this point, the process can be repeated to form long strings of amino acids.
When a so-called "stop" codon is positioned in the A site, a special releasing factor enters the A site, causing the entire amino acid chain to be released into the endoplasmic reticulum, eventually making its way to the Golgi bodies where it is packaged in a vesicle and sent to other parts of the cell.