Elongation cycle of a ribosome (gray dome) translating an mRNA (black-green-purple line).

<p>Aminoacyl-tRNA (small gray, green, purple, or orange sphere) is delivered to the ribosome in a ternary complex with the elongation factor EF-Tu (larger blue sphere) and GTP (not shown). In addition to the initial binding site, the ribosome has three tRNA binding sites, the A, P, and E sites. The elongation cycle of translation starts when the A site of the ribosome has arrived at a new codon (green) of the mRNA. The ribosome then binds a ternary complex with a tRNA that may be cognate, near-cognate, or non-cognate to this codon. As a consequence, the elongation cycle exhibits three different branches corresponding to three different reaction pathways: (left) A non-cognate ternary complex is again released from the initial binding site of the ribosome; (top) A near-cognate ternary complex is usually rejected but is very rarely used to elongate the peptide chain; and (bottom) A cognate ternary complex may also be rejected but is typically used for elongation of the peptide chain. The two dotted arrows correspond to additional intermediate states and transitions as explained in more detail in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003909#pcbi-1003909-g003" target="_blank">Fig. 3</a> below.