As translation begins, the small subunit brings in the first tRNA and aligns it on the start codon. The first tRNA occupies the A site, one of two little slots on the small subunit. At this point we’ve formed the initiation complex—mRNA, the small subunit, and the first aminoacyl tRNA. Now the large subunit attaches. Translation can begin.

Figure 18. Translation Initiation. An amino acid (blue) is coupled to a specific tRNA, which has an anticodon complementary to the codon in mRNA which codes for that amino acid. When the tRNA binds to the p site in the small ribosomal subunity and binds to the anticodon, the initiation complex is formed, and translation is almost ready to begin.