The ribosome assembly moves down the mRNA to the next codon, and the first tRNA ratchets down to the P site, the other slot on the small subunit. Now the second tRNA comes in, carrying with it the second amino acid. The anticodon on the tRNA recognizes the codon on the mRNA, and now you have the first two amino acids lined up next to each other. The ribosome then uses its peptidyl transferase activity to bind the two amino acids together—forming the peptide bond we talked about in Fundamentals II.  We’ve begun to make a protein.

Figure 19. Translation underway. The large ribosomal subunit has joined the initiation complex. Three amino acids are joined by peptide bonds, and the small subunity is about to join the amino acid in the p-site (blue square) to the amino acid in the a-site (blue triangle) with a peptide linkage. A spent tRNA, which carried the amino acid represented by the blue circle, has just been ejected from the p-site. Confused? Check out the cartoon in the next frame.

The ribosome moves down to the third codon. The first tRNA, stripped of its amino acid, is ejected from the P site like a spent casing. The second tRNA is still bound to its amino acid, which is in turn bound to the first amino acid. It moves to the P site, and a new aminoacyl tRNA, carrying the third amino acid, moves into the A site to recognize the third codon.