Protein Synthesis - the process

First, an initiation codon (usually AUG, which codes for methionine) on an mRNA molecule binds to a special site on a small ribosome subunit. There are two such special sites on ribosomes. After the mRNA is attached, a large ribosome subunit attaches to the mRNA. Transfer RNA with initiation anticodon UAC bind to the mRNA on the ribosome. The amino acid methionine is attached to this tRNA. THis signals the start of the amino acid chain. The second special site on the ribosome is lined up with the next codon on the mRNA molecule. A tRNA molecule with the correct anticodon binds to the codon at the second site. This tRNA has an amino acid attached to it. Methionine now has another amino acid next to it. Enzymes from the ribosome form a bond between the two amino acids. The tRNA that had been attached to the methionine moves away from the ribosome. The ribosome now shifts so that the second tRNA moves to the first site. Another tRNA, with the correct anticodon, binds to the mRNA codon that is now at the second site. This means that a third amino acid is now present. Enzymes join it to the second amino acid. The second tRNA leaves, and the ribosome shifts again.

In this way a chain of amino acids is built up. The sequence is determined by the codons on the mRNA. An mRNA unit can have many ribosomes attached to it. As each ribosome moves along the mRNA, it builds up proteins. The chains usually start with methionine and end when a "stop" codon on the mRNA is reached.

A single mRNA molecule may have up to 100 ribosomes attached to it. Therefore, as the ribosomes move along, many molecules of the same protein are being synthesized at the same time.