Twenty different amino acids are used to build proteins, whether those proteins are constructed in you and me, or in a worm, bacterium, whale, or any other living creature on Earth. Even an entity that can hardly be classified as "alive," namely a virus, has a protein coat that uses the same amino acids we do. Apparently, 20 of these building blocks represent enough variety to build all the proteins in all life forms.
Proteins are long chains of amino acids that fold into specific three-dimensional structures based on the order in which the amino acids occur along the chain. All of the 20 amino acids are variations of the same basic structural plan: a central carbon atom, called the a (alpha) carbon, which has four chemical groups attached directly to it. Three of the chemical groups are common to all amino acids (although one particular amino acid combines two of its groups in an unusual way, as you'll see.) The fourth group, called the 'R' group, varies between the amino acids; it is what distinguishes one amino acid from another.
In this tutorial, we will explore both the similarities of the amino acids and the R groups that differ between them.