Now, by convention, the beginning of the polypeptide is always set to be at the positive end at the alpha amino group, and the end is always set to be at the negative side at the alpha carboxyl group. So we always read the amino acids beginning from the alpha amino side and ending at the alpha carboxyl side. Now, notice if we go along the polypeptide chain, we will see a repeating unit of three atoms. So we have nitrogen, carbon, carbon, nitrogen, carbon, carbon, nitrogen, carbon, carbon, nitrogen, carbon, carbon, nitrogen, carbon, carbon. And this can continue if we continually add amino acid. So this repeating section of the polypeptide is known as the backbone.