This unusual process is needed .
The D - alanine in this peptide is not among the 20 amino acids coded for in the genetic code and thus the peptide can not be synthesized in the usual way from the encodings in the genome of an organism .
This unusual process is needed because the D - alanine in this peptide is not among the 20 amino acids coded for in the genetic code and thus the peptide can not be synthesized in the usual way from the encodings in the genome of an organism .