DNA, sugars and proteins are biopolymers that are vital to life. The salient difference is the monomer from which the biopolymer is made. Due to the salient difference, these critical biopolymers function differently in the cell and in cosmetic products.
Proteins use amino acids as monomers and are arranged in specific amide structures known as peptide bonds. The convention for a polypeptide is to list its constituent amino acid residues as they occur from the amino terminus to the carboxylic acid terminus. The amino acid residues are always joined by peptide bonds. Protein, though used colloquially to refer to any polypeptide, refers to larger or fully functional polypeptides and can consist of several polypeptide or single chains. Proteins can also be modified to include non-peptide components, such as saccharide chains and lipids.
DNA uses nucleic acids as monomers. DNA makes up the genetic apparatus used to make proteins. The convention for a nucleic acid sequence is to list the nucleotides as they occur from the 5' end to the 3' end of the polymer chain, where 5' and 3' refer to the numbering of carbons around the ribose ring that participate in forming the phosphate diester linkages of the chain. Such a sequence is called the primary structure of the biopolymer.
Sugar-based biopolymers are difficult with regard to convention. Sugar polymers can be linear or branched, and they are typically joined with glycosidic bonds. However, the exact placement of the linkage can vary, and the orientation of the linking functional groups is also important, resulting in α- and β-glycosidic bonds with numbering definitive of the linking carbons' location in the ring. In addition, many saccharide units can undergo various chemical modification, such as amination, and can even form parts of other molecules, such as glycoproteins.