Primary Structure
The linear sequence of amino acids in a polypeptide chain.
Unit
Dimensionless
In depth
Written by convention from the free amino end to the free carboxyl end, the sequence is dictated by the gene and determines everything else about the protein. A single substitution can be catastrophic, which is the basis of many inherited diseases.
Examples in the real world
Insulin's 51 residues, the first protein sequenced; one glutamate-to-valine change causing sickle cell anaemia.