A Codon is a set of three RNA bases, which represents an amino acid.
A sequential combination of codons represents a specific sequence of a polypeptide.
S. Brenner, L Barnett, and R.J Watt-Tobin were the first to find out that a 3-nuclear base sequence in RNA represented a specific amino acid.
Protein sequencing refers to the identification of the sequence of amino acids that constitute the protein.
There are three major methods used to understand the sequencing of a protein:
Sanger’s method
Edman degradation
Mass spectrometry

Sanger’s method is used for the determination of the N-terminal amino acids.
Edman degradation helps to obtain the complete sequence of the protein.
For very long protein sequences, enzymatic or chemical sequencing methods are used.