Peptides are naturally occurring biological molecules. Peptides are found in all living organisms and play a key role in all manner of biological activity.
Like proteins, peptides are formed (synthesized) naturally from transcription of a sequence of the genetic code, DNA. Transcription is the biological process of copying a specific DNA gene sequence into a messenger molecule, mRNA, which then carries the code for a given peptide or protein.
Reading from the mRNA, a chain of amino acids is joined together by peptide bonds to form a single molecule.
There are 20 naturally-occurring amino acids and, like letters into words, they can be combined into an immense variety of different molecules. When a molecule consists of 2-50 amino acids it is called a peptide, whereas a larger chain of > 50 amino acids generally is referred to as a protein.