Polypeptides

Polypeptides
Amino acids can be joined together to form a peptide or polypeptide. They are called peptides because when the carboxyl group of one amino acid joins to the amino group of another, a peptide bond is formed. Chemically this is an amide bond but when it occurs in proteins it is given the name peptide bond.

Polypeptides
Polypeptides are chains of amino acids. Proteins are made up of one or more polypeptide molecules. The distinction is that peptides are short and polypeptides/proteins are long. There are several different conventions to determine these, all of which have caveats and nuances. In a polypeptide, there are many peptide bonds. These bonds are rigid and planar, due to electron sharing between the carboxyl carbon and the amide nitrogen which contribute to the bond and give it a partial double-bond character.

Primary Structure
A protein has a completely defined order of amino acids, called its sequence. The primary structure is specified by the sequence of the piece of DNA containing a gene for that protein, and the sequence is unique to the individual protein. The primary structure of a protein (amino acid sequence) contains all the information needed to make the protein into a complicated 3 dimensional shape (similar to those you saw in the first tutorial). This shape is essential to the function of the protein.