Final answer:
The backbone of all protein molecules is formed by peptide bonds, which are a specific type of covalent bond establishing the primary structure of proteins.
Step-by-step explanation:
The backbone of all protein molecules is made up of peptide bonds. These are a specific type of covalent bond formed during a dehydration reaction, where a carboxyl group of one amino acid combines with the amino group of an incoming amino acid, releasing a water molecule. Although other types of bonds like hydrogen bonds and ionic bonds are crucial for the secondary and tertiary structures of proteins, it is the peptide bond that forms the primary structure of proteins by linking amino acids together into polypeptides.