Which chemical bond specifically links amino acids together in a protein chain?

• A. Hydrogen bonds between backbone amide hydrogens and carbonyl oxygens that covalently attach one amino acid to the next in a primary sequence of the polypeptide chain.
• B. Glycosidic bonds connecting individual monosaccharide units into a long polymer that then attaches to amino acids to form proteins.
• C. Phosphodiester bonds linking amino acid side chains through phosphate bridges in the primary structure.
• D. Peptide bonds formed between the amino group of one amino acid and the carboxyl group of the next.

Answer: (D)