Which of the following describes the structure of protein molecules?
O
A. Proteins are long chains of nucleotides.
O
B. Proteins are long chains of fatty acids.
O
C. Proteins are long chains of nucleic acids.
D. Proteins are long chains of amino acids.

D. Proteins are long chains of amino acids

Explanation: Amino acids are building blocks of proteins. Amino acids are joined covalently by a peptide bond to form polypeptide chains. A peptide bond is formed by removal of elements of water from an alpha carboxyl group of an amino acid and an alpha amino group of another amino acid. These polypeptide chains formed by joining of many amino acid are packaged to form proteins.