Primary structure of protein. This is the simplest level of protein structure. This is the sequence of amino acids in the polypeptide chain. Protein sequence is determined by the DNA of the gene which also encodes the protein. Any changes in the gene's DNA sequence lead to a change in the amino acid sequence protein. An example is sickle cell anemia where only one amino acid is changed. Glutamate is replaced by Valine resulting a defect in the hemoglobin, the oxygen carrying protein that is present in our blood.