Proteins

The unique sequence of amino acids in a protein is termed the primary structure. When amino acids form a protein chain, a unique bond, termed the peptide bond, exists between two amino acids. The sequence of a protein begins with the amino of the first amino acid and continues to the carboxyl end of the last amino acid.

The unique sequence of amino acids results from the translation of codons present in messenger RNA (mRNA). The mRNA, in turn, is a complementary copy of the gene that codes for that protein. Protein structure and function can change when "misspellings" occur in the order of amino acids during their transcription and translation. Sickle-cell hemoglobin, for example, is "misspelled" in only one amino acid; the sixth amino acid in the beta chain, where a valine is substituted for a glutamic acid. This occurs because the codon for valine, GUG, has replaced the codon for glutamic acid, GAG. This change from acidic to basic amino acid causes the hemoglobin molecules to stick to one another, forming long chains and blocking oxygen binding. These chains of hemoglobin precipitate in the cell, causing the red blood cells to assume a sickle shape. All of these structural and functional changes occur because of the mutation in the hemoglobin gene and a "misspelling" in the hemoglobin's amino acid sequence.