DNA molecules are made up of nucleotides, which each consist of a nitrogenous base, a pentose sugar, and a phosphate group. Nucleotides are linked together through covalent bonds between the sugar and phosphate groups to form polynucleotides. The sequence of bases along the DNA polymer is unique to each gene. DNA contains the genetic instructions to direct the development, functioning and reproduction of living organisms. While only about 1.5% of human DNA actually codes for proteins, recent evidence indicates non-coding DNA also plays important roles in the cell.