DNA and its building blocks. DNA is made of four types of nucleotides, which are covalently linked together into polynucleotide chains with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. A DNA molecule is composed of two polynucleotide chains (DNA strands) held together by hydrogen bonds between the paired bases. The arrows on the DNA strands indicate the polarities of the two strands, which run antiparallel to each other in the DNA molecule. In the diagram at the bottom left of the figure, the DNA is shown straightened out; in reality, it is twisted into a double helix, as shown on the right