DNA sequencing autoradiogram. DNA consists of two sugar-phosphate backbones, arranged in a double helix, linked by nucleotide bases. There are 4 types of base; adenine (A), cytosine (C), guanine (G) and thymine (T). Sequences of these bases make up genes, which encode an organism's genetic information. The bands (black) on the autoradiogram show the sequence of bases in a sample of DNA.