DNA autoradiogram of a forensic soil sample. Researcher pointing over a DNA (deoxyribonucleic acid) autoradiogram. Most soil samples contain organic matter such as fungi, plant debris and bacteria. A DNA 'profile' of the soil sample, obtained using DNA sequencing, can be compared to geographical data and provide a location match. 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. Photographed at the Macaulay Institute, Aberdeen, UK.