DNA sample. Computer artwork of a sample of DNA (deoxyribonucleic acid) in a test tube seen on some text defining the word genetic. DNA encodes the hereditary characteristics of living organisms such as the foetuses at bottom right. It consists of two strands that wind around each other in a double helix. The strands each have a sugar- phosphate backbone and are connected by four types of molecular groups known as bases. The sequence of bases determines the genetic code. Samples of DNA are used to screen foetuses for inherited diseases, to carry out paternity tests and for general genetic research.