DNA is present in all cellular organisms and in viruses. It contains the genetic information of the organism. DNA, the abbreviation for Deoxyribonucleic Acid, consists of two antiparallel chains (double helix) consisting of sugar, a phosphate-unit and 4 different bases. The two chains are held together by hydrogen bonds between base pairs, while the sugar-phosphate backbone is on the outside. The sugar-phosphate backbone stays the same within a strand, whereas the bases can differ between Adenine, Thymine, Guanine and Cytosine. The order of the bases in the double-chain determines the genetic code, that means the order and the distribution of the bases is responsible for all heritable traits of an organism. It is a very precise code, and a mistake in even a single letter of the DNA code can have disastrous consequences.
A unit (sequence of nucleotides) of the double-helical chain, responsible for a certain trait, is called a gene.