Deoxyribonucleic acid (DNA) is a nucleic acid in the form of a double helix. DNA contains genetic instructions for the specific biological development of cellular life forms and most viruses, is a long polymer nucleotide, and encodes the amino acid sequence in proteins using a genetic code, i.e. a triple nucleotide code. It is a polymer of nucleotides made up of pentose de-oxyribose, a phosphate group and a nitrogen base which in DNA can be adenine, guanine, cytosine and thymine. In eukaryotic organisms such as plants, animals, fungi, and protests, most of the DNA is located in the cell nucleus. In simpler organisms called prokaryotes, DNA is not separated from the cytoplasm by a nuclear envelope. Mitochondria and chloroplasts, which are organelles of eukaryotic cells, also contain DNA.