DNA

Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and functioning of living organisms. All living things contain DNA genomes. A possible exception is a group of viruses that have RNA genomes, but viruses are not normally considered living organisms. The main role of DNA in the cell is the long-term storage of information. The genome is often compared to a set of blueprints, since it contains the instructions to construct other components of the cell, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the expression of genetic information.

In eukaryotes such as animals and plants, DNA is stored inside the cell nucleus, while in prokaryotes such as bacteria, the DNA is in the cell's cytoplasm. Unlike enzymes, DNA does not participate directly in most of the biochemical reactions it controls; rather, various enzymes act on DNA and copy its information into either more DNA, in DNA replication, or transcribe and translate it into protein. In chromosomes, chromatin proteins such as histones compact and organize DNA, which helps control its interactions with other proteins in the nucleus.

DNA is a long polymer of simple units called nucleotides, which are held together by a backbone made of sugars and phosphate groups. This backbone carries four types of molecules called bases, and it is the sequence of these four bases that encodes information. The major function of DNA is to encode the sequence of amino acid residues in proteins, using the genetic code. To read the genetic code, cells make a copy of a stretch of DNA in the nucleic acid RNA. Some RNA copies are used to direct protein biosynthesis, but others are used directly as parts of ribosomes or spliceosomes.