DNA

Deoxyribonucleic acid; a.k.a. DNA, is an organic (i.e. carbon-based) molecule that stores genetic information and allows cells to function and reproduce. Eukaryotic organisms have it located within the nuclei of their cells. It forms the primal basis of the metabolism of all known life forms on Earth, with the exception of some viruses, which use the simpler ribonucleic acid (RNA) instead; and possibly the proteic prions, which aren't usually regarded as life forms.

Unlike RNA, DNA molecules consist of two strands which wrap around each other, forming a double helix structure, that makes it more stable and allows information to be backed up more effectively. The whole molecule is composed of nucleotides made from four different types of nitrogenous bases: adenine, thymine, cytosine and guanine; along with deoxyribose (a type of sugar) and a phosphate compound. The base pairs unite the two strands by means of hydrogen bounds: adenine only linking with thymine while cytosine only links with guanine; and vice-versa.

In popular culture
Extraterrestrial life forms in science fiction are often portrayed as having either the same kind of DNA as earthly life forms; or one that differs in some aspect, but is still referred to as "DNA" regardless. A compatible DNA is essential for inter-species reproduction to take place; at least in theory. However, some media will ignore scientific rigor and employ the term DNA in situations where it shouldn't be possible (i.e. a non-carbon-based form of life), using it as a generic term for any natural system of storing biological information.
 * AVP Universe: The Engineers are confirmed to have DNA; the analysis of which has proven that they are genetically-related to mankind. The fact that the Xenomorph inherits genetic traits from its hosts (such as Humans, Dogs or Yautja; the latter also implied to possess DNA) suggests that the Xenomorph itself either has DNA, or at least some means to convert DNA information into its own genetic structure.
 * Babylon 5 Universe: References suggest that the major humanoid races, such as the Minbari, Centauri and Narn, all possess DNA, and the telepath gene created by the Vorlons can manifest in numerous species.
 * Ben 10 Universe: The Omnitrix contains DNA samples from countless different species, allowing the wearer to assume the form and abilities of the alien in question. The sheer biological absurdity of species such as Galvanic Mechamorphs (made of nanotechnology) or Sonorosians (made of sound waves) having DNA has never been commented on; suggesting that the term is used to refer to a physical way to store biological information, rather than actual DNA molecules. Anodites, on the other hand, explicitly lack DNA, but are still capable of reproducing with Humans, though the offspring has no detectable Anodite heritage and isn't considered a hybrid.
 * Calculating God Universe: The biological structure of Humans, Forhilnors and Wreeds are all based on DNA, with the same four base pairs found on Earth, and also the same twenty aminoacids that are common to Earth life. This is interpreted as evidence that all three races were created by a single deity.
 * Doctor Who Universe: The Kinda, despite being a seemingly primitive civilization, wear necklaces showcasing a double helix, suggesting an intrinsic comprehension of DNA. The Moon Germs posses non-chromosomal DNA.
 * The Draco Tavern Universe: Some races, such as the Gligstith(click)optok, have DNA, while others, like the Chirpsithra, have different means of storing genetic information.
 * End of an Era Universe: The Het are DNA-based viral agents.
 * Evolution Universe: The aliens' DNA has ten base pairs, rather than four, although it's been stated that their biochemistry is nitrogen-based, rather than carbon-based.
 * Fifth Element Universe: Leeloo's DNA has "200,000 memo groups".
 * Saturn Rukh Universe: The Rukhs possess DNA, just like Earth organisms, and are in fact theorized to have evolved from a common ancestor via panspermia.
 * Star Trek Universe: All major humanoid races are believed to have DNA, as their homeworlds have been seeded, billions of years ago, by the Ancient Humanoids, who found a way of "programming" these DNA molecules to give rise to a humanoid body plan and intelligence. Even the Changelings have been confirmed to have DNA, despite being non-humanoids.
 * The X-Files Universe: The Syndicate acquired alien DNA from a fetus given to them by the Colonists in exchange for Human children. While the original plan was to use this DNA to create hybrids, members of the Syndicate secretly used it in an attempt to develop a vaccine against the Purity.