Life on Earth either possess DNA or RNA in its cells. These are composed of multiple nucleobases made of nitrogen, oxygen, and hydrogen in different formations. These are connected to a backbone made of sugar (deoxyribose) and phosphates. In the case of DNA, which exists as a double helix, these nucleobases are adenine, cytosine, guanine, and thymine. They link together to form base pairs, which, barring coding errors or mutations, are consistently paired A-T and C-G. In RNA, which exists as a single strand, thymine is replaced with a similarly structured nucleobase called uracil.
There are different variations of how these genetic materials may be stored. Eukaryotes have their DNA wrapped up chromosomes, which are contained inside the nucleus. Prokaryotes, on the other hand, have their DNA freely floating inside the cytoplasm.
Neither of these could be the case with aliens. Xeno Nucleic Acid, or XNA, is a blanket term for any alternative biopolymers to DNA or RNA. This could mean the presence of other elements, different numbers of base pairs, a different overall structure, etc. Since aliens will likely have evolved in a very different environment, their genetic material could be unlike anything we've seen on Earth.
It's common to hear the term "Alien DNA" in fiction. This can be justified in the case of Human Aliens; if they experienced a case of convergent evolution on a planet identical to Earth, or both species were just engineered by the same precursors, then their genetic material could conceivably have a similar composition to ours. However, in the case of Starfish Aliens, it's almost guaranteed to be Artistic Licence Biology. This is especially true in the case of any human-alien hybrids.
- Elfen Lied: Among the many other things the Vector Virus does, such as granting Psychic Powers and causing the development of large horns, it also causes the Diclonius to experience a "DNA Voice". This is basically a natural force they all experience that compels them to Kill All Humans and replace them with their own species. It's left ambiguous whether this voice actually exists or it's actually the result of the constant abuse, neglect, and hatred the Dicloni experience from humans that causes it.
- Neon Genesis Evangelion: The Angels have some truly bizarre genetics, assuming that's even the right word to use. They're physically made of an exotic form of matter with properties of both waves and particles, just like light. However, their molecules are arranged in a pattern that is 99.89% similar to human DNA, just with a different color wavelength. Wrap your head around that one.
- Alien: The Xenomorphs have what's called a "DNA Reflex," which is apparently why they end up looking similar to their host species. This is further elaborated on in novels and guides, such as The Weyland-Yutani Report. The Chestburster functions much like a cancer, being built from the body's own cells and integrating 10-15% of its DNA in order to prevent an immune response. This causes the Xenomorph to develop similar physical traits, possibly mental ones as well, such as a specific gait or physical features. The reverse is also true; the host has some of the Xenomorph's genetic material integrated into its body during the Chestburster's incubation. This is why Ripley's clone in Alien: Resurrection is a Half-Human Hybrid.
- In Evolution, when Ira examines the microbes from an asteroid that fell to Earth, he finds that they have DNA with ten base pairs.
- In The Fifth Element, Leeloo's DNA is briefly shown containing numerous helices compacted together. A scientist also comments that she has 200,000 "memo groups" compared with the 40 of humans,note which is pure technobabble. These are supposed to be signs of Leeloo being a genetically engineered perfect being.
- The Half-Human Hybrid Sil from Species is the product of human gametes matched with engineered gametes received from extraterrestrial transmissions. Sil grows from zygote to prepubescent girl in one day, then escapes from the lab and goes running around loose in society.
- Greg Egan addresses this at times:
- In Incandescence, the far-future galactic civilization of the Amalgam is so diverse that asking someone whether they're "a child of DNA" is weird and intrusive. The person who asks had found a new DNA-based species and thought it a rare curiosity, worth sharing.
- Distress is set before First Contact, but a Crazy Survivalist had genetic engineering to replace his own DNA with custom-designed nucleotides all the same, purely to improve his odds in some hypothetical space-plague apocalypse.
- The Humanoid Cylons in Battlestar Galactica (2003) are a race of synthetic beings. Their DNA is completely artificial. Subverted in that they're still genetically compatible with humans and a genetic test would indicate they are indeed human, though they still identify as machines.
- Doctor Who:
- Time Lords have triple-stranded DNA.
- The Daleks have had all emotions except hatred removed from their very genome by Davros. They also don't seem to possess base pairs, but instead have large spikes along the backbones. Their DNA, and presumably that of all life from planet Skaro, is described as being Fundamental Type 467-989.
- On Star Trek: Deep Space Nine, Sloane says that the cure for the Changeling virus uses adenine, radodine, lidestolinine, and asporanine.
- In The X-Files, alien organisms are discovered that have two additional bases in their DNA.
- Warhammer 40,000:
- The Orks have a strand of fungal/algal DNA symbiotically intertwined with their own. This grants them traits such as chlorophyll in their skin and the ability to dispense spores wherever they go, which can create more Orkoid life. The knowledge on how to build weapons and vehicles is also passed down genetically among their species.
- Eldar have triple-helix DNA, and their method of reproducing involves the males repeatedly adding their genetic material.
- Genestealers infest people by injecting small nodules into the target and releasing it into the population (the target doesn't remember what happened, but sees their wounds and assumes they got in a fight). From then on, every generation starting from that individual is entirely devoted to the Hive Mind and acquires ever more Tyranid traits such as needle teeth or extra arms until they give birth to purestrain genestealers, who cannot pass for human and are hidden by the cults.
- The Kroot are able to identify useful genetic codes in their prey, and the Shapers direct their kin as to what they can and cannot eat and rewrite their DNA for whatever evolutionary purpose is needed (and have a Berserk Button towards genestealers, who also mess around with genes). Kroot that eat too much of a certain species become locked in a non-sentient evolution (such as the apelike Krootoxen, Kroothounds, and the T Rex in all but name Knarlocs). It's believed the greenish tint and gunsmithing ability of Pech kroots came from their defeating and eating an ork invasion.
- Space Marines are made by implanting them with vat-grown organs derived from the genetic material of their superhuman Primarch, which greatly increases the growth of muscle and bone, along with other abilities like spitting acid and being immune to poisons. When a Marine dies, the Chapter's top priority is to retrieve the progenoid gland (the organ that creates all others, and hosts the genetic memorynote ) from their bodies, both to use them to make more Marines and to prevent them from falling into the hands of Chaos.
- Assassin's Creed: Members of the First Civilization had triple-helix DNA.
- Fallout: In a world set after a nuclear war, there are bound to be countless genetic abnormalities out in the wasteland. However, one pre-war example would be the experiments performed in the Pan-Immunity Virion Project. In their goal to protect America's population from Chinese bioweapons, they created a retrovirus that could transform all the DNA in a host cell into a quad-helix structure, which they hoped would prevent diseases from taking hold. Their crowning achievement, however, is the Forced Evolutionary Virus. The FEV is described as a Megavirus with a protein sheath reinforced by ionized hydrogen that protects it from radiation. The virus has pre-programmed introns specified for the species it infects. It works by copying the host cell's DNA and placing the data into exons, which is taken back by the virus and then re-introduced into the host's cells alongside the introns, which induce the radical mutations the FEV is known for. note
- Mass Effect:
- Protheans are mentioned as having quad-strand DNA, which is unique among known species. It's also how Shepard and company discover the Collectors are actually heavily modified Protheans in the second game.
- Asari have the ability to mate with a partner of any gender and any species. Their actual method of reproduction is a form of parthenogenesis, where they scan DNA via biotics and modify their offspring's genome using any useful patterns from their partner. Physical mating is optional.
- The Ukra-Tal, a race of worm-like aliens in the Space Empires series, possess a triple-helix DNA which is said to baffle geneticists of other races.
- StarCraft has the Zerg, who famously have the ability to bypass biochemical barriers and steal genetic material from their prey, allowing them to rapidly evolve. Their alpha amino acids are said to have unique "R groups" that allow damaged cells to fuse with protein to repair themselves, and "survival of the fittest, nature red in tooth and claw" is apparently imprinted into them on the genetic level.
- Several types of XNA have been created synthetically, which tend to be named after the different molecular compositions for their backbones. These include threose nucleic acid (TNA), glycol nucleic acid (GNA), peptide nucleic acid (PNA), etc.
- Quadruple-strand DNA does exist, and has been observed in human cells.
- Most species, including humans, have diploid genomes — i.e. the genome has two sets of paired chromosomes, one from each parent. Some, however, have only a single set of chromosomes (i.e. monoploidy/haploidy, depending on the circumstancesnote ), while others have more than two sets, a phenomenon known as polyploidy. And they form n-tuples instead of pairs, with n being the number of chromosome sets; i.e. a triploid organism has its chromosomes form into triples, a tetraploid one has them form into quadruples, and so on so forth.