Xeno Nucleic Acid

"T.N.A. Tri-helix nucleocarbonic acid? It's like your D.N.A., but succubi have a triple helix. A normal double helix contains our succubus code and a haploid helix from our father attaches to that and is selectively called to pull attributes. That's how succubi can mate with a huge variety of species, and it's why we have such a broad diversity in our appearance."

— Dabbler, Grrl Power

Life on Earth encodes its genetic information in two related polymers: DNA^note deoxyribonucleic acid and RNA^note ribonucleic acid. These are composed of a backbone — made of sugar (deoxyribose for DNA, ribose for RNA) and phosphates — and a sequence of nucleobases made of nitrogen, carbon, oxygen, and hydrogen in different formations. In the case of DNA, which exists as a double helix, the four available 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 in chromosomes, which are contained inside the nucleus. Prokaryotes, on the other hand, have their DNA freely floating inside the cytoplasm.

With aliens, either or neither of these could be the case. 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 License – Biology. This is especially true in the case of any human-alien hybrids.

A Sub-Trope of Bizarre Alien Biology. Genetic Memory, Superpowerful Genetics and Unstable Genetic Code could also apply. Related to Mirror Chemistry.

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Examples:

Anime and Manga

• 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.

Film — Live-Action

• 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.
• Evolution (2001): When Ira examines the microbes from an asteroid that fell to Earth, he finds that they have DNA with ten base pairs.
• 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 Or in the French version, 200 billions to the humans' four which is pure technobabble. These are supposed to be signs of Leeloo being a genetically engineered perfect being.
• Species: The Half-Human Hybrid Sil 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.

Literature

By Author:

• Greg Egan addresses this at times:
  • 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.
  • 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.

By Work:

• In the Doctor Who Missing Adventures novel The Man in the Velvet Mask, two of the characters travel to Paris and scoff at the idea of DNA, finding it ridiculous that humans have two coiled-up snakes inside them.
• Known Space: Most species descend from food microorganisms seeded by a long-vanished precursor empire, and thus the same DNA. Trinocs do not, and instead store genetic information with a system of chemicals that fold themselves in complex three-dimensional shapes.

Live-Action TV

• Battlestar Galactica (2003): The Humanoid Cylons 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.
• Star Trek: Deep Space Nine: Sloane says that the cure for the Changeling virus uses adenine, radodine, lidestolinine, and asporanine.
• The X-Files: Alien organisms are discovered that have two additional bases in their DNA.

Radio

• The Hitchhiker's Guide to the Galaxy (1978) features a race of aliens called the Haggunenons, whose DNA is described as "a quadruple-striated octo-helix" and is so chronically unstable that their bodies undergo evolution into a new form several times over lunch.

Tabletop Games

• 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 memory^note Each Space Marine has two of them, but only one is extracted before they serve in battle; in hopes of getting strong genetic memories, the other is kept in until death) from their bodies, both to use them to make more Marines and to prevent them from falling into the hands of Chaos.

Toys

• Transformers: Depending on the continuity, the franchise flip flops on what exactly it is that make up the Cybertronian equivalent of DNA, but often it's 'CNA', an abbreviation which can either stand for 'Cyber/Nano Algorithms' or 'Cybernucleic Acid' Depending on the Writer. Note that Cybertronians are Mechanical Lifeforms.

Video Games

• Assassin's Creed: The Isu Abusive Precursors were Human Aliens with triple-helix DNA. Fragments of this can be found in some humans, distant descendants of Isu who interbred with their human servants.
• 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 This is actually a case of Artistic License – Biology; viruses, by definition, can't edit genetic material or replicate on their own.
• 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 kett from Mass Effect: Andromeda have an unusual DNA sequence, in part due to their method of reproduction: namely, stealing the best bits of alien genomes they find interesting and splicing it into their own. When Ryder tries scanning it, their omni-tool can't make head nor tails of it.
• Space Empires: The Ukra-Tal, a race of worm-like aliens, have triple-helix DNA that 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.

Webcomics

• Grrl Power: Discussed when Dabbler reveals how she can learn languages so quickly. She, like other succubi, has T.N.A (tri-helix nucleocarbonic acid), consisting of a double helix that contains the succubus genes and an extra helix from the father that can be selectively used to determine physical traits and help the succubus mate with so many different species. She and Sydney then waste no time in making "T&A" jokes.
• Schlock Mercenary: Discussed while researching an alien biosphere.
  Lieutenant "Ebby" Ebbirnoth: Samples inbound. Look for common chains of compounds across all the samples. Replication encoding often manifests as a ladder-like super-molecule, a long chain of smaller molecules that—
  Kevyn: You mean DNA?
  Ebby: Well, in Terran life it's DNA, but not—
  Kevyn: I get it. I'm looking for the alien DNA.
  Ebby: My professors would have had you bodily ejected from the school.
  Kevyn: Yup. That was a thing that almost happened.

Web Original

• Whateley Universe: Tennyo, despite appearing human, has some very strange internal features. In particular, her DNA is completely alien and isn't even arranged in a helix. And that's without getting into the antimatter in her blood.

Western Animation

• Ben 10: It's revealed that an Ectonurite's consciousness is somehow present within every single strand of their DNA, allowing them to be reborn From a Single Cell. This is a very bad thing when the protagonist has a device containing various samples of extraterrestrial DNA, and the Ectonurite sample was taken from a very bad alien.

Real Life

• 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 circumstances^note "Monoploidy" refers to when the species' complete genome has only one set. "Haploidy", on the other hand, only applies when the organism's genome is reduced from the normal state; e.g. in ants, females possess two sets of chromosomes, whereas males only possess one (due to being produced from unfertilized eggs).), 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.
• Before the role of both DNA and RNA were known, it was thought proteins stored genetic information.
• While all life on Earth uses DNA and RNA, there are more than 30 different codes for converting from them to proteins.
• Some models of life's origin on Earth suggest that RNA, alone, was employed to encode protein-assembly instructions by the planet's earliest life forms. DNA came later, as a more stable storage-medium for RNA's base patterns.

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