Most replicators in biology — bacteria, viruses, even loose strands of replicating RNA — copy themselves by copying a sequence. The information being propagated is a chain of letters in DNA or RNA. Prions are different. Prions propagate a shape.
This is so unusual that when Stanley Prusiner proposed the prion concept in 1982, he was widely disbelieved. The dominant assumption was that anything capable of self-propagation in biology must carry a nucleic acid template. Prusiner spent fifteen years accumulating evidence that prions are pure proteins with no DNA or RNA component. He won the Nobel Prize for it in 1997, by which point the evidence had become overwhelming.
What a prion is
Established A prion is a misfolded version of a protein that normally exists in your body. The normal version has one shape; the prion version has a different, abnormal shape. When the prion form contacts the normal form, it forces the normal one to refold into the prion shape.
That's the entire mechanism. There is no copying of sequence, no transcription, no machinery. The misfolded protein acts as a template, and the normal protein folds itself into the new shape because of the weak forces (M-Chem-04) that govern protein folding — the hydrogen bonds, van der Waals interactions, and hydrophobic effects that determine which 3D shape is most stable when the protein is touching a prion-shaped neighbour.
This is why you needed the previous module. A prion is not violating the rules of chemistry. It's exploiting them. The misfolded shape changes the local energy landscape around the normal protein, and the normal protein folds itself into the lower-energy configuration available in that altered landscape — which happens to be the prion shape.
The diseases
Established Prion diseases are uniformly devastating. They include:
Creutzfeldt-Jakob disease (CJD) in humans — rapid neurodegeneration, fatal within months of symptom onset. Bovine spongiform encephalopathy (BSE, "mad cow disease") — caused panic in the UK in the 1980s and 1990s when contaminated beef transmitted prions to humans. Kuru — once endemic in a Papua New Guinean community where ritual cannibalism transmitted prion-infected brain tissue between people. Scrapie in sheep. Chronic wasting disease in deer and elk, currently spreading through wild populations in North America.
The diseases share a pattern: misfolded proteins accumulate in the brain, form sponge-like cavities in nervous tissue, and the patient progressively loses motor and cognitive function.
Why prions are so strange
Prions challenge several assumptions about what a biological agent has to be.
They have no genome. They cannot be killed by anything that targets DNA or RNA. Standard sterilisation procedures — autoclaving, formaldehyde, ultraviolet light — don't destroy them. Prions can survive temperatures and chemical treatments that obliterate every other known biological agent. Surgical instruments that have contacted prion-infected tissue often have to be destroyed because they can't be reliably decontaminated.
They have no metabolism. They don't eat, they don't grow, they don't move on their own. They just exist, and when they touch the right substrate, they convert it.
They have no obvious upper limit on host range within reason. The species barrier exists but is sometimes weak. BSE crossed from cattle to humans. CWD's potential to cross to humans is a current research concern.
Why this matters for the protomolecule
The protomolecule's behaviour on first contact with biological matter, as shown in The Expanse, is fundamentally a prion mechanism scaled up dramatically and given direction. The brown goo conversion scenes are a prion cascade with two extensions added: the converted matter is then organised into coordinated structures, and the conversion is directed toward specific endpoints rather than just propagating the misfold.
Handwave The "given direction" part is where biology runs out and the fiction takes over. Real prions just propagate. They don't have goals, they don't build things, and they don't coordinate. But the underlying mechanism — propagating a shape via local protein-folding effects — is real, and the protomolecule is a recognisable extrapolation from it.
This is what the Institute means by the speculative edge. Prions are real biology, well-established and Nobel-recognised. The protomolecule is not real. But the conceptual ladder from one to the other is short, and walking that ladder tells you exactly which pieces of the fictional entity are extrapolated from real science and which pieces have no current basis.
Why are prions so resistant to standard sterilisation methods, when bacteria and viruses generally are not?
Show answer
Most sterilisation methods target nucleic acids — DNA or RNA. UV light damages DNA. Heat denatures proteins but also fragments DNA. Formaldehyde cross-links nucleic acids. Prions have no nucleic acids to attack. They're pure protein, and the misfolded shape is unusually stable against denaturation. Destroying a prion requires either chemical conditions extreme enough to break protein structure entirely (concentrated bleach, sodium hydroxide) or prolonged high-temperature autoclaving at higher temperatures than standard protocols use.