When your sons and daughters proudly display their pierced body parts and colorful tattoos, you have at least one consolation: those adornments, no matter what, will not be passed on to the grandchildren.

Biologically, this comforting thought is expressed by the statement that there can be no inheritance of acquired characteristics. It is by no means obvious that this is always true, and scientists argued about it for over a century before accepting it.

The same idea today forms part of what is often termed the Central Dogma of molecular biology; namely, the DNA (or, for some viruses, the RNA) of the organism decides the nature of the adult being. Furthermore, no matter what that being does, it cannot modify the structure of its DNA or the DNA of its offspring.

The assumption, of course, is that you need to have DNA or RNA in order to have offspring at all. This seems reasonable, since DNA carries the entire genetic code that defines the creature. Twenty-five years ago, there was probably not a biologist in the world who would have disagreed with the idea that reproduction needs either DNA or RNA.

Then a baffling thing happened. A man named Stanley Prusiner had been studying certain peculiar diseases with a long incubation period. They include "scrapie", mostly affecting sheep and goats; "kuru," the "laughing death" disease of the natives of New Guinea that became famous because cannibalism was involved in its spread; "Creutzfeldt-Jakob" disease, a rare and fatal form of dementia and loss of coordination in humans; and, more recently, the "mad cow disease" (bovine spongiform encephalopathy) that required the killing by farmers of millions of cattle in Great Britain a few years ago.

These diseases had a long latency period, years or even decades, before the infected animal or human showed symptoms. The standard theory was that a "slow virus" was responsible for them. In order to reproduce itself, such a virus must possess either DNA or RNA, known together as nucleic acids.

The trouble was, Stanley Prusiner looked long and hard at the infecting materials for these diseases, and found no sign of either nucleic acid. Finally, in 1982, he made a bold suggestion: the infectious agent for scrapie and related diseases had no DNA or RNA; it consisted exclusively of some form of protein. He introduced the term "prion" (pronounced pree-on), for "proteinaceous infectious particle."

Soon afterwards, Prusiner and his co-workers discovered a protein that always seemed to be present in the infectious agent for scrapie. Not only that, something very close to the same protein occurred naturally in animals that were not sick. The difference between the two proteins, for well and sick animals, seemed to be only in the shape of their molecules. Prusiner made another extraordinary suggestion: the infecting protein had the power to change protein in the healthy animal to its own form. Prions were, in effect, reproducing themselves without possessing either DNA or RNA.

Since this runs counter to the Central Dogma, according to which DNA, working via RNA, produces proteins, Prusiner's idea at the time he proposed it looked like pure scientific heresy. However, it is the fate of all great truths to begin as heresy. In 1997, Stanley Prusiner received the Nobel Prize for Medicine for the concept of prions. Once again, something that everyone "knew" to be the case now looks as though it's not always so.

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