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  B. How Prions Jump the Species Barrier

Narrator: This is Science Today. Prions are a type of infectious agent and unlike viruses or bacteria, which have DNA or RNA as a genome, prions are made solely of proteins. For the first time, Jonathan Weissman of the University of California, San Francisco, demonstrated that a change in the folded shape of a prion protein can change its infectious properties, including the prion's ability to jump the “species barrier”.

Weissman: There's this phenomena called the species barrier, which is supposed to protect different animals from infecting each other. So, sheep don't infect people even though people have been living with and eating scrapie-infected sheep for 200 years.

Narrator: It was thought that cows could not infect people, but it turns out that mad cow disease – which is a prion strain – can jump the species barrier.

Weissman: We now believe that the reason mad cow jumped the species barrier to people had less to do with cows being more like people than sheep are like people and more to do with the fact that the shape or the strain of the mad cow was one that was able to be more compatible with the human protein.

Narrator: For Science Today, I'm Larissa Branin.