you heard of "mad cow" disease? This disease can come from
eating beef or sheep that have been infected with an unusual
infectious agent called a "prion". Prions are an
odd form of protein and contain no nucleic acid (DNA
or RNA), yet they are still infectious.
So how do
prions reproduce without having DNA to provide instructions? Prions
reproduce by converting certain cellular proteins into an infectious
form. You might say that it makes the cell infect itself (and
other cells that get exposed from a person eating prion-contaminated
meat). The infectious prion is a changed structural form, dominated
by conversion of protein helix structure into flat sheets. The
infectious prion is extremely insoluble and resistant to
protein-digesting enzymes, heat or other denaturing
attack nervous tissue, especially the brain. That is why the
disease is called "mad cow". Affected cows run around
uncontrollably and act crazy. When you examine the brain of such
cows, the brain is full of holes, like a sponge. That is why
they call it a Spongiform Encephalopathy ("enceph"
refers to the head and "pathy" refers to "diseased").
detail can be found at: