FoldIt is an experimental game where you fold proteins, hopefully finding the forms they would naturally take in living beings.
We understand the process by which living beings create the primary structure of proteins based on genes quite accurately, and we know how to read DNA sequences. What's currently difficult for science is figuring out how the primary structure turns into the three-dimensional structure of actual, functioning proteins. The general process is known, but it takes enormous amounts of CPU time to transform the primary structure of a protein to the tertiary structure computationally.
Protein folding is a very important, central process of biological life, and information about this process is a basic building block of the science of biology. It can help researchers find cures to diseases - AIDS, cancer, Alzheimer's - among many, many other things.
FoldIt is an attempt to use the human brain's natural three-dimensional pattern-matching abilities to take shortcuts that a computer wouldn't find. The researchers behind it are currently compiling information on whether this is possible at all by posting puzzles based on various well-researched proteins, letting people fold them, and analysing the results.
The server's been taking a fair beating, so you might want to download the game by torrent (Windows, OS X). I wasn't able to try it out myself because Wine didn't run it properly on my system - other people have gotten better results, though...
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