Kate asks:

>A fellow science fiction fan once asked me why ribose was the 
>sugar in RNA - what was special about ribose? Could a biology 
>exist somewhere which was based around another sugar, such 
>as xylose? I was intrigued by the question, but have never come 
>across an answer in my reading. Can anyone help?

Other sugars have indeed been considered. Ribose is considered to be a
difficult molecule to form abiogenically (ribose, btw, derives its name from
the "Rockefeller Institute of Biology"), thus other, simpler
sugars have been
investigated. See, e.g.:

   http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=189

Wirt Atmar
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