Linking Subunits into Chains

Mineral catalysts have also been useful in forming chains of RNA-like molecules from the activated precursors. James Ferris has focused on the ability of montmorillonite (a type of clay) to promote the assembly of the ribose-phosphate backbone, starting with a special, uracil-containing compound known as a phosphorimidazolide. Although phosphorimidazolides do not occur in nature today, they are highly reactive species and closely related to the modern building blocks of RNA. Most scientists do not maintain that these compounds were present in the primordial soup, but they are convenient substitutes for the natural precursors of RNA.

When the adenine derivative of this compound binds to the mineral surface, the products include chains of up to ten units long, primarily with the "biologically correct" bonds between adjacent riboses. By repeated additions of the starting material, the process can extend the structure up to fifty units. The uracil derivative reacts in a similar fashion, although the chains are somewhat shorter. These data suggest that binding to mineral surfaces may have been important in controlling the proximity and orientation of molecules that could give rise to the first RNA-like fragments, and set the stage for subsequent replication.

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