Here Be Dragons

ORIGINS
1.3 Chemical structure of
DNA. In this diagram a
short stretch of the familiar
double helix, formed by
twin sugar-phosphate
backbones, has been untwisted
to reveal the molecule's
ladder-like structure.
The four bases are adenine
(A), thymine (T), cytosine
(c), and guanine (c). In
RNA, the sugar is ribose
rather than deoxyribose,
and thymine is replaced by
uracil.
uracil replaces thymine in RNA) linked to a sugar (deoxyribose in DNA,
ribose in RNA), which in turn is linked to a phosphate group (PO 4 ).
(Nucleosides are nucleotides without a phosphate group.) As we've
mentioned, the bases may have been available from reactions in the
primordial soup, or may have been imported on micrometeorites. But
making ribose and adding it on to the bases is more problematic. The
difficulty is that, while it is possible to find circumstances in which ribose
is made, it generally shows up as a small constituent in a mixture
of many different molecules. Some of these molecules are likely to interfere
with subsequent processes, just as mixing different-sized ball
bearings will bring a machine grinding to a halt. Still, Orgel is reasonably
optimistic that a plausible pathway will eventually be found, perhaps
involving inorganic catalysts such as mineral surfaces.
Mineral surfaces might also play a role in the polymerization of
RNA. James Ferris, who directs the New York Center for Studies of the
Origins of Life, has managed to get ribonucleotides to assemble into
short chains on the surface of a kind of clay called montmorillonite. 15
Some slightly altered ribonucleotides will form chains of an RNA-like
polymer containing more than fifty bases. Again, it seems as if further
research may find circumstances in which long chains of RNA will be
formed.
Getting RNA to replicate is the toughest problem. For this to happen,
new ribonucleotides must bind to the existing chain, following the
rules of base pairing: uracil binds to adenine, cytosine to guanine.
31