Article: Origin Of Life - The Search For The First Genetic Material

Origin Of Life: The Search For The First Genetic Material

How did life originate on Earth? Until now, there have only been theories to
answer this question. One of the fundamental steps leading to living
organisms is the development of molecules that can replicate and multiply
themselves-the first genetic material. A team led by Ramanarayanan
Krishnamurthy and Albert Eschenmoser at The Scripps Research Institute in La
Jolla, California, is researching how this molecule might have looked.

Our own genetic material is DNA. Its backbone is made of sugar and phosphate
building blocks. Like a strand of pearls, the four "letters" of the genetic
code are arranged along this backbone. Two complementary strands of DNA form
a double helix because the purine bases adenine (A) and guanine (G) form
specific pairs with the pyrimidine bases thymine (T) and cytosine (C),
attaching to each other through two or three docking sites. This type of
structure could also be the basis for the first genetic material. However,
it is doubtful that its backbone consisted of sugar and phosphate; it may
have consisted of peptide-like building blocks. Amino acids, from which
peptides are made, were already present in the "primordial soup". However,
the bases may also have looked different in their primitive form.

To find the right track in searching for the origins of life, the team is
trying to put together groups of potential building blocks from which
primitive molecular information transmitters could have been made. The
researchers have taken a pragmatic approach to their experiments. Compounds
that they test do not need to fulfill specific chemical criteria; instead,
they must pass their "genetic information" on to subsequent generations just
as simply as the genetic molecules we know today-and their formation must
have been possible under prebiotic conditions. Experiments with molecules
related to the usual pyrimidine bases (pyrimidine is a six-membered aromatic
ring containing four carbon and two nitrogen atoms), among others, seemed a
good place to start. The team thus tried compounds with a triazine core (a
six-membered aromatic ring made of three carbon and three nitrogen atoms) or
aminopyridine core (which has an additional nitrogen- and
hydrogen-containing side group). Imitating the structures of the normal
bases, the researchers equipped these with different arrangements of
nitrogen- and hydrogen- and/or oxygen-containing side groups.

Unlike the usual bases, these components can easily be attached to many
different types of backbone, for example, a backbone made of dipeptides or
other peptide-like molecules. In this way, the researchers did indeed obtain
molecules that could form specific base pairs not only with each other, but
also with complementary RNA and DNA strands. Interestingly, only one
sufficiently strong pair was formed within both the triazine and
aminopyridine families; however, for a four-letter system analogous to the
ACGT code, two such strongly binding pairs are necessary.

"Our results indicate that the structure of the bases, rather than the
structure of the backbone, was the critical factor in the development of our
modern genetic material," says Krishnamurthy. Many chain molecules are able
to adopt a suitable spatial structure, but only a few bases can enter into
the necessary specific pairing. In this, our alternative bases are clearly
inferior to the usual Watson-Crick bases. "Based on our observations, we are
beginning to understand why the natural bases are optimal with regard to the
function they perform."

Source: John Wiley & Sons, Inc.
http://www.sciencedaily.com/releases/2006/11/061117121623.htm

Posted by
Robert Karl Stonjek


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