Wednesday, May 30, 2001
How pre-life chemicals may have become biologically significant
By Robert C. Cowen Special to The Christian Science Monitor
Chemists studying the rise of life on earth have penetrated a little deeper
into its mystery. Research reported earlier this month shows how a common
mineral could have sorted some of life's precursor chemicals into biologically
significant groups. Other recent experiments demonstrate how the earliest life
forms might have reproduced themselves without the help of DNA or proteins,
two ingredients thought to be essential in the evolution of life.
Molecular chemist David Bartel notes that "we will never be able to prove" how
primordial life actually worked "because we can't go back in time." But
scientists can do the next best thing. They can study the basic properties of
relevant biological chemicals and "see if these are compatible" with the
scenarios scientists invent, he says.
Dr. Bartel and associates at the Whitehead Institute for Biomedical Research
in Cambridge, Mass., are exploring reproduction in this way. Today, DNA
molecules encode an organism's genetic instructions. The closely related
chemical RNA reads those instructions and transfers the information to the
chemical machinery that makes proteins. These, in turn, enable life processes
to work. When living cells reproduce, protein catalysts help their DNA make
new copies of itself. The early-life puzzle challenges scientists to explain
how the first life forms reproduced when DNA and proteins had not yet emerged.
Chemists have suspected that RNA might both encode genetic instructions and
promote its own replication. If so, then RNA alone would have been able to
jump-start organic life. Now, the Bartel team has shown, for the first time,
that RNA can indeed replicate itself. No natural form of RNA can do this
today. As the team explained in the journal Science, they used a process that
mimics natural evolution to create an artificial RNA to do the job. The result
is a form of RNA that can carry out the reactions needed to synthesize its own
building blocks and hook these together.
The team has not yet copied a complete RNA molecule. But it has copied enough
of an RNA template to feel it is on the right track. Team members consider
this some of the strongest evidence yet that RNA chemistry could have promoted
early life. The early life puzzle also challenges scientists to explain how
certain pre-life chemicals became biologically significant. Non-life chemistry
produced many different organic materials some 4 billion years ago on earth.
These would have included amino acids, some of which are building blocks of
proteins today.
Amino acids come in two forms that are mirror images of each other. Chemists
call them left-handed and right-handed molecules. Non-life chemistry produces
these forms in equal amounts, but life chemistry uses mainly left-handed
forms. The challenge for scientists is to explain how left-handed forms stood
out in the primordial 50-50 mix.
Robert Hazen thinks minerals are the key. Dr. Hazen and Timothy Filley at the
Carnegie Institution in Washington, and Glenn Goodfriend at nearby George
Washington University recently described research in the Proceedings of the
National Academy of Sciences that makes this point. They worked with calcite,
a mineral that forms limestone and seashells. When immersed in a 50-50 mix of
amino acids, calcite crystals preferentially segregated left-handed amino
acids on one crystal face and right-handed acids on another face.
Again, this doesn't prove pre-life chemistry worked that way. But it does
encourage Hazen to pursue his thesis.
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