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Posted by baalke on November 14, 2006, 6:19 pm
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http://www.carnegieinstitution.org/dna_microarray/default.html
Carnegie Institution News Release
November 9, 2006
Contact:
Robert Hazen
1-202-478-8962
rhazen@gl.ciw.edu
PIO Contact:
Tina McDowell
1-202-939-1120
tmcdowell@ciw.edu
For a copy of the paper, contact Rachel Russell at
rrussell@minsocam.org
See http://ammin.geoscienceworld.org
Selecting life: Scientists find new way to search for origin of life
Washington, D.C. - Over the last half century, researchers have found
that
mineral surfaces may have played critical roles organizing, or
activating,
molecules that would become essential ingredients to all life-such as
amino
acids (the building blocks of proteins) and nucleic acids (the essence
of
DNA). But which of the countless possible combinations of biomolecules
and
mineral surfaces were key to this evolution? This vexing question has
stumped scientists for years because of the sheer volume of
possibilities.
Now an interdisciplinary team of researchers led by Robert Hazen, of
the
Carnegie Institution's Geophysical Laboratory and former president of
the
Mineralogical Society of America, has developed new protocols and
procedures for adapting DNA microarray technology to rapidly identify
promising molecule/mineral pairs.
Hazen's Presidential Address in the November/December issue of American
Mineralogist describes this work. It sets out a first-of-its-kind
comprehensive survey into research that has identified processes by
which
minerals may have prompted the transition from a geochemical world to a
biological one almost four billion years ago.
Scientists understand several probable steps in the origin of life,
notably
how the first organic molecules could have formed. In fact, prebiotic
synthesis processes are now thought to have been so productive that the
ancient Earth must have had far more different kinds of molecules than
could have been used by early life. One of the biggest questions in
origins
research, therefore, is how just the right blend of critical
biomolecules
was selected, concentrated, and organized from the diverse primordial
"soup." Previous research by the Carnegie team and others has shown
that
many molecules, including amino acids, can adhere to mineral surfaces,
prompting further organic reactions. These findings have made
surface/molecule interactions the subject of intense study.
Scientists suspect that organic material was likely introduced to Earth
from many complementary sources. Abundant biomolecules form in
molecular
clouds in deep space, and these extraterrestrial compounds must have
rained down on the early Earth. Other molecular synthesis was driven by
lightning and ultraviolet radiation in the atmosphere or volcanic heat
and
chemical reactions in the deep oceans. Some of these building blocks of
life were attracted to specific mineral surfaces, where they collected,
concentrated, and underwent further reactions.
"Some 20 different amino acids form life-essential proteins," Hazen
explained. "In a quirk of nature, amino acids come in two mirror-image
forms, dubbed left and right-handed, or chiral molecules. Life, it
turns
out, uses the left-handed varieties almost exclusively. Non-biological
processes, however, do not usually distinguish between left and right
variants. For a transition to occur between the chemical and biological
eras, some process had to separate and concentrate the left- and
right-handed amino acids. This step, called chiral selection, is
crucial to
forming the molecules of life."
Like amino acids, some minerals have pairs of crystal surfaces that
have a
mirror relationship to each other, called left and right faces.
Calcite,
one such mineral, is common today and was prevalent during the Archean
Era
when life first emerged. In 2001 Hazen and colleagues performed the
first
experiments showing that the left-handed amino acid, aspartic acid,
preferentially adhere to left-faced calcite. That study confirmed
previous
theoretical suggestions of a plausible process by which the mixed
right-
and left-handed -amino acids in the primordial soup could be
concentrated
and selected on a readily available mineral surface. The challenge
since
has been to determine which of the countless biomolecules/surface
interactions are the most likely candidates to the first steps to life.
"Crystal surfaces are complicated," Hazen continued. "They have
crevices
and craters, and are seldom flat. We need to find which surface types
are
the best 'docking stations' for different biomolecules. However, there
are
hundreds of mineral surface types and thousands of plausible prebiotic
molecules, making literally millions of possible biomolecule/mineral
pairs.
It's an overwhelmingly large number of possibilities."
DNA microarrays provide a means to address this problem. Microarrays
are
produced robotically to spot tens of thousands of microscopic droplets
of
DNA from as many genes onto a slide, enabling scientists to measure
which
genes are turned on. This rapidly developing technology can be used to
identify, for example, the genes involved in disease. The
high-throughput
has revolutionized biotechnology research.
Hazen, working with Carnegie staff scientist Andrew Steele and his
team,
has developed modifications of this tool to study molecule/mineral
interactions. The scientists have devised protocols for cleaning
mineral
surfaces, spotting the surfaces with up to 96 different organic
species,
washing the surfaces to remove molecules that don't adhere to a mineral
surface, and locating the remaining adsorbed molecules.
To discover "which molecules stick and which don't," as Hazen says, the
Carnegie scientists are also collaborating with a team at the
Smithsonian
Institution led by Edward Vicenzi to employ a workhorse of chemistry
called
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The
instrument
effectively blasts a sample with ionized particles and fragments the
surface-bound molecules and topmost mineral layer. The mass
spectrometer
allows the researchers to determine what's there. "ToF-SIMS will also
allow
us to detect the organic molecules that bind most strongly to mineral
surfaces," commented Hazen.
"In many ways, Hazen's approach cuts to the chase of addressing this
problem," said Patricia Dove, professor of geochemistry at Virginia
Tech.
"By adapting the microarray approach from molecular biology, his
research
group can identify up to one million types of biomolecules very quickly
to
learn which have the strongest interactions with mineral surfaces. It
doesn't stop there though-another real advance lies in analyzing their
experiments by the ToF-SIMS. This eliminates the need for chemical tags
whose own properties could influence the results."
David Deamer, professor of chemistry and biochemistry at U.C. Santa
Cruz,
commented that "Bob Hazen is boldly asking a fundamental question
related
to the origin of life. We know that organic compounds were present in
the
early Earth environment, but as dilute solutions of thousands of
different
species in the global seas. How were specific kinds of organics
selected
to assemble into the first forms of life, and by what process were they
sufficiently concentrated to initiate a primitive version of
metabolism?
We now know that minerals select specific organic compounds out of
solution, and can even distinguish between subtle properties such as
chirality, binding a left-handed amino acid in preference to one that
is
right handed. These are very significant results that are guiding my
own
research as well as many other investigators in the field."
Once Hazen and coworkers have identified molecule/surface pairs of
interest
with the DNA microarray and ToF-SIMS, an arsenal of other techniques
can be
used to look at the details of the interactions.
"What's particularly rewarding about this research is that it's an
interdisciplinary effort from different areas of science-biology,
chemistry
and geology," reflected Hazen. "It marries them to search for an answer
to
a question that has intrigued humanity since the birth of
consciousness:
How did we get here?"
_________________________________________
This work was supported by the NSF Geobiology and Low-temperature
Geochemistry program of the Division of Earth Sciences, the NASA
Astrobiology Institute (NAI), and the Carnegie Institution.
The NAI was founded in 1997. It is a partnership between NASA, 12 major
U.S. teams, and six international consortia. NAI's goal is to promote,
conduct, and lead integrated multidisciplinary astrobiology research
and
to train a new generation of astrobiology researchers. For more
information about the NAI on the Internet, visit: http://nai.nasa.gov/
The Carnegie Institution of Washington www.carnegieinstitution.org, a
private nonprofit organization, has been a pioneering force in basic
scientific research since 1902. It has six research departments: the
Geophysical Laboratory and the Department of Terrestrial Magnetism,
both
located in Washington, D.C.; The Observatories, in Pasadena,
California,
and Chile; the Department of Plant Biology and the Department of Global
Ecology, in Stanford, California; and the Department of Embryology, in
Baltimore, Maryland.
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