News: DNA-like Molecule Replicates Without Help
Robert Karl Stonjek
By Robert F. Service
ScienceNOW Daily News
11 June 2009
Researchers pondering the origin of life have long struggled to crack the
ultimate chicken-and-egg paradox. How did nucleic acids like DNA and
RNA--which encode proteins--first form, when proteins are needed for their
synthesis? Now, scientists report that they've cooked up molecular hybrids
of proteins and nucleic acids that skirt the dreaded paradox. Although it's
unknown whether such molecules existed prior to the emergence of life, they
offer insight into a chemical pathway that might have helped life arise.
DNA and RNA sport a backbone of sugar and phosphate groups linked to the
nucleotide bases that spell out the genetic code. Certain proteins help copy
nucleic acids by fashioning complementary strands that carry matching
nucleotides. But how could nucleic acids originate without proteins, and
vice versa? Proponents of the "RNA World" hypothesis argue that RNA itself
was the key because of its dual abilities: It not only carries genetic
information but also can catalyze chemical reactions. That view received a
big boost earlier this year, when researchers at The Scripps Research
Institute in San Diego, California, showed that small RNA fragments can
catalyze their own reproduction. "The question remains, how those first RNA
molecules appeared," says Luke Leman, a chemist at Scripps who was not part
of the study. Other researchers have synthesized DNA and RNA analogs with
simpler sugar backbones that may have done the job. Yet those are still
complex, lessening the chance that they were the primordial replicating
molecules, Leman says.
In hopes of finding something simpler, Leman and colleagues did away with
the sugar-phosphate backbones altogether. Instead, they turned to amino
acids, protein building blocks that have been shown to assemble under
prebiotic conditions. The researchers report online today in Science Express
that when they combined just two amino acids, a backbone readily assembled
without the need for additional enzymes. They then found that DNA bases
could bind to a sulfur group in one of the amino acids, cysteine, creating a
protein-DNA hybrid strand. But because the nucleic acid bases attach weakly
to the cysteines--think Velcro instead of glue--the bases can jump on and
off in solution. As a result, when the researchers placed their hybrids in
solution with single strands of DNA and RNA, the hybrids were able to
rearrange their nucleic acid makeup to form complementary strands that would
bind to the DNAs and RNAs. The researchers discovered that the hybrids could
also form strands that would bind to other complementary hybrids, which
shows that such molecules have the potential to copy themselves.
"This is very interesting and creative," says Eric Kool, a chemist at
Stanford University in Palo Alto, California, who studies nucleic acid
analogs. These particular hybrids change so rapidly in solution, it's
unclear if they would remain stable long enough to propagate genetic
information over several generations. However, Kool says, "It's an idea
worth considering."
Source: Science
http://sciencenow.sciencemag.org/cgi/content/full/2009/611/1?etoc
Posted by
Robert Karl Stonjek