Science News, May 28, 1994, page 349, reports: Tricks to make DNA beget DNA For scientists
_Science News_, May 28, 1994, page 349, reports:
Tricks to make DNA beget DNA
For scientists interested in how life came about, the
chicken-and-egg controversy boils down to a question of
molecular replication. Modern DNA molecules -- the stuff of
genes -- encode information about other molecules, including
enzymes that enable DNA to replicate, mutate, and evolve as
conditions change. But how did DNA -- or perhaps RNA --
replicate before there were enzymes?
Several research groups already have mimicked many of the
necessary steps for molecular evolution (SN 8/7/93, p91) in
their atempt to re-create conditions leading to the origin of
life. But in their experiments they make new copies of these
molecules artificially, with enzymes helping.
Now, two groups have tricked small pieces of DNA into making
copies by themselves, without enzymatic assistance. Both teams
report their results in the May 19 _Nature_.
As a result of this work, "We are a step closer to
understanding possible pathways to life," comments James Ferris
of Rensselaer Polytechnic Institute in Troy, N.Y.
DNA and RNA are made up of long chains of nucleotides. In
cells, each link in the chain readily pairs off with its
complement: purines with pyrimidines and vice versa.
These connections give rise to DNA's typical structure -- a
double- stranded helix -- which enzymes help split apart during
cell division. The newly created single strands then act as
templates. Each nucleotide seeks out a new partner, and these
partners align to form a complementary strand, thereby creating
two new double helices.
In test tubes, single purine nucleotides redily assemble on a
pyrimidine template, but the reverse doesn't occur, so
replication comes to a halt with mixed templates. Also, even
when scientists could get molecules to replicate, those
molecules could not make copies of their complements.
However, using DNA fragments with three nucleotides overcomes
this obstacle, leading to the formation of complements on an
ongoing basis, says Guenther von Kiedrowski from Albert-Ludwigs
University in Freiberg, Germany.
For their experiments, von Kiedrowski and a collegue put
nucleotide threesomes into a solution that also contained a six
nucleotide strand. The matching threesomes then lined up to
make a complementary six-nucleotide strand. This strand, too,
began serving as a template for new strands.
Von Kiedrowski thinks that life's earliest molecules arose when
small DNA fragments came together and served as templates for
longer ones. Such fragments could have formed on clay
substrates, adds Ferris.
Bigger nucleotide fragments also work, report Tianhu Li and
Kyriaou C. Nicolaou, chemists at the Scripps Research Institute
in La Jolla, Calif. They started with a palindromic sequence of
24 nucleotides: The order of purines and pyrimidines reads the
same from either end of the strand.
In a slightly acidic solution, a double-stranded DNA fragment
attracted two shorter 12-nucleotide fragments, which assembled
into a third 24-nucleotide strand upon the addition of a
chemical reagent, the scientists report. Making the test-tube
solution less acidic or adding more of the 12-nucleotide
fragments causes that third strand to separate from the
original double strand and to act as a template for a second
stratnd complementary to itself.
"We're not saying that we've created life," says Nicolaou, "but
this is perhaps the first example that molecules can replicate
themselves without the help of enzymes."
Living systems expand exponentially: Two DNA strands beget
four, which beget eight, then 16, then 32, and so on. Chemical
systems increase incrementally, from one to to to three and so
on. These new processes yield molecules at an in-between rate,
say the scientists.
E-Mail Fredric L. Rice / The Skeptic Tank