Subject: DNA Replicates Without Enzymes Keywords: DNA,replication,enzymes Date: Fri, 10 Ju
From: firstname.lastname@example.org (Karl Hahn)
Subject: DNA Replicates Without Enzymes
Organization: Loral Data Systems
Date: Fri, 10 Jun 1994 16:59:51 GMT
_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 attempt 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 live," 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 live," 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