电脑模型解释单分子如何进化为复杂的生命

Nearly four billion years ago, the earliest precursors of life on Earth emerged. First small, simple molecules, or monomers, banded together to form larger, more complex molecules, or polymers. Then those polymers developed a mechanism that allowed them to self-replicate and pass their structure on to future generations. We wouldn't be here today if molecules had not made that fateful transition to self-replication. Yet despite the fact that biochemists have spent decades searching for the specific chemical process that can explain how simple molecules could make this leap, we still don't really understand how it happened. 

Now Sergei Maslov, a computational biologist at the U.S. Department of Energy's Brookhaven National Laboratory and adjunct professor at Stony Brook University, and Alexei Tkachenko, a scientist at Brookhaven's Center for Functional Nanomaterials (CFN), have taken a different, more conceptual approach. They've developed a model that explains how monomers could very rapidly make the jump to more complex polymers. And what their model points to could have intriguing implications for CFN's work in engineering artificial self-assembly at the nanoscale. Their work is published in the July 28, 2015 issue of The Journal of Chemical Physics.

To understand their work, let's consider the most famous organic polymer, and the carrier of life's genetic code: DNA. This polymer is composed of long chains of specific monomers called nucleotides, of which the four kinds are adenine, thymine, guanine, and cytosine (A, T, G, C). In a DNA double helix, each specific nucleotide pairs with another: A with T, and G with C. Because of this complementary pairing, it would be possible to put a complete piece of DNA back together even if just one of the two strands was intact. 

While DNA has become the molecule of choice for encoding biological information, its close cousin RNA likely played this role at the dawn of life. This is known as the RNA world hypothesis, and it's the scenario that Maslov and Tkachenko considered in their work. 

The single complete RNA strand is called a template strand, and the use of a template to piece together monomer fragments is what is known as template-assisted ligation. This concept is at the crux of their work. They asked whether that piecing together of complementary monomer chains into more complex polymers could occur not as the healing of a broken polymer, but rather as the formation of something new.

"Suppose we don't have any polymers at all, and we start with just monomers in a test tube," explained Tkachenko. "Will that mixture ever find its way to make those polymers? The answer is rather remarkable: Yes, it will! You would think there is some chicken-and-egg problem--that, in order to make polymers, you already need polymers there to provide the template for their formation. Turns out that you don't really."