A new scenario for the beginning of the formation of life on Earth

A new feature discovered in short DNA segments suggests a new scenario for the assembly of long and complex DNA strands. The researchers believe that the linear shape of today's long DNA strand is actually a sign and remnant of its formation from liquid crystals

Image 1: A liquid crystal when all its molecules are 'standing'.
Image 2: The same liquid crystal when all its molecules changed position together.
A research group from the University of Colorado in Boulder and the University of Milan discovered a new form of liquid crystals composed of extremely short DNA molecules. Since DNA molecules carry the genetic information of the cell, this discovery sheds light on a new scenario for the way life was created on Earth.

Noel Clark, a professor of physics at the University of Colorado, said that the group was surprised to find that very short sequences of DNA can bond together into organic crystals that line up parallel to each other and connect into columns when soaked in water. The popular belief today is that life began in the form of DNA or RNA molecules, created in a primordial 'soup' of organic molecules.

In practice, it is impossible to create uniform molecular chains like DNA. Thus, according to Clark, scientists have been trying to understand how simple molecules can spontaneously and efficiently connect, and then replicate themselves. The new research proves that tiny pieces of DNA can form liquid crystals and thus compress and turn into droplets. The chemical environment inside the droplets encourages them to connect with more molecules, thus creating longer and longer molecules, which tend to become liquid crystals.

"We discovered that even the smallest pieces of double-stranded DNA can spontaneously connect to strings containing many molecules," said Clark, "We believe that in the past, out of all those random molecules, short pieces of RNA or similar molecules emerged. These parts were compressed into droplets of liquid crystals, then selectively evolved into long molecules.

Liquid crystals are organic substances similar to soap, in that they have the properties of a solid and a liquid at the same time. Their main use is in computer monitors, flat TVs, cell phones, calculators and watches. Most of the molecules in the crystalline-liquid state are in the form of a straight line, and are able to spontaneously change their position. When one molecule changes its position, it also causes those around it to rotate in the same direction, thereby creating a 'domino effect' in which all the molecules change their direction almost at the same time. As a result, liquid crystals are particularly sensitive to environmental stimuli, such as changes in temperature or voltage applied to them.

RNA and DNA are chain-like polymers, with side groups called nucleotides, or bases. The bases are only willing to attach to certain other bases on the second chain. When the base sequences in the two chains match each other, the chains can connect to each other and form the famous double helix structure. The genetic information is encoded in sequences consisting of millions of bases along the chains. If we were to unfold the DNA found in one cell, we would get a chain that would exceed two meters in length. Despite this, when the DNA is compressed in the cell nucleus, the length of all the chains is several millionths of a meter.

Clark admits that it has already been proven in the past that DNA is able to organize itself as a liquid crystal, in which the chains line up parallel to one another. The researchers believe that the shape of the liquid crystal is due to the elongated shape of the DNA molecule, because of which the molecules prefer to line up parallel to each other. The process can be compared to spaghetti sticks in a box, which even after a thorough shaking will remain parallel to each other.

The research teams from the University of Colorado and Milan began a series of experiments to see how much the DNA segments can be shortened before they stop organizing as liquid crystals. The teams discovered that even very short DNA segments - six bases long - can assemble into a liquid crystal shape, even though their shape is hardly elongated. The condition for the process is the pairing of the DNA strand with its corresponding strand.

Analysis of the liquid crystal structure showed that the short pairs of DNA strands joined together end to end, thus creating straight and elongated aggregates that could behave similarly to longer segments of DNA. The adhesion occurred as a result of the presence of small, fatty dots at the ends of the short DNA segments, which helped them stick to each other in a reversible way, while keeping the water between the two segments away.

The group continued with additional tests, during which they mixed together solutions containing matching and non-matching segments of DNA. The results showed that virtually all the matching DNA segments were compressed into droplets of liquid crystals, moving away from the non-matching DNA segments.

"This is a one-of-a-kind phenomenon," Clark said. "The interpretation is that small molecules that are able to connect in the right way can scan one after the other, and connect together into droplets with a very ordered internal structure. These droplets allow even larger double-stranded molecules to form.

"Actually, the liquid crystal droplets select by themselves the molecular components that are suitable for them. With the right chemistry, larger and larger molecules can develop that stabilize the liquid crystal structure. If the model is correct, then the linear shape of the DNA polymer is itself a sign and a remnant of its formation from a liquid crystal."

The study was published in the latest issue of Science, on November 23. The authors of the article are Noel Clark, Michi Nakata and Christopher Jones from the University of Colorado, Giuliano Zankata and Tommaso Bellini from the University of Milan, Brandon Chapman and Ronald Findak from the Brookhaven National Laboratory and Julie Cross from the Argonne National Laboratory. Michi Nakata died in September 2006.

For information on the University of Colorado website