The bridge between the still and the living
6.6.2000
By: Marit Selvin
How life originated is one of the most difficult questions in science. Scientists have been dealing with it for hundreds of years, but so far have not reached a satisfactory answer. Since the XNUMXs, there has been active research on this important question, in which researchers from various fields of science participate, but despite the extensive achievements in the field of molecular biology, there is still no convincing script that explains the processes that led to the formation of life.
The conventional view claims that living beings are characterized by the ability to reproduce themselves and pass on the information stored in them to their offspring. All living things - bacteria, plants, animals, as well as each and every cell in all multicellular creatures - have the ability to self-replicate. Replication includes all the components of the cell, which receive the instructions from the information stored in the nucleic acids - the DNA and the DNA. During replication, an exact copy of the sequence of building blocks is made, but every now and then mistakes are made: these are the mutations. The new copies carrying the mutations are sometimes more successful and manage to survive when the environmental conditions change, and thus a process of natural selection is obtained.
Replication of molecular systems is linked, as mentioned, to the ability to copy information and pass it on from generation to generation. Thus many scientists have so far tended to assume that life began with the spontaneous formation of long chain molecules such as DNA, R-NA or proteins. Experiments designed to mimic the conditions that existed on the ancient Earth about four billion years ago showed that the building blocks of these molecules could indeed have formed spontaneously at that time. But many scientists claim that the chances that all of these will join together and produce a primary key molecule capable of self-replication are extremely low, and hence the assumption that a successful chance encounter between the right building blocks gave rise to a functional nucleic acid is rejected as improbable.
What, then, was the first step that eventually led to the creation of living cells? The attempts to find a way out of this impasse led Prof. Doron Lantz from the Department of Molecular Genetics at the Weizmann Institute and his research students, Daniel Segera and Dafna Ben-Eli, to search for simpler molecular structures capable, under the conditions that prevailed on the surface of the ancient Earth, of replicating themselves and forming the basis for the formation of life. The three researchers developed a model that describes a new way of understanding the beginning of life. According to this model, life arose from the organization of structures made of lipid molecules (a fatty substance that is the main component of the cell membrane). Their research work was published last month in the journal "Academy of Sciences Proceedings of the National".
"I assumed that a chemical mechanism must be sought that leads a random collection of chemical substances to entities capable of self-replication. And so we started looking for simple molecules, albeit the most primitive ones, that can create structures with such an ability", Lantz says. Lipids are a main component of the membranes surrounding the cell and the nucleus. The structure of the lipids is relatively simple, and it has been proven that they are found in comets, and could have formed under the conditions that prevailed on the early Earth.
Due to their special composition, the lipids can spontaneously form tiny droplets, which are clusters containing thousands of molecules. This is how they are also organized in the membranes of living cells. "Scientists did not take lipids seriously, since in today's cells they play only auxiliary roles in creating a separation between the parts of the cell. But we have shown that under certain conditions the lipid clusters can contain information and also undergo replication-like processes," says Lantz. Lantz and his students built a computer model fed by the conditions that prevailed on the surface of the ancient Earth, statistical laws and interrelationships between molecules, and from it they get a new and intriguing process.
In the first step of the process, the lipid molecules connect to each other. Later, the computerized model presents a new phenomenon that belongs to the field of complex systems: the clusters of lipid molecules show the ability to store information and replicate. How does this happen? Lantz: "Our assumption was that under the conditions that prevailed on the surface of the ancient Earth, thousands and even millions of different types of lipid-like molecules were created. It turns out that for droplets below a certain size there is an astronomical number of connection possibilities between the individual lipid molecules and supramolecular structures, and thus spontaneously different lipid clusters are obtained. An evolution-like process can be based on this diversity."
Out of the great versatility of the lipid clusters, some show special properties. In these clusters, a network of mutual effects forms between the molecules, resulting in the entire cluster growing while maintaining its original composition. When the collection of molecules exceeds a certain size, the cluster splits into two parts (a well-known phenomenon in the world of lipids) that are similar in composition. Lentz calls the phenomenon "copying of a composite genome". According to the researchers, this is the simplest mechanism for self-replication, and it is the one that could have started natural selection processes.
Lantz and his students showed, through simulations on supercomputers, that the connection between all these properties creates a life-like phenomenon. On the computer screen you see how lipid clusters with a unique composition are created, grow, split, replicate, accumulate "mutations" and participate in complicated processes of selection and evolution.
How can this "lipid world" eventually lead to the creation of complex nucleic acid molecules, as we know them today? "We think that in these processes, based on the system's ability to undergo natural selection, more complex materials are created over time that are more and more similar to what we see today in living things. We are trying to simulate this development using a computer," says Lantz. "According to our approach, nucleic acids are the result of an evolutionary process and not its beginning. The starting point of our model is based on simple chemistry, but the result is complex life-like behavior. Therefore, we hope that the model can be the bridge that many were looking for between the still world and the living world."
{Appeared in Haaretz newspaper, 6/6/2000}
