A study carried out by physicists shows that basic properties of polymer molecules, such as their subunit composition, are sufficient to trigger selection processes in a plausible prebiotic environment.
Before life arose on earth, many physicochemical processes on our planet were highly chaotic. In every conceivable combination there was a multitude of small compounds and polymers of different lengths made up of subunits (such as the bases found in DNA and RNA). Before lifelike chemical processes could arise, the chaos in these systems had to be reduced. In a new study, LMU physicists led by Dieter Braun show that basic characteristics of simple polymers, together with certain aspects of the prebiotic environment, can lead to selection processes that reduce interference.
In previous publications, Braun's research group investigated how the spatial order in narrow, water-filled chambers in porous volcanic rocks on the sea floor could have developed. These studies showed that in the presence of temperature differences and a convective phenomenon known as the Soret effect, RNA strands can be locally accumulated by several orders of magnitude in a length-dependent manner. "The problem is that the base sequences of the longer molecules you get are completely messy," says Braun.
Evolved ribozymes (RNA-based enzymes) have a very specific base sequence that allows molecules to fold into specific shapes, while the vast majority of oligomers formed on early Earth were most likely random in sequence. "The total number of possible base sequences, known as 'sequence space', is incredibly large," says Patrick Kudella, first author of the new report. "This makes it practically impossible to put together the complex structures that are characteristic of functional ribozymes or comparable molecules through a purely random process." This led the LMU team to suspect that the expansion of molecules to form larger “oligomers” was subject to a kind of preselection mechanism.
Since there were few, very simple physical and chemical processes at the time of the emergence of life compared to the sophisticated replication mechanisms of cells, the selection of the sequences must be based on the environment and the properties of the oligomers. This is where research by the Braun Group comes into play. For the catalytic function and stability of oligomers it is important that they form double strands, like the well-known helical structure of DNA. This is an elementary property of many polymers and enables complexes with double and single-stranded parts. The single-stranded parts can be reconstructed by two processes. First, through so-called polymerization, in which strands are completed with individual bases to form complete double strands. The other is the so-called ligation. Longer oligomers are linked to one another. Here, both double-stranded and single-stranded parts are formed, which allow further growth of the oligomer.
"Our experiment begins with a large number of short strands of DNA. In our model system for early oligomers, we only use two complementary bases, adenine and thymine," says Braun. "We believe that the ligation of strands with random sequences leads to the formation of longer strands whose base sequences are less chaotic." Braun's group then analyzed the mixture of sequences produced in these experiments using a method also used to analyze the human genome. The test confirmed that the sequence entropy, i.e. H. The degree of perturbation, or randomness, within the obtained sequences, in these experiments was actually reduced.
The researchers were also able to identify the causes of this "self-generated" order. They found that the majority of the sequences obtained fell into two classes – with base compositions of either 70% adenine and 30% thymine or vice versa. "With a significantly larger proportion of one of the two bases, the strand cannot fold on itself and remains as a reaction partner for the ligation," explains Braun. Thus, hardly any strands with half of the two bases are formed in the reaction. "We also see how small distortions in the composition of the short DNA pool leave behind different position-dependent motif patterns, especially with long product strands," says Braun. The result surprised the researchers, since a strand made up of only two different bases with a certain base ratio offers only limited possibilities for differentiation. "Only special algorithms can recognize such amazing details," says Annalena Salditt, co-author of the study.
The experiments show that the simplest and most basic properties of oligomers and their environment can form the basis for selective processes. Even in a simplified model system, different selection mechanisms can come into play that affect strand growth on different length scales and are the result of different combinations of factors. According to Braun, these selection mechanisms were a prerequisite for the formation of catalytically active complexes such as ribozymes and therefore played an important role in the emergence of life out of chaos.