abput a theoretical, chemical system reproducing some biological features


Here, a chemical system based on polymers is discussed that contains some elements typically associated with life. The system may provide insights into evolution.


Introduction

Life comprises a series of highly complex phenomena, distributed in multiple forms. Plants, animals, fungi, bacteria or archaeobacteria are all living organisms, as they share some common characteristics. Specifically, living organisms are entities capable of growing and reproducing autonomously. Moreover, they do it in such a way that replicates the organism’s composition and the behavior across generations. In these processes, both matter and energy need to be obtained from the environment [1].

Today, it is known that DNA plays a fundamental role in life, as it controls the cellular machinery and is responsible for the transmission of the genetic information [2]. Another key element of living systems is the presence of cells. Every organism is composed of one or more cells [1].

There are other complex molecular entities, in particular viruses [3] and prions [4] , which lack certain biological features. Viruses cannot replicate autonomously, although they do contain nucleic acids [3]. Prions are proteins that propagate by transforming other previously non-prion proteins, however this cannot be considered replication. In addition, no growth is associated with this process [4].

Pseudo-living (Pl) polymers

A polymer is generally conceived as a molecule composed of multiple subunits (monomers) [5]. The simplest polymers are composed of only one type of monomers. These are called homopolymers. In a homopolymer, each monomer must have at least two functional groups. The functional groups enable the growth of the polymer. Each new monomer is added by binding one functional group to another one in the growing polymer [5]. The chemical reaction forming the bond between two monomers may increase the entropy of the system formed by the polymer and the new monomer. However, if this is not the case a third element is needed (e.g a parallel chemical reaction or the incidence of light) to complete the reaction.

Alternatively, polymers may lose monomers as well. This can be due to a series of factors, for example a temperature increase or the unstability of the polymer when reaching a certain size.

The above referred system can be considered to a certain extent a pseudo-living entity. The pseudo-living (Pl) polymer i) grows when incorporating new monomers, also it ii) consumes matter from the environment (new monomers). In addition, it may iii) consume energy -for example a light catalyzed reaction to incorporate new monomers. The polymer can also iv) reproduce if it is divided in two or more parts. That can be an effect of reaching a certain critical size when the molecule is no longer stable. At the same time, the new “individuals” generated are also composed of one or more monomers; i.e they share the same chemical composition as the previous generation. This is a consequence of the presence of v) “genetic information”, in the sense that the nature of the polymer, and especially of its functional groups ensures that only new monomers can be incorporated into the molecule. Nevertheless, the system is not 100% reliable so that imperfections

(“mutations”) can occur. In other words, the population of polymers can potentially vi) evolve. Finally, vii) “regulation of gene expression” is also possible, since environmental conditions may affect the addition of new monomers; for example the polymer may grow only at certain pH conditions.


Discussion

Life is based on repetition, both at the spatial scale and the temporal scale. Spatially, organisms are composed of one or multiple cells. At the same time, the members of a population are repetitions of one basic unit (the individual). In terms of time, living systems are able to replicate themselves, leading to different generation of a same species.

The fact that polymers are also intrinsically repetitive may explain the biological-like properties in these systems.

In this regard, it is noteworthy the polymeric nature of main biological biomolecules such as DNA, RNA or proteins. Of course, these molecules are not pseudo-living polymers, as they require complex machineries to grow and propagate. DNA or RNA molecules encode proteins, whilst enzymatic proteins in turn catalyze nucleic acid replication and expression. In this manner, neither nucleic acids nor proteins by themselves grow and replicate autonomously.

Nevertheless, it could be possible that proto-nucleic acids and proto-proteins existed that were in fact pseudo-living polymers. These would be related although chemically different from nucleic acids and proteins. A singular symbiosis between these hypothetical Pl polymers may have lead to current living organisms. For example, the referred Pl polymers may have catalyzed the incorporation of new monomers in the other Pl polymer or have offered protection from environmental conditions. In the course of evolution, thus, the central dogma of molecular biology could have emerged potentially from the symbiosis of Pl polymers.


References

[1] Lodish, U. (2016). Molecular Cell Biology. W.H. Freeman.

[2] Griffiths, A., Miller, J., Suzuki, D., Lewontin, R. and Gelbart, W. (2000). An Introduction to Genetic Analysis. 7th ed. New York.

[3] Payne, S. (2018). Viruses. From understanding to investigation. 1st ed. Academic Press.

[4] Baker, H. (2001). Molecular pathology of the prions. Totowa, N.J.: Humana Press.

[5] Carraher, C. and Seymour, R. (2010). Seymour/Carraher's polymer chemistry. Boca Raton, Fla.: CR