Otsuka J, Nozawa Y
Department of Applied Biological Science, Faculty of Science and Technology, Science University of Tokyo, Noda 278, Japan.
J Theor Biol. 1998 Sep 21;194(2):205-21. doi: 10.1006/jtbi.1998.0755.
The recent discovery of polymerase activity in a ribosomal RNA intervening sequence as well as other studies of RNA-replicating systems suggest that the first living molecules were RNAs called replicases. According to this suggestion from biochemical studies, the replicase system is chosen as the simplest case of self-reproducing systems, and the fundamental problem of "what is life" is theoretically investigated by analysing the behavior of a replicase system with a supply of organic materials under prebiotic conditions. In this analysis, it is essential to consider (i) self-reproduction on the basis of its own information, (ii) maintenance and improvement of the information by selection and (iii) environmental event of non-biologically generating organic materials from inorganic matter by photochemical reactions, probably occurring in prebiotic conditions on the Earth. The replicases can retain and further elevate their self-reproducibility through competition among their descendant mutants for acquiring a limited quantity of materials, if the initial ability of self-reproduction and the concentration of replicases are above some critical values. By this selection, the replicase molecules retain a narrowed range of nucleotide sequences, or a state of lower entropy, against the natural tendency of sequence divergence, but this entropy reduction is sufficiently compensated by the entropy production in the environmental event of energy conversion from photons to heat. Once the stability of a self-reproducing system is established in the above sense, the self-reproducing system can operate as Maxwell's demon to regulate the outside flow of matter by its catalytic function without any contradiction to the second law of thermodynamics.
核糖体RNA间隔序列中聚合酶活性的最新发现以及对RNA复制系统的其他研究表明，最早的生命分子是被称为复制酶的RNA。根据生化研究的这一观点，复制酶系统被选为自我复制系统的最简单例子，并通过分析在益生元条件下有有机物质供应时复制酶系统的行为，从理论上研究了“生命是什么”这一基本问题。在该分析中，必须考虑：（i）基于自身信息的自我复制；（ii）通过选择对信息的维持和改进；（iii）可能发生在地球益生元条件下的由光化学反应将无机物质非生物生成有机物质的环境事件。如果自我复制的初始能力和复制酶的浓度高于某些临界值，复制酶可以通过其后代突变体之间为获取有限数量物质的竞争来保留并进一步提高其自我复制能力。通过这种选择，复制酶分子相对于序列发散的自然趋势保留了较窄的核苷酸序列范围，即较低的熵状态，但这种熵的降低通过从光子到热的能量转换的环境事件中的熵产生得到了充分补偿。一旦在上述意义上建立了自我复制系统的稳定性，该自我复制系统就可以充当麦克斯韦妖，通过其催化功能调节物质的外部流动，而与热力学第二定律没有任何矛盾。
