Self-replication: spelling it out in a chemical background.

Self-replication, an important concept abstracted from reproduction, the key feature of life, remains vague in definition and lacking in clear interpretation in terms of its chemical mechanism. Mentioned frequently in discussions concerning the essence of life and its origin, the vague concept has caused a lot of uncertain statements, confusable references, and malposed debates, and has seriously held back efforts in this field. In this article, we try to improve the situation by a conceptual analysis in a more fundamental and clearer background. Self-replication in the substantial world could not mean anything but that "an entity favors the production of its own." The major chemical mechanism for such favoring is catalysis, which can be classified into speed- and direction-favoring types (the template-directing function is actually a type of direction-favoring catalysis). Molecular self-replication could be based on autocatalysis or self-metabolism; the self-replication of a complex entity could be based on autocatalytic and/or self-metabolic sets, and should involve a mechanism of self-division. This conceptual clarification sheds light on the dim areas concerning the essence of life and its origin.