An autocatalytic cycle is a set of reactions or other processes that form a special kind of loop- with each completion of the loop, at least one of the compounds is duplicated. A good schematic representation of this phenomenon was made in a paper by Zachar and Szathmáry (2010):
In this representation, the X products are inputs that drive the cyclic process, the Y products are other outputs of the cyclic process, and A is the compound that is produced in excess with each turn of the loop.
This basic relationship goes far beyond chemistry, and is actually found throughout all of biology. Cell replication and species reproduction are the most obvious and familiar examples of organismal autocatalysis: a parent organism or organisms consumes food and results in the replication of its pattern, i.e., the production of offspring.
At the subcellular level, the reductive TCA cycle is a striking example of a metabolic autocatalytic relationship. The replication of the ribosome or the replication of strands of DNA are striking examples of subcellular autocatalytic relationships in genetics. The complex dynamics seen in ecosystems and economic systems are arguably underpinned and made possible by coupling between different population-level autocatalytic relationships such as mutualism, symbiosis and predation.
Encyclopedia Autocatalytica 