On producing more complexity than entropy in replication.

RNA replication in the bacteriophage Q beta system can, in principle, transmit sequence complexity at a higher rate than it increases entropy. Expanding the variety of nucleotides, through novel base-pair interactions, would move the threshold at which synthesis produces more complexity than entropy away from near equilibrium while accelerating the system approach to equilibrium. A decrease in sequence complexity during polymerization, leading to a many-to-one monomer correspondence with template, cannot be reversed, owing to symmetry restrictions. In terms of the kinetic mechanism, uncertainty associated with the the path of depolymerization yields a path entropy which selectively prolongs the reverse reaction. Together with an elevation in thermodynamic entropy, therefore, there are two possible sources of irreversibility in a physical process. Some implications of kinetic irreversibility are considered in relation to the second law of thermodynamics and to the processing and translation of mRNA.