Mathematical models to describe transcription (Arnold et al. (2001); Biotech Bioeng 72:548-561) and translation (Mehra et al. (2003); Biotech Bioeng 84:822841) in bacteria are modified in order to improve reaction kinetics and to include the number of polymerase molecules that are active on the DNA, as well as to include the number of ribosomes that are active on the nascent and on the completed mRNA, respectively. (c) 2006 Wiley Periodicals, Inc.