  In genetic networks, information of relevance to the organism is represented
by the concentrations of transcription factor molecules. In order to extract
this information the cell must effectively "measure"' these concentrations, but
there are physical limits to the precision of these measurements. We explore
this trading between bits of precision in measuring concentration and bits of
relevant information that can be extracted, using the gap gene network in the
early fly embryo as an example. We argue that cells in the embryo can extract
all the available information about their position, but only if the
concentration measurements approach the physical limits to information
capacity. These limits necessitate the observed proliferation of enhancer
elements with sensitivities to combinations of transcription factors, but fine
tuning of the parameters of these multiple enhancers is not required.
