  Plasmids are extra-chromosomal DNA molecules which code for their own
replication. We previously reported a setup using genes coding for fluorescent
proteins of two colors that allowed us, using a simple model, to extract the
plasmid copy number noise in a monoclonal population of bacteria [J. Wong Ng et
al., Phys. Rev. E, 81, 011909 (2010)]. Here we present a detailed calculation
relating this noise to the measured levels of fluorescence, taking into account
all sources of fluorescence fluctuations: the fluctuation of gene expression as
in the simple model, but also the growth and division of bacteria, the
non-uniform distribution of their ages, the random partition of proteins at
divisions and the replication and partition of plasmids and chromosome. We show
how using the chromosome as a reference helps extracting the plasmid copy
number noise in a self-consistent manner.
