Bacteria are arguably the simplest living organisms that are able to reproduce independently. Most bacteria possess one circular chromosome that is 1000 times compacted occupying only about  15% of cell volume. This pseudo-compartment of chromosome is  known as nucleoid and is maintained in absence of any nuclear membrane, unlike eukaryotes. Surprisingly, we know more about cells of higher organisms (
eukaryotic) than we do in bacteria (prokaryotic). How the E. coli DNA, e.g., that comprises of 4.6 Mbp and has a length of about 1.5mm in the extended state fits into a cell of linear dimension of about  is not well understood. The nucleoid was earlier  perceived to be an amorphous entity composed of random positioning of ill-defined domains of supercoils. Recent experiments, however, showed a remarkable degree of spatial organization [e.g, Berlatzky et. al. PNAS 105, 14136 (2008), where from the figure on the left is adapted]


We have recently shown that entropic forces can drive such spatial organization of chromosome [Phys Rev Lett, (2012)]. We considered a bottle-brush arrangement of DNA with a main-chain attached with side loops to model cross-linking in DNA due to chromosome modification proteins. We showed that such a model with an open linear backbone spontaneously adopts a helical structure with a well-defined pitch when confined to small cylindrical volume of the bacterial cell. The results were analyzed in terms of the interplay between the effective stiffness and actual intra-chain packing effects caused by the side loops in response to the confinement.  We envisage that this structural arrangement plays a crucial role in the segregation dynamics of the  newly replicated bacterial chromosome.