a three-dimensional form. The haploid circular chromosome in E. coli consists of ~ 4.6 x 106 bp. If DNA is relaxed in the B form, it would have a circumference of ~1.5 millimeters (0.332 nm x 4.6 x 106). However, a large DNA molecule such as the E. coli chromosomal DNA does not remain a straight rigid molecule in a suspension. Brownian motion will generate curvature and bends in DNA. The maximum length up to which a double-helical DNA remains straight by resisting the bending enforced by Brownian motion is ~50 nm or 150 bp, which is called the persistence length. Thus, pure DNA becomes substantially condensed without any additional factors; at thermal equilibrium, it assumes a random coil form. The random coil of E. coli chromosomal DNA would occupy a volume (4/3 π r3) of ~ 523 µm3, calculated from the radius of gyration (Rg = (√N a)/√6) where a is the Kuhn length (2 x persistence length), and N is the number of Kuhn length segments in the DNA (total length of the DNA divided by a). Although DNA is already condensed in the random coil form, it still cannot assume the volume of the nucleoid which is less than a micron. Thus, the inherent property of DNA is not sufficient: additional factors must help condense DNA further on the order of ~103 (volume of the random coil divided by the nucleoid volume). The second essential aspect of nucleoid formation is the functional arrangement of DNA. Chromosomal DNA is not only condensed but also functionally organized in a way that is compatible with DNA transaction processes such as replication, recombination, segregation, and transcription. Almost five decades of research beginning in 1971, has shown that the final form of the nucleoid arises from a hierarchical organization of DNA. At the smallest scale (1 kb or less), nucleoid-associated DNA architectural proteins condense and organize DNA by bending, looping, bridging or wrapping DNA. At a larger scale (10 kb or larger), DNA forms plectonemic loops, a braided form of DNA induced by supercoiling. At the megabase scale, the plectonemic loops coalesce into six spatially organized domains (macrodomains), which are defined by more frequent physical interactions among DNA sites within the same macrodomain than between different macrodomains. Long- and short-range DNA-DNA connections formed within and between the macrodomains contribute to condensation and functional organization. Finally, the nucleoid is a helical ellipsoid with