Practice question

Bacterial genome organization contrasts sharply with eukaryotic chromatin packed into chromosomes with histones. Escherichia coli, Bacillus subtilis, and most eubacteria possess single circular chromosome ranging 1 to 6 megabases, contour length about 1 millimeter compacted over 1000-fold into irregular nucleoid region via negative supercoiling induced by ATP-dependent DNA gyrase and topoisomerase I relaxation, bridging by nucleoid-associated proteins HU, H-NS, Fis, and condensin MukBEF complex. DNA replication initiates at single origin oriC regulated by DnaA-ATP binding to DnaA boxes, proceeds bidirectionally with leading and lagging synthesis coupled to membrane anchoring. Plasmids are extrachromosomal small circles conferring antibiotic resistance, virulence factors, conjugation ability. Linear chromosomes exist as rare exceptions in Borrelia burgdorferi spirochete causing Lyme disease and Streptomyces coelicolor, but require specialized telomere-like proteins and linear replication mechanisms. Claiming bacterial DNA is generally linear misrepresents dominant circular topology, which impacts replication termination at dif site resolved by XerCD recombinases and segregation by ParABS partitioning. Lack of nucleus enables coupled transcription-translation, and reproduction by binary fission using FtsZ tubulin homolog constricting septum without mitotic spindle typical for bacteria.