DNA gyrase is an essential bacterial enzyme that catalyzes negative supercoiling of plasmid and chromosomal DNA. In the supercoiled state, DNA adopts a branched and interwound conformation that allows the large chromosome to function in the highly constrained space of a bacterial cell. In living cells, many enzymes move along the DNA, causing rapid rotation of the double helix. Gyrase and a family of related enzymes called topoisomerases prevent hypersupercoiling and keep sister chromosomes from becoming intertwined or knotted together, which would prevent chromosome segregation during cell division. Topoisomerases carry out their reactions by forming reversible covalent protein-DNA complexes with the phosphodiester backbone. The supercoil level of microbial chromosomes is highly regulated and varies in different bacterial species with different optimal growth rates. Gyrase activity is under strong genetic selection to match the catalytic rate of RNA polymerase chain elongation during rapid growth. Because gyrase is not present in eukaryotes, potent antibiotics that block (Ciprofloxacin) or slow (Novobiocin) gyrase supercoiling activity have been developed to treat patients infected with a wide range of pathogenic bacteria.