Bacterial DNA gyrase enzyme plays a role in replication, transcription, recombination and repair. Escherichia coli gyrase enzyme is composed of two types of subunit, the gyr A and gyr B gene products called subunit A and B. Each type of subunit can be inhibited with temperature-sensitive mutations or by treating cells with specific antibiotic. Quinolones class of drugs such as nalidixic acid, oxonilic acid and norfloxacin affect gyr A protein. This effect due to inactivation of gyrase arises from the change in DNA superhelical tension. Mutant bacteria resistant to nalidixic acid show structural alterations in subunit A. Nalidixic acid resistant mutants of E. coli strains showed lower recombination efficiency, linkage, DNA repair, and mutation frequency than their wild types.