Releases the supercoiling and torsional tension of DNA introduced during the DNA replication and transcription by transiently cleaving and rejoining one strand of the DNA duplex (By similarity). Introduces a single-strand break via transesterification at a target site in duplex DNA (By similarity). The scissile phosphodiester is attacked by the catalytic tyrosine of the enzyme, resulting in the formation of a DNA-(3'-phosphotyrosyl)-enzyme intermediate and the expulsion of a 5'-OH DNA strand (By similarity). The free DNA strand then undergoes passage around the unbroken strand thus removing DNA supercoils (By similarity). Finally, in the religation step, the DNA 5'-OH attacks the covalent intermediate to expel the active-site tyrosine and restore the DNA phosphodiester backbone (By similarity). ATP-independent breakage of single-stranded DNA, followed by passage and rejoining. Interacts with Topors. During early cell divisions of developing embryos, it is detected in the nucleus at interphase then diffusely dispersed in the cytoplasm at metaphase. Detected in germline and follicle cells. Expressed in follicle cells throughout oogenesis. In germline cells, expression levels increase as cells develop and move to the posterior region of the germarium, then at stage 5 of egg chamber development levels decrease until it is no longer detected (at protein level). Eukaryotic topoisomerase I and II can relax both negative and positive supercoils, whereas prokaryotic enzymes relax only negative supercoils. Belongs to the type IB topoisomerase family.