Changing levels of UV light and carcinogen-induced unscheduled DNA synthesis in mouse oocytes during meiotic maturation

Abstract

Mouse oocytes were exposed in vitro to various DNA-damaging agents at the germinal vesicle (GV), metaphase I (M I) or metaphase II (M II) stage of meiotic maturation and then labelled with [ 3H]thymidine for 2.0 h following their exposure. Variable levels of unscheduled DNA synthesis were induced in the oocytes at the different stages of maturation by the damaging agents. These levels may be ranked in the following order: GV>M I=M II when exposed to UV light; GV>M I>M II when exposed to 4-nitroquinoline-1-oxide (4-NQO); GV=M I=M II when exposed to methyl methanesulfonate (MMS). When the oocytes were exposed to UV light or 4-NQO at the M II stage the levels of unscheduled DNA synthesis in the polar bodies were considerably lower than those in the oocytes, but MMS exposure at the M II stage resulted in the same levels of unscheduled DNA synthesis in both the polar bodies and the oocytes. Factors which may account for both the variable levels of unscheduled DNA synthesis at the different stages as well as the differential response of the oocytes and the polar bodies are discussed.