Fig. 6.

Age-related DNA fragmentation occurred in somatic cells, but not in sperm. The undigested genomic DNAs of sperm, embryos (h; hours post fertilization), and adult muscle of the indicated ages were run on a 1 % agarose gel and stained with ethidium bromide (a). In addition to a high molecular weight DNA band (arrowhead), a small DNA fragment was seen in an aged fish (arrow). Small DNA fragments in the tissues of an aged zebrafish were visualized by ethidium staining (b) and by the end-labeling method we developed (c). The same procedure was employed to reveal the stage at which small DNA fragments appeared (d–f). The times for exposure to X-ray films were 20 and 60 s for e and f, respectively. Similar to DNA hypomethylation, this age-related phenomenon was observed in all of the somatic cells examined, but not in male germ cells. The fragments showed a 180 ~ 200-bp periodicity, resembling the DNA cleavage observed in apoptosis. Lane M, 1 kb or 100 bp DNA ladder