Origin and fate of the small molecular weight DNA of the silkworm, Bombyx mori, during metamorphosis

Abstract

The origin and fate of small molecular weight DNA components of Bombyx, that have been demonstrated in degenerating tissues, were examined. These small components were exclusively labelled when 3H-TdR was injected at rather early stages of the fifth instar, whereas injection into pharate pupae or newly ecdysed pupae resulted in the labelling of the normal DNA only. After injecting 3H-TdR into newly ecdysed fifth instar larvae, it was found that radioactivity in the DNA fraction decreased gradually during pupal-adult development, increasing the radioactivity in acid-soluble fraction of blood. These findings indicate that the small DNA components originate not from the DNA synthesized after larval-pupal ecdysis but from the DNA synthesized in the young fifth instar larvae, and that they are degraded to acid-soluble (i.e., small mol. wt) substances to be liberated into the blood during pupal-adult development. It may be reasonable to conclude that the small DNA components are nothing but partly-degraded DNAs which occur reflecting the chromatin structure.