ON THE IMPORTANCE OF DNA STRAND BREAKS AS THE FIRST EVENT TO INITIATESISTER-CHROMATID EXCHANGE (SCE) - EXPERIMENTS WITH RESTRICTION-ENDONUCLEASE BGLI

The exact molecular mechanism of sister chromatid exchange (SCE) is st ill unknown, despite the many reports dealing with this cytogenetic en d point published in the last 40 years. One point to be investigated i s the nature of the original lesion(s) in DNA leading to the productio n of SCE. Whereas, for chromosomal aberrations, the importance of DNA double-strand breaks has bean well established, there is still controv ersy about: the relative Importance of strand breaks end base modifica tions for triggering the process of SCE formation. In the present pape r, we have taken advantage of the ability of the restriction endonucle ase BgII to induce SCE and have exploited the fact that preincubation with 2,3-butanedione results in the loss of BgII ability to cut DNA, w hile it is still able to recognize its sequence in DNA and bind to it, to see whether this alone is enough to initiate SCE formation, or if a physical DNA double-strand break is required. Our results seem to su pport the necessity of DNA breaks for SCE production.