DNA fragmentation during apoptosis is caused by frequent single-strand cuts

Nucleic Acids Res. 1993 Sep 11;21(18):4206-9. doi: 10.1093/nar/21.18.4206.


One of the hallmarks of apoptosis is the digestion of genomic DNA by an endonuclease, generating a ladder of small fragments of double-stranded DNA. We have examined the nature of the DNA breaks produced in mouse thymocytes triggered to undergo apoptosis by steroids or by stimulation of the T cell receptor. Whereas the typical ladder pattern of oligonucleosomal fragments was observed after agarose gel electrophoresis, numerous single-strand cuts were detected after electrophoresis under denaturing conditions. Single-strand nicks were found to be very frequent in the internucleosomal regions, but also to occur in the core particle-associated DNA. An identical pattern of single-strand nicks was obtained when chromatin DNA was exposed to the single-strand cleaving deoxyribonuclease I. The nicked DNA fragments, extracted from apoptotic thymocytes, were sensitive to the action of S1-nuclease. We propose that DNA fragmentation induced during apoptosis is not due to a double-strand cutting enzyme as previously postulated, but rather is the result of single-strand breaks. This ensures the dissociation of the DNA molecule at sites where cuts are found within close proximity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis / drug effects
  • Apoptosis / genetics*
  • CD3 Complex / physiology
  • Cells, Cultured
  • Computer Simulation
  • DNA Damage*
  • Dexamethasone / pharmacology
  • Electrophoresis, Agar Gel
  • Mice
  • Signal Transduction


  • CD3 Complex
  • Dexamethasone