The linker sites are the only parts of the DNA strand that are exposed and thus accessible to CAD. Furthermore, the myeloid-origin group showed marked DNA fragmentation but less chromatin condensation during apoptosis. This is because the DNA is normally tightly wrapped around histones, the core proteins of the nucleosomes. CAD cleaves the DNA at the internucleosomal linker sites between the nucleosomes, protein-containing structures that occur in chromatin at ~180-bp intervals. In apoptotic DNA fragmentation, the DNA is cleaved in the internucleosomal linker region, which is the part of the DNA not wrapped around the histones. ДЄ nucleosome, consisting of DNA (grey) wrapped around a histone tetramer (coloured). During apoptosis, the apoptotic effector caspase, caspase 3, cleaves ICAD and thus causes CAD to become activated. CAD is normally inhibited by another protein, the Inhibitor of Caspase Activated DNase (ICAD). The enzyme responsible for apoptotic DNA fragmentation is the Caspase-activated DNase. In dying cells, DNA is cleaved by an endonuclease that fragments the chromatin into nucleosomal units, which are multiples of about 180-bp oligomers and appear as a DNA ladder when run on an agarose gel. DNA fragmentation is a biochemical hallmark of apoptosis. In non-apoptotic cells, DFF is a nuclear heterodimer consisting of a 45 kD chaperone and inhibitor subunit. This might also provide an explanation of why Jurkat/ICAD-DM and MCF-7/caspase-3 cells displayed some histone release but not DNA fragmentation under apoptotic conditions (Figs. Apoptotic DNA fragmentationВ is a natural fragmentation that cells perform in apoptosisВ (programmed cell death). DNA fragmentation in response to apoptotic signals is achieved, in part, through the activity of apoptotic nucleases, termed DNA fragmentation factor (DFF) or caspase-activated DNase (CAD) (reviewed in Widlak and Garrard, 2005). Taken together, it appears that, unlike DNA fragmentation, CAD is indirectly involved in the dissociation of histone from chromatin in apoptotic cells.

Dna chromatin histone cad icad