![]() In 1972, Kerr, Wyllie, and Currie coined the term apoptosis and distinguished this type of cell death from necrosis based on morphological features. This finding was consistent with the hypothesis that these DNA fragments were a specific degradation product of nuclear DNA. In 1970, Williamson described that cytoplasmic DNA isolated from mouse liver cells after culture was characterized by DNA fragments with a molecular weight consisting of multiples of 135 kDa. The discovery of the internucleosomal fragmentation of genomic DNA to regular repeating oligonucleosomal fragments generated by Ca/Mg-dependent endonuclease is accepted as one of the best-characterized biochemical markers of apoptosis (programmed cell death). Often this may complicate identification of apoptotic cells if cell populations are analyzed only at a single time point e.g. The presence or absence of particular apoptotic event(s), including DNA fragmentation, depends on the "time window" at which the kinetic process of apoptosis is being investigated. Apoptosis can be initiated by a myriad of different mechanisms in different cell types, and the kinetics of these events vary widely, from only a few minutes to several days depending on the cell system. Cells that lack ICAD or that express caspase-resistant mutant ICAD thus do not show DNA fragmentation during apoptosis, although they do exhibit some other features of apoptosis and die.Įven though much work has been performed on the analysis of apoptotic events, little information is available to link the timing of morphological features at the cell surface and in the nucleus to the biochemical degradation of DNA in the same cells. When cells are induced to undergo apoptosis, caspase 3 cleaves ICAD to dissociate the CAD:ICAD complex, allowing CAD to cleave chromosomal DNA. CAD is synthesized with the help of ICAD (inhibitor of CAD), which works as a specific chaperone for CAD and is found complexed with ICAD in proliferating cells. Molecular characterization of this process identified a specific DNase (CAD, caspase-activated DNase) that cleaves chromosomal DNA in a caspase-dependent manner. It occurs in response to various apoptotic stimuli in a wide variety of cell types. The linker sites are the only parts of the DNA strand that are exposed and thus accessible to CAD.ĭegradation of nuclear DNA into nucleosomal units is one of the hallmarks of apoptotic cell death. This is because the DNA is normally tightly wrapped around histones, the core proteins of the nucleosomes. 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.ĬAD cleaves DNA at internucleosomal linker sites between nucleosomes, protein-containing structures that occur in chromatin at ~180-bp intervals. A 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 (CAD). The apoptotic DNA fragmentation is being used as a marker of apoptosis and for identification of apoptotic cells either via the DNA laddering assay, the TUNEL assay, or the by detection of cells with fractional DNA content ("sub G 1 cells") on DNA content frequency histograms e.g. ![]() Apoptosis is characterized by the activation of endogenous endonucleases, particularly the caspase-3 activated DNase (CAD), with subsequent cleavage of nuclear DNA into internucleosomal fragments of roughly 180 base pairs (bp) and multiples thereof (360, 540 etc.). A 1 kb marker (middle) and control DNA (right) are included for comparison.Īpoptotic DNA fragmentation is a key feature of apoptosis, a type of programmed cell death. ( February 2012) ( Learn how and when to remove this template message)Īpoptotic DNA fragmentation, visualised by the DNA laddering assay (left). Please help improve this article if you can. This article may require cleanup to meet Wikipedia's quality standards.
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