page_title: DNA fragmentation
section_title: Spontaneous
hierarchical_section_title: DNA fragmentation / Spontaneous
caption_attribution_description: Schematic representation of the nucleosome, the smallest subunit of the chromatin. The DNA (grey) is wrapped around the nucleosomal core proteins, the histones H2A (yellow), H2B (red), H3 (blue), and H4 (green). The nucleosomal linker is the section of the DNA that is not wrapped around the core. It is normally attached to the histone H1 (not shown). The total length of the DNA per nucleosome is ~180 bp.
context_page_description: DNA fragmentation is the separation or breaking of DNA strands into pieces. It can be done intentionally by laboratory personnel or by cells, or can occur spontaneously. Spontaneous or accidental DNA fragmentation is fragmentation that gradually accumulates in a cell. It can be measured by e.g. the Comet assay or by the TUNEL assay.
  Men with sperm motility defects often have high levels of sperm DNA fragmentation.
  The degree of DNA fragmentation in sperm cells can predict outcomes for in vitro fertilization and its expansion intracytoplasmic sperm injection. The sperm chromatin dispersion test and TUNEL assay are both effective in detecting sperm DNA damage. Using bright-field microscopy, the SCD test appears to be more sensitive than the TUNEL assay.
  Its main units of measurement is the DNA Fragmentation Index. A DFI of 20% or more significantly reduces the success rates after ICSI.
  DNA fragmentation was first documented by Williamson in 1970 when he observed discrete oligomeric fragments occurring during cell death in primary neonatal liver cultures.
context_section_description: Apoptotic DNA fragmentation is a natural fragmentation that cells perform in apoptosis (programmed cell death). DNA fragmentation is a biochemical hallmark of apoptosis. 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. The enzyme responsible for apoptotic DNA fragmentation is the Caspase-activated DNase. CAD is normally inhibited by another protein, the Inhibitor of Caspase Activated DNase (ICAD). During apoptosis, the apoptotic effector caspase, caspase 3, cleaves ICAD and thus causes CAD to become activated.
  CAD cleaves the DNA at the internucleosomal linker sites between the nucleosomes, protein-containing structures that occur in chromatin at ~180-bp intervals. This is because the DNA is normally tightly wrapped around histones, the core proteins of the nucleosomes. The linker sites are the only parts of the DNA strand that are exposed and thus accessible to CAD.
caption: A nucleosome, consisting of DNA (grey) wrapped around a histone tetramer (coloured). 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.
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