Unformatted text preview: program. The survival of many
other types of cells in animals is similarly dependent on growth factors or
contacts with neighboring cells or the extracellular matrix, so programmed
cell death is thought to play an important role in regulating the associations
between cells in tissues. The Events of Apoptosis
In contrast to the accidental death of cells that results from an acute injury
(necrosis), programmed cell death is an active process, which usually proceeds by a distinct series of cellular changes known as a poptosis , first
described in 1972 (Figure 17.1). During apoptosis, chromosomal DNA is
(A) (B) (C) Hours after induction of apoptosis
3 DNA fragmentation
Chromatin condensation Normal
Fragmentation of nucleus Fragmentation of the cell Apoptotic FIGURE 17.1 Apoptosis (A) Diagrammatic
representation of the events of apoptosis.
(B) Light micrographs of normal and apoptotic
human leukemia cells illustrating chromatin
condensation and nuclear fragmentation.
(C) Gel electrophoresis of DNA from apoptotic
cells, showing its degradation to fragments
corresponding to multiples of 200 base pairs
(the size of nucleosomes) at 1–4 hours following induction of apopto...
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