Crisis periods in development are critical periods of cell death that have long been suggested as “epigenetic crises” which are central to normal and abnormal embryogenesis. Under in-vitro culture conditions, there are similar crisis periods or Hayflick limits of culture senescence. Epigenetic modulations from CpG methylation coupled to DNA replication provide an alternate timing mechanism to the telomere/telomerase biological clock. Physiological cell death in both development and in in-vitro isolates is primarily apoptotic. Arguments of divergent apoptotic death commitments as caspase-dependent and -independent pathways seem to suggest that there is no possibility of a global life-and-death signal. However recent reports implicating CpG-specific cleavage in apoptosis implies that the powerful imprint mark that silences genes, protects genes from nuclease restriction, and modulates chromatin conformations, could provide a common commitment pathway of convergence in the death cascade. If the imprint mark were central to the apoptotic commitment, then apoptosis is Lamarckian not Darwinian.
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