Delinking of S phase and cytokinesis in the protozoan parasite Entamoeba histolytica

The alternation of DNA replication in S phase and chromosome segregation in M phase is a hallmark in the cell cycle of most well-studied eukaryotes and ensures that the progeny do not have more than the normal complement of genes and chromosomes. An exception to this rule has been described in cancer cells that occasionally become polyploid as a result of failure to restrain S phase despite the failure to undergo complete mitosis. Here, we describe the cell division cycle of the human pathogen, Entamoeba histolytica, which routinely accumulates polyploid cells. We have studied DNA synthesis in freshly subcultured cells and show that, unlike most eukaryotes, Entamoeba cells reduplicate their genome several times before cell division occurs. Furthermore, polyploidy may occur without nuclear division so that single nuclei may contain 1-10 times or more genome contents. Multinucleated cells may also accumulate several genome contents in each nuclei of one cell. Thus, checkpoints that normally prevent DNA reduplication until after cytokinesis in most eukaryotes are not observed in E. histolytica.