Endoreplication

Endoreduplication (also referred to as endoreplication or polytenization) is replication of the nuclear genome in the absence of cell division, which leads to elevated nuclear gene content and polyploidy. Endoreplication can be understood simply as a variant form of the mitotic cell cycle (G1-S-G2-M) in which mitosis is aborted prior to cytokinesis or circumvented entirely, due in part to modulation of cyclin-dependent kinase (CDK) activity. Examples of endoreplication characterized in arthropod, mammalian, and plant species suggest that it is a universal developmental mechanism responsible for the differentiation and morphogenesis of cell types that fulfill an array of biological functions. While endoreplication is often limited to specific cell types in animals, it is considerably more widespread in plants, such that polyploidy can be detected in the majority of plant tissues.

Endoreplicating cell types that have been studied extensively in model organisms

Endoreplication typically results in elevated cellular DNA content, but the manner in which the genetic material is configured varies depending on whether mitotic events are allowed to occur.Endocycling is a form of endoreplication in which the cell largely avoids mitosis and duplicated chromatids remain physically associated. Repeated rounds of endocycling can lead to the production of polytene chromosomes in which sister chromatids are tightly configured in parallel arrays. In contrast, endomitosis is a form of endoreplication in which cells undergo aspects of mitosis but fail to execute telophase and/or cytokinesis. Duplicated chromosomes produced by endomitosis exist as discrete units in a single polyploid nucleus or may be packaged into separate nuclei, depending on the phase at which mitosis is aborted. In certain instances (e.g. trophoblast giant cells), the distinction between endocycling and endomitosis is clouded, as the cell undergoes early mitotic events such as chromosome condensation that tend to diminish the physical association among homologous chromosomes. It should also be noted that endocycling does not always involve replication of the entire genome, and that certain regions are often replicated more frequently than others.

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