Complex Relationships Between DNA Methylation Status and Chromosome Compaction and Cohesion

Mitotic chromosomes are generally studied on cultured cells. For instance, almost all data collected in human cytogenetics since the early 1960s were obtained from fibroblast, lymphocyte, amniotic and trophoblastic cell cultures. Even for cancer cells, which are spontaneously proliferating, cultures are almost always performed before metaphase harvesting. Consequently, the chromosomes of spontaneously dividing cells are rarely studied. When such studies are successfully performed, chromosomes of “poor quality” are invariably obtained. Indeed, the notion of quality for mitotic chromosomes is purely subjective, the goal of cytogeneticists being the obtention of sharply delineated chromosomes, which can resist to treatments inducing banding or allowing “in situ” hybridization. The “poor quality” of chromosomes from non-cultured cells may have several explanations. It is more difficult to standardize the cytological techniques. Mitotic indexes are generally low, which reduces the choice of mitotic cells. It is also possible that mitotic chromosome structure is different.