An hypothesis about genome structures in mammalian polyploid cells based on a new concept that genome is fractal of six hierarchies

A new model for the spatial configurations of DNA is proposed to solve the problem of DNA loss in mammalian polyploid cells. The ordinary concept that chromosomes are situated independently in nuclei cannot account well for the DNA loss in polyploid cells. A new concept about the DNA configurations in diploid cells is constructed based on observations that have been reported. Briefly, the DNA structure is self-similar fractal with a unit of opposite-handed twin-circles. In human diploid cells, DNA is constructed with six hierarchies whose sizes are 325, 324, 323, 322, 321 and 320 with a unit of 200 DNA base pairs, corresponding to a genome, a chromosome, a chromosome band, a replicon, a rosette loop (a gene) and a nucleosome, respectively. A model assuming particular spatial configurations of chromosomes in polyploid cells is deduced from this new concept about chromosome configurations in diploid cells. It can account satisfactorily for the problem of DNA loss in polyploid cells. When cell division is inhibited and DNA synthesis progresses, replicated DNA will be stacked. When inhibitors are removed, the polyploidized cells may return to the initial ploidy, because the stacked DNA loops have not been linked. When the stacked DNA twin-loops are linked with a proper configuration, the cells may become polyploid cells. There is a distinct difference in genome structure between polyploidized and polyploid cells. The homologous chromosomes of polyploid cells are arrayed mirror-symmetrically and they can come close to each other in the folded structure. If DNA synthesis is bypassed at the paired homologous chromosomes, DNA content is lost at every cell division. As the DNA loss progresses, the chromosome configuration of polyploid cells deviates gradually from mirror-symmetry and the DNA loss ceases, resulting in the establishment of semi-stable hypoploid.