Position of Labelled Chromatids in Diplochromosomes of Endo-reduplicated Cells after Uptake of Tritiated Thymidine

Abstract

ENDO-REDUPLICATION as defined by Levan and Hauschka¹ consists of a microscopically non-visible duplication of the chromosomes within the cell nucleus leading to the formation of so-called ‘diplochromosomes’ in the following mitosis. Endo-reduplication, has been reported to occur regularly, although rarely, in human tissues in vitro^2,3. In a previous paper³, we showed that in cultured human fibroblasts diplochromosomes originate by two separate and successive chromosome duplications. Diplochromosomes of endo-reduplicated cells which took up tritiated thymidine during the second chromosome replication show all four chromatids labelled in the following metaphase, whereas when tritiated thymidine was given during the first chromosome replication the label was restricted to but one chromatid of each of the two chromosomes forming a diplochromosome. These observations lead to the conclusion that the chromatids of diplochromosomes are, like chromatids of ordinary human chromosomes⁴, composed of two sub-units which are distributed in a semi-conservative mode during chromosome duplications. The duration of one endo-reduplication cycle is in the range of two cycles of a diploid cell.