The splitting of human chromosomes into chromatids in the absence of either DNA or protein synthesis.

TLDR: The experiments indicate that the human somatic chromosome is a multistranded structure that loosens in late G 1 to form functional chromatids.

Abstract: If cells are irradiated during most of the G 1 portion of the cell cycle, chromosome aberrations are produced. If, however, they are irradiated while in S or G 2 , chromatid aberrations are formed. The best estimate of when the transition occurs is late G 1 . That is, in both plant and animal cells, the chromosome splits into chromatids before DNA synthesis. This doubling of the chromosome conceivably could be caused by either a synthesis of chromosomal protein prior to S, or by a loosening of a multistranded struture. Experiments were carried out with phytohemagglutinin-stimulated human lymphocytes to test which of the two possible explanations cited above would be the more likely. Phytohemagglutinin-stimulated lymphocytes were cultured in the presence of 2·10 −3 M hydroxyurea and triated thymidine for 40 h. The hydroxyurea was used to inhibit DNA synthesis and to enrich the population of cells at the end of G 1 . Cells were then irradiated with 200 R of X-rays and recultured in the absence of tritium. When the cells reached metaphase, unlabeled cells were scored for the types of aberrations induced by the prior irradiation. Chromatid aberrations were found, indicating that in human cells, too, the split occurs in G 1 in the absence of DNA synthesis. Similar experiments in which the protein synthesis inhibitor cycloheximide was added after the cells were in culture for 16 h (before any DNA synthesis had occurred) and kept in until the fortieth hour showed that the transition occurred when protein synthesis was inhibited, too. The experiments indicate that the human somatic chromosome is a multistranded structure that loosens in late G 1 to form functional chromatids.