SEQUENCE OF DNA REPLICATION IN THE ISO-X CHROMOSOME FROM X/ISO-X HUMAN FEMALES DURING THE INITIAL STAGES OF THE SYNTHETIC PERIOD

Abstract

Tritiated thymidine was added to the leucocyte cultures from two X/iso-X human females, when they were first established so that the cells were exposed to H3TdR from the beginning of their DNA synthetic period. Following incubation for 50-55 hr., the cells were washed and allowed to complete their DNA synthesis in an H3TdR-free medium containing un-labelled thymidine. Colchicine was added one hour after the washing. The iso-X which is a good marker chromosome and which is destined to finish its DNA replication after all the other chromosomes in the complement was not found to be correspondingly late in beginning its replication; in fact a number of autosomes began repolication later than the late-terminating iso-X.