Periodicity of Nuclear DNA Synthesis in the Intraerythrocytic Cycle of Plasmodium knowlesi

SYNOPSIS. Preparations of the various intraerythrocytic stages of Plasmodium knowlesi nearly free of host‐cell material were made from infected rhesus monkey blood. Analyses of these revealed that late trophozoite and schizont stages contain similar amounts of DNA and that ring stages contain much less DNA. This indicates that DNA synthesis occurs mainly during the ring and trophozoite stages of the cycle. Attempts to confirm these results using a quantitative Feulgen staining technique yielded anomalous results. The analytical data indicated that incorporation of 3H‐adenosine rather than of 3H‐orotic acid, was the better indicator of DNA synthesis in P. knowlesi in culture. A detailed study with time course and pulse‐labelling experiments showed that incorporation of 3H‐adenosine occurred at a similar rate throughout the intraerythrocytic cycle until schizogony began, that it ceased during this period, and began again as soon as the parasites had invaded new cells. It is concluded that the S‐phase of the cell cycle occupies most of the ring and trophozoite stages and that the G1 and G2 phases, if they occur, are of short duration. The amount of DNA in the nucleus is increased 10–20 fold during the S phase. At schizogony, the nucleus divides without further DNA synthesis to yield a 16‐nuclear schizont.