Circadian Cell Cycle

The marker rhythms of an in vivo circadian cell cycle (Figure 2/III) time-specify the classical cell cycle and fill in some of its gaps (G). Circadians are revealed by a combination of histology for mitotic counts, differential centrifugation for cellularly localized wet chemistry and tracer studies for the determination of the relative specific activity of phospholipid, RNA and DNA phosphorus. In growing mouse liver, as in the organ regenerating from partial hepatectomy, daily recurring RNA formation precedes DNA formation, at variance with the former dogma postulating information flow from DNA to RNA to protein without any consideration of rhythmicity and its timing. The circadian rhythmicity of each variable and the sequence of acrophases, that is the timing of overall high values recurring in each cycle, are reproduced in separate experiments (Figure 2/I). The reproducibility improves when the data are expressed as a percentage of the mean, whereby modulations of the mean by rhythms other than circadian may be reduced. These bioperiodicities can be quantified by the cosinor method, which describes the 24-hour data set as a whole ratherthan by the inspection of a single value, the peak in a plot of the data as a function of time. Indeed, a polar cosinor plot (Figure 2/II) shows the sequence of events that start at the level of the membrane by phospholipid labelling. RNA formation in the cytoplasm follows promptly, and, with a lag, DNA formation and eventually mitosis.

The sequence from RNA formation to DNA formation to mitosis here demonstrated in the intact immature mouse liver is also found in regenerating liver (not shown) and in Euglena. The time relation between DNA and mitosis is also mapped in hamsters (and humans; not shown). These findings attest to the generality of the rhythmicity in DNA formation, whereas earlier DNA was regarded as the most constant feature of organisms. The critical importance of a circadian cell cycle lies in its eventual use for cancer therapy targeted in time.