The discrete phases of the cell cycle: autoradiographic, physical, and chemical evidences

Recently the discrete model of the cell cycle, described as a series of metabolic progressions through four distinct phases, has been challenged by a few curves of percent labeled mitoses (PLM) and related autoradiographic data, which have been questionably interpreted in terms of a "continuous" cell cycle model. This conclusion is based in part on a few questionable observations which we summarize, along with our objections and explanations. We present a critical analysis of the autoradiographic data obtained with the PLM, in terms of the limitations and inconveniences of the technique itself. Some attention is paid to the usefulness of the "radioautographic transfer function," which seems an arbitrary mathematical expression of old autoradiographic observations. Finally, a review is presented of the old (by autoradiography) and new (by physical, chemical, and computer image analysis) evidences of the existence of dicrete phases in the cell cycle, which in turn support the idea of an all-or-none phenomenon for DNA synthesis, and which rule out the hypothesis of a continuous cell cycle model with a varying DNA synthesis rate function.