Normal versus abnormal cell proliferation

The most recent findings on the molecular and cellular characterization of normal and abnormal cell proliferation are summarized. They include molecular spectroscopy, nucleic acid conformation, protein modifications, premature chromosome condensation, nuceoli changes, nuclear and cell morphometry, image analysis, flow microfluorimetry, and time-lapse cinematography. Biophysical and biochemical evidence in favor or against two cycles of chromatin condensation, followed by two abrupt random decondensations, per cell cycle are presented. Other biphasic changes at the molecular and cellular levels that favor the existence of two random transitions, or restriction points, per cell cycle are discussed. A comprehensive unitary model of the cell cycle is then outlined; this model is able to explain most findings on continuously dividing cells and on quiescent cells induced to proliferate. Within this analytical framework the physical-chemical and biological properties are given, in either normal or tumor cells, for the various types of “noncycling” cells that are here viewed as necessary steps in mammalian cell growth rather than separate states. The implications of the coupling of higher-order chromatin structure with cell geometry and growth, high in fibroblast-like cells but low in transformed cells, are also discussed. Molecular mechanisms likely responsible for the chromatin conformational changes occurring at the G₀→G₁, G₁→S, G₂→M transitions are finally discussed in terms of polyelectrolyte theory.