[Possible mechanisms regulating the entry of normal and neoplastic cells into the DNA synthesis phase].

Literature data concerning some aspects in the regulation of the entry of normal and neoplastic cells into the phase of DNA synthesis are reviewed. Basing on the analysis of data reported by different authors, a hypothetical model is suggested for regulation of cell passing from G- to S-period. The main chain in the regulation is the constitutive synthesis of protein-repressor which blocks the expression of genes whose products are necessary for replication ("replication genes"). The repressor inactivation is achieved by growth factors through protein kinases. A comparison is made between the transcriptional mechanisms in eukaryotic and prokaryotic cells. Four hypothetic types of genes have been isolated responsible for regulation of progression of the cell to phase S. The role of quantitative and/or qualitative changes in the products of these genes in neoplastic transformation is discussed.