The relationship between DNA synthesis and the synthesis of nuclear proteins in rat brain during development

—The incorporation of [³H]thymidine into nuclear DNA of rat brain progressively increased from birth until the 8th postnatal day and it was lowest in the adult brain. When isolated nuclei from brain cells were separated into a neuronal- and a glial-rich fraction (composed of glial and neuroblast nuclei in young animals), the specific radioactivity of the DNA was higher in the glial-rich fraction at all ages investigated. The incorporation of [³H]leucine into proteins of rat brain was considerably higher in the 8-than in the 1-day-old rat. The greatest difference in the rate of protein synthesis between 8- and 1-day-old brain occurred in the nuclear proteins, especially those associated with DNA. There was an accumulation of protein and RNA in nuclei from brain cells from birth to the 8th postnatal day. The increased content of proteins occurred primarily in the fraction soluble in buffered saline (nuclear sap).