NUCLEIC ACID SYNTHESIS AND TRANSFER IN NORMAL, SECOND GENERATION CHICK EMBRYO FIBROBLASTS

Abstract

The stripping film technique of autoradiography has been used to study some aspects of RNA and DNA metabolism in chick embryo fibroblasts in second generation tissue culture.Approximately one third of the cells incorporated thymidine-H³into DNA in a 20-minute uptake period. The duration of DNA synthesis, the generation time (time elapsing between two successive cell divisions), and the duration of mitosis have been calculated and are very similar to the values obtained for pure strains of hamster cells maintained in culture for several months by another author.RNA synthesis, as measured by uridine-H³incorporation, occurred only in the nucleus and nucleolus. Both sites synthesized RNA simultaneously beginning at zero time. The ratio of the number of grains over the nucleolus to that over the whole nucleus remained constant throughout the uptake of uridine-H³and its transfer to the cytoplasm.A small amount of labelled soluble RNA precursors remain in the nucleus after removal of uridine-H³from the medium. This results in a slight rise in radioactivity of the nucleus after uridine-H³removal. RNA then leaves the nucleus rapidly and appears in the cytoplasm. The half life of RNA in the nucleus is about 4 hours. Some turnover of cytoplasmic RNA seems to occur after 8 hours but quantitative estimates of its rate cannot be made due to changing geometry of the cells as the RNA migrates from the nuclear to the peripheral parts of the cell.Finally, little or no RNA synthesis occurs for a period of about 30 minutes during contraction of the chromosomes in mitosis.