High-resolution autoradiography as a tool for the localization of nucleic acid synthesis and distribution in the mammalian cell nucleus.

Several examples of the application of high resolution autoradiography to the study of nucleic acid distribution in ultrathin sections of fixed and plastic embedded or frozen material are presented. Newly-synthesized DNA, labelled by very short pulses of 3H-thymidine is found to be localized throughout the nucleus. In the blastomeres of early mouse embryos developing after fertilization by 3H-thymidine-labelled spermatozoa, the labelled paternal DNA is distributed non-homogeneously in the nucleus. Finally some results obtained by using a cytochemical enzymatic technique for visualization of DNA directly in ultrathin sections of fixed and embedded cells are demonstrated. Concerning the distribution of RNA, perichromatin localization of rapidly transcribed RNA is described in different cell systems. This label can be attributed to perichromatin fibrils. In cells labelled for longer periods of time, sometimes followed by prolonged postincubations, a residual label is always found over the nucleus. Clusters of interchromatin granules are usually only weakly labelled or unlabelled. These structures probably contain a limited amount of rather slowly labelled RNA. The present results are discussed in the context of some biochemical evidence and of the data described by other investigators.