In spite of many attempts at resolution, the problem of the chromosome topography in interphase nuclei and its possible functional significance (Lewin, 1981; Vogel and Kruger, 1983) has remained a matter of great controversy and uncertainty (for reviews see Comings,1980; Avivi & Feldman, 1980). In particular, the relationship between metaphase and interphase arrangements of chromosomes is still largely unknown. A solution of this problem seems especially urgent, since the spatial relationship of chromosomes at metaphase has been extensively studied in many species during the last twenty years. Recently, Bennett and coworkers have demonstrated a highly ordered arrangement of centromeres in metaphase plates of certain plant species (Bennett, 1983) and stimulated fresh interest in the development of techniques for mapping the arrangements of individual chromosomes directly within the interphase nucleus. In this paper we describe a laser-uv-microbeam approach which allows selective microirradiation of a small amount of chromatin in the interphase nucleus of cultured cells and study of its distribution in chromosomes during the subsequent mitosis. Alternatively, chromatin can be microirradiated during mitosis and its distribution can be studied in the subsequent interphase nucleus.
