The arrangement of chromatin in the interphase nucleus with reference to cell differentiation and repression in higher organisms.

HeLa cells were grown as monolayer cultures in Spinner medium and given 12-minute pulse labels of (3)H thymidine. Synchronous crops of cells labelled in the last minutes of S showed a metaphase pattern of chromosomes hot-labelled close to the kinetochores (centromeres). Such cells in subsequent stages of interphase showed a concentration of label at the nuclear envelope. A control experiment with cells labelled in the first minutes of S yielded chromosomes labelled at the tips and in the nucleolar organizers. These cells showed label concentrated in the central area of the nucleus in the next interphase. The results are compatible with the view that the kinetochore regions of chromosomes are attached to the nuclear envelope in interphase. The significance of these results is considered with reference to interpreting DNA replication patterns, the segregation of chromosomes, eukaryote differentiation and repression, and the rationale for chromosome numbers. It is argued that the nuclear envelope serves as a 'repressor organelle' in higher organisms, and that DNA methylation may be involved in the control of RNA transcription, and hence gene expression.