CRISPR-based genomic loci labeling revealed ordered spatial organization of chromatin in living diploid human cells.

The eukaryotic genome is compacted in the form of chromatin within the nucleus. Whether the spatial distribution of the genome is ordered or not has been a longstanding question. Answering this question would enable us to understand nuclear organization and cellular processes more deeply. Here, we applied a modified CRISPR/dCas9 system to label the randomly selected genomic loci in diploid living cells, which were visualized by high-resolution wide-field imaging. To analyze the spatial positions of three pairs of genomic loci, three sets of parameters were progressively measured: i) the linear distance between alleles; ii) the radial distribution of the genomic loci; and iii) the linear distances between three pairs of genomic loci on nonhomologous chromosomes. By accurate labeling, geometric measuring and statistical analysis, we demonstrated that the distribution of these genomic loci in the 3D space of the nucleus is relatively stable in both late G1 and early S phases. Collectively, our data provided visual evidence in live cells, which implies the orderly spatial organization of chromatin in the nucleus. The combination of orderliness and flexibility ensures the methodical and efficient operation of complex life systems. How the nucleus adopts this ordered 3D structure in living cells is thought-provoking.