Common pathway of chromosome condensation in mammalian cells

Chromatin folding in the interphase nucleus is not known. We compared the pattern of chromatin condensation in Indian muntjac, Chinese hamster ovary, murine preB, and K562 human erythroleukemia cells during the cell cycle. Fluorescent microscopy showed that chromosome condensation follows a general pathway. Synchronized cells were reversibly permeabilized and used to isolate interphase chromatin structures. Based on their structures two major categories of intermediates were distinguished: (1) decondensed chromatin and (2) condensed chromosomal forms. (1) Chromatin forms were found between the G1 and mid-S phase involving veil-like, supercoiled, fibrous, ribboned structures; (2) condensing chromosomal forms appeared in the late-S, G2, and M phase, including strings, chromatin bodies, elongated prechromosomes, precondensed chromosomes, and metaphase chromosomes. Results demonstrate that interphase chromosomes are clustered in domains; condensing interphase chromosomes are linearly arranged. Our results raise questions related to telomer sequences and to the chemical nature of chromosome connectivity.