Prolonged treatment of Allium cepa root meristems with changing concentrations of hydroxyurea (HU) results in either premature chromosome condensation or cell nuclei with an uncommon form of biphasic chromatin organization. Żabka et al. show that extended stress gives rise to axially elongated cells with half interphase/half mitotic structures, having both decondensed and condensed domains of chromatin. During recovery, some cell nuclei resuming S phase reveal gradients of BrdU labelling. Long-term HU treatment brings about an increase inboth the relative time span and the spatial scale of H3S10 phosphorylation.