A limited number of chromosomes makes a nucleus competent to respond to inducers of replication and mitosis in a plant.

Multinucleate tetraploid cells with unbalanced chromosomal distribution in aneuploid nuclei were obtained in Allium cepa L. root meristems. For this, their natural diploid cells were treated with a multipolarizing agent (1 h carbetamide) followed by an inhibitor of cytokinesis (1 h caffeine). Data from these multinucleate cells with aneuploid nuclei suggest that only four out of the thirty-two chromosomes of their autotetraploid complement possess DNA sequences making the nucleus competent to respond to inducers of replication and mitosis. Direct observation of cells where a single replicated chromosome had reached mitosis showed that this chromosome was the one bearing the nucleolar organizer. Six specific chromosomes would confer competence to the nucleus to respond to inducers of replication but not to those producing chromosome condensation. Another four different chromosomes would confer the nucleus with the ability to respond to mitotic inducers but not to replication inducers. The rest of the chromosomal complement seemed to lack any of the DNA sequences needed for these two important cycle transitions. In a nutshell, certain DNA sequences distributed in a few chromosomes of the onion complement are an intranuclear requirement to initiate replication and mitosis in these plant cells.