The chromosomal subunits that segregate from each other during chromosome replication are shown to be uniquely different. An analysis of tritiated thymidine labeled chromosomal subunits in dicentric chromosomes, generated by isolocus breakage followed by proximal reunion, shows that the end-to-end association of the subunits is not random but is strictly preferential. The data suggest that the functional subunit in the formation of these particular chromosomal aberrations might be single polynucleotide chains of deoxyribonucleic acid, although other molecular species cannot be disregarded. The suggestion that the DNA is the molecule that is involved is based on the fact that the two chains of the DNA double helix exhibit reverse polarity and this property predicts the observed autoradiographic patterns.