Chromosome replication in somatic hybrids of mouse and temperature sensitive Chinese hamster cells

Hybrid clones were obtained between a mouse cell line (3TP) and a temperature‐sensitive Chinese hamster cell line (K12) unable to grow at 40° because of a ts defect apparently located at the G1/S transition. The great majority of hybrid clones grew at 40°, showing the ts defect to be “recessive.” Chromosome DNA replication was analyzed in some detail in three hybrid clones with balanced complements. Although the S period of these hybrids was longer than that of K12, DNA replication in mouse and hamster chromosomes started and ended synchronously. Upon prolonged culture, mouse chromosomes were lost as they are in hybrids involving a non ts Chinese hamster partner, in which case asynchronous chromosome replication appears to be the rule. It seems therefore that asynchronous replication is not the determining factor in chromosome loss from cell hybrids.