DNA Replication Patterns in Somatic and Germ-line Cells of the Male Chinese Hamster

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Abstract
DNA replication patterns in bone marrow (somatic) and spermatogonial (gonadal) cells of the Chinese hamster were studied with autoradiographic methods. The generation time of bone marrow cells was approximately 10 h, and the G2 phase was less than 2 h. The DNA replication sequence in bone marrow cells was similar to that of somatic cells in tissue culture, i.e., the long arm of the X and the entire Y replicate in late S, but not in early S phase. The average G2 phase of the spermatogonia was approximately 6 h. Its DNA replication sequence was different from that of the somatic cells. The long arm of the X and the entire Y were not labeled in late S phase. The late labeling pattern of autosomes resembled that of the somatic cells. Since the heteropycnosis, late DNA replication, and genetic inactivation appear closely related, the DNA replication sequence of spermatogonial cells suggests that the sex chromosomes are metabolically active, at least in some stages of germ cell development.