Late DNA replication in male mouse meiotic chromosomes.

Parts of the male mouse meiotic complement comprising the Y chromosome, the whole X chromosome, and near-centromeric parts of autosomal bivalents are synthesized late, as judged by tritiated thymidine autoradiography. This confirms the occurrence of end-to-end association between X and Y chromosomes and suggests that paired heterochromatic segments in autosomes must synthesize DNA at the same time.