The establishment of the complete karyotype of human pachytene spermatocytes reveals differences in stretching of chromosomes between meiosis and mitosis. Bivalents or specific regions of bivalents which exhibit many R-bands are particularly elongated. In mitotic chromosomes, the DNA contained in such bands is known to be early replicating. The study of variations in the total length and the centromeric index of bivalent 1 suggests that differential elongation of pachytene bivalents is a premeiotic event, taking place during the last DNA replication.

In order to sublocalize NCAM, ETSl, and the anonymous DN A fragment D11S29 within 11 q23, in situ hybridization was performed on pachytene bivalents. Analysis of the grain distribution within the band 11q23 indicated that the chromosomal sublocalization of both NCAM and D11S29 was in llq23.1, whereas ETSl was found to be localized in 1 lq23.3. These results clearly demonstrate the usefulness of in situ hybridization applied to pachytene bivalents to obtain accurate gene sublocalization.