Amplification of (GACA)n simple repeats in an exceptional 14p+ marker chromosome.

An inherited 14p+ marker chromosome with an unusually large differentially staining region (DSR) on the short arm was examined with a number of banding techniques and by non-radioactive in situ hybridization using various repetitive DNA probes. The increase in the size of this variant chromosome was 40% that of a normal chromosome 14. The extra chromosomal material in the DSR consisted mainly of GC-rich constitutive heterochromatin within which two equally sized clusters of 18S + 28S ribosomal RNA genes were located. In situ hybridization demonstrated that the DNA in the DSR was highly enriched in simple tetrameric (GACA)n sequences, whereas the centromeric alphoid sequences and the (TTAGGG)n telomeric repeats were not amplified. Silver staining of the two nucleolus organizer regions (NORs) within the DSR showed that the telomerically located NOR was always more active than the paracentromerically located NOR. A comparison with other DSRs found in human acrocentric autosomes revealed a gradient of transcriptional activity of adjacent multiple NORs. This gradient decreased in the order of their telomeric-paracentromeric-interstitial position, regardless on which acrocentric chromosome the DSR was located.