A MOLECULAR-BASIS FOR DISCRETE SIZE VARIATION IN HUMAN RIBOSOMAL DNA

  • 1 January 1985
    • journal article
    • research article
    • Vol. 37  (2) , 311-325
Abstract
The tandemly repeated human RNA genes contain a region of size heterogeneity that is present in the nontranscribed spacer of every individual examined. This heterogeneity was previously examined by Southern analysis of BamHI-digested human DNA. Using a ribosomal DNA (rDNA) probe specific for the 3'' end of the 28S rRNA gene, at least 4 discrete sizes of BamHI fragments were seen in human populations. Molecular analysis of the cloned DNA from this region reveals tandem duplication of a segment of spacer rDNA located 388 base pairs (bp) 3'' to the end of the 28S rRNA gene. DNA (550 bp), flanked on either side by a 150-bp repeated element, is either duplicated or deleted to produce a series of spacers that differ in size by 850 bp. These duplications/deletions appear to be the product of unequal homologous exchange, mediated by the small repeated element. Human rDNA fragments cloned in < vectors and propagated in Escherichia coli generate the same apparent size variation seen in genomic DNA. Unequal homologous exchange may be the molecular basis for the observed length heterogeneity in the spacer rDNA and may be a common mechanism for the generation of human genetic diversity.