Evidence for Single Copies of Globin Genes in the Mouse Genome

Abstract

IT is crucial to all our ideas about the structure of eukaryotic chromosomes that we should have estimates of the numbers of copies of typical genes in the genome. Genetic evidence argues for a single copy of each gene but this is difficult to reconcile with experimental evidence that some genes, for example those specifying histones¹ and ribosomal RNA², are reiterated some hundreds or thousands of times without significant divergencies. In a previous report from this laboratory³, it was estimated that globin mRNA hybridized with an amount of DNA corresponding to 50,000 copies of globin genes. These experiments were performed in conditions of RNA excess and the hybrids were of poor quality; hence they were probably relatively non-specific and the results are readily explained in terms of hybridization of a component of the RNA with a family of repetitive sequences⁴. Similar considerations also apply to the estimate of 60,000 copies of globin genes obtained by hybridization of avian blood 10S RNA⁵. More recently, in experiments performed in conditions of DNA excess, Bishop et al.⁶ obtained a much lower estimate, of about five globin genes per genome, when duck 9S RNA was hybridized to excess duck DNA. These experiments have been criticized⁷ because the relationship between the rate constants for RNA–DNA and DNA–DNA hybridization reactions is uncertain and could be changed by partial degradation of RNA; moreover, a considerable fraction of the RNA remained unreacted in the presence of excess DNA even at very high C₀t values (the product of concentration of nucleic acid and incubation time expressed as mole nucleotide per litre × s).