The dawn of gene isolation.

Ever since it became clear through the work of Watson and Crick that the gene is a stretch of double stranded helical DNA and is understandable in chemical terms, biochemists have striven to get their hands on isolated genes. The isolation of the ribosomal genes of Xenopus laevis in 1966 provided a first instance where a purified DNA of known function could be investigated, long before the advent of gene cloning technologies. The second instance was the purification of the Lac operon from Escherichia coli. Later, but still before the gene cloning days the 5S RNA genes of X. laevis and the histone genes of the sea urchin Psammechinus miliaris were isolated by physico-chemical methods, but their isolation marked the end of an era. By 1975, gene cloning technology was well established and the isolation of genes quickly became an everyday occurrence.