ABSTRACT
The veil on genetic information was lifted by the advent of gene cloning and sequencing technologies in the 1970s. The discovery and purification of bacterial nucleases capable of cutting DNA at specific sequences and ligases capable of joining them enabled DNA from any species - or mRNAs converted to complementary ‘cDNA’ by reverse transcriptase - to be recombined into plasmids or phages and replicated to high copy number in bacterial hosts. Cloned genes were identified by screening with oligonucleotide probes and various genetic strategies. Heterologous expression systems produced medically important proteins, such as human insulin and growth hormone, kick starting the biotechnology industry. The following development of DNA sequencing technology led to an explosion in knowledge, including the discovery of oncogenes and tumor suppressors, and the rationalization of immunology. It also led to the characterization of homeotic and other genes that regulate differentiation and development, cell signaling and trafficking pathways, among many others, revealing their strong evolutionary conservation across wide ranges of developmental complexity and phenotypic diversity. The development of sensitive and specific exponential amplification of target DNA sequences in vitro by ‘polymerase chain reaction’ transformed DNA technology and diagnoses of genetic variants and pathogens. Protein and genetic engineering became possible. While an exhilarating period of discovery, gene cloning was biased toward protein-coding sequences. By the mid-1990s shotgun cDNA cloning and sequencing was being used to characterize the many RNAs that had eluded identification by biochemical and genetic screening, setting the stage for the genome sequencing projects that followed at the turn of the century.
