The beginnings of molecular biology as a distinct discipline occurred in the late 1940s and early 1950s with the recognition that polynucleotides were the repository of genetic information in the form of DNA and the transmitters of genetic information in the form of messenger RNA (mRNA), and that transfer RNAs are fundamental for the assembly of amino acids into proteins. Detailed descriptions of the historical developments of this modern era of molecular biology are provided in several books.1,2,3 and 4 These were exciting times, as understanding progressed rapidly from the discovery by Avery and Brundage that DNA was a genetic substance; Chargaff established that DNA is composed of four different deoxyribonucleotides (dATP, dGRP, dTTP, dCTP); Watson and Crick elucidated the double-helical structure of DNA; Jacob and Monod identified mRNA as the intermediary in the transfer of information encoded in DNA to the assembly of amino acids into proteins; Holly discovered transfer RNAs; and Nirenberg et al. discovered the genetic code (i.e., each of the 21 amino acids is specified by a triplet of nucleotides, or codons, within the mRNA to be translated into a protein).

In the 1960s, several major discoveries paved the way for the development of recombinant DNA technology and genetic engineering. Two of the major breakthroughs that made this possible were the discoveries of reverse transcriptase5 and restriction endonucleases,6,7 and 8 and techniques for determining the precise sequence of nucleotides in DNA.9,10 Reverse transcriptase, which is found encoded in the RNA of certain tumor viruses, is the means by which the virus makes DNA copies of its RNA templates. It allows molecular biologists to copy mRNA into complementary DNA (cDNA), which is an essential step in the preparation of recombinant DNA for purposes of cloning.