The DNA molecule was discovered in the mid-1800s. It was not proven to be critical to the mechanisms of inheritance until the 1940s. Since the 1950s, and the seminal work of Watson and Crick to characterize the chemical structure of DNA and how it could be copied, the understanding of the biochemistry of DNA and the ability to manipulate it in the laboratory has grown exponentially. Breakthroughs studying DNA now come fast and furious, but several key discoveries are largely responsible for the development of modern recombinant DNA technology.
The Genetic Code
Knowing that DNA is the molecule of heredity is one thing. Knowing how it maintains and transmits crucial genetic information is something else entirely. In the early 1960s, Marshall Nirenberg and colleagues working at the US National Institutes of Health cracked the genetic code. Using synthetic RNA molecules and radioactively labeled amino acids they proved that DNA used a three letter code (three nucleotide bases made a “codon”) to specify individual amino acids, the codons in RNA molecules were contiguous, they did not overlap, and there were punctuation marks, places specifying “start” and “stop”.
Episomes and Plasmid DNA
In the 1950s and 1960s, numerous scientists were studying how DNA was handled in bacteria, viruses and yeast. From their combined work came the knowledge that bacteria could carry small, circular molecules of DNA that were not integrated as part of their chromosomal DNA. Most important of all, these “plasmids” or “episomes” could be isolated and more importantly they could be transferred back into other bacteria.
Restriction Endonucleases
One of the key factors in recombinant DNA technology is the ability to “cut” DNA molecules and to be able to “paste” pieces together, often in a new order. Werner Arber, a Swiss microbiologist, was the first to recognize that there were enzymes that would cut DNA molecules in specific ways. Shortly after this work was published studying the bacterium E. coli, Hamilton Smith and colleagues identified a “restriction endonuclease” from another bacterium, Haemophilus influenza, and then showed that it cut at a very specific sequence of DNA bases. Today there are hundreds of restriction enzymes known to cut DNA at specific occurrences of base sequences.
Recombinant DNA Molecules
In 1972, scientists Herbert Boyer and Stanley Cohen were in Hawaii attending a scientific meeting about plasmids when they met to discuss the work that they were each pursuing. What ultimately came from this meeting was the birth of recombinant DNA cloning, the start of the biotechnology industry and a new era in molecular biology. Collaborating on the use of newly identified restriction enzymes and DNA manipulation techniques, they published seminal papers showing that DNA from one source could be cut with restriction enzymes and then placed into the midst of cut DNA from another source, and these could be placed into bacteria and grow stably with the “recombined” DNA maintained in the newly arranged form.
Herbert Boyer went on to found the biotech company Genentech and the research world has never been the same. Recombinant DNA methods are now used to make things from insulin for diabetics to proteins that can make individual cells, or even whole organisms, glow green, blue or red under just the right light. Remarkable.
To read more about the cracking of the genetic code by Nirenberg and colleagues visit the history page at the US National Institutes of Health.
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