Berg, Gilbert, and Sanger Develop Techniques for Genetic Engineering
The development of genetic engineering techniques by Paul Berg, Walter Gilbert, and Frederick Sanger has significantly advanced the field of molecular biology. These scientists contributed to methods that enable the sequencing and manipulation of DNA, the molecule that encodes genetic information. In the 1970s, Gilbert and his team developed a chemical cleavage approach to sequence DNA, allowing researchers to determine the order of nucleotides in a DNA strand. Similarly, Sanger introduced a simpler biological method involving cloning, which involved linking DNA from different sources and using DNA polymerase to create complementary strands for sequencing.
Berg's work focused on gene cloning, enabling the creation of hybrid DNA molecules that could be analyzed for gene function and mutations. These techniques laid the groundwork for genetic engineering, where genes can be transferred between organisms to confer new traits. The implications of these advancements are profound, facilitating the production of human proteins like insulin, enhancing crops for resilience, and paving the way for genetic therapies to treat various diseases. The sequencing techniques developed by these scientists have become standard in laboratories worldwide and were crucial in mapping the human genome, a milestone achieved in 2000. The collective contributions of Berg, Gilbert, and Sanger have revolutionized our understanding of genetics and its applications in medicine and biotechnology.
Berg, Gilbert, and Sanger Develop Techniques for Genetic Engineering
Date 1980
Paul Berg, Walter Gilbert, and Frederick Sanger initiated a new field of research when they developed techniques for sequencing and manipulating DNA.
Locale United States; England
Key Figures
Paul Berg (b. 1926), professor of biochemistry at Stanford Medical Center who was a cowinner of the 1980 Nobel Prize in ChemistryWalter Gilbert (b. 1932), professor of biochemistry at Harvard University who was a cowinner of the 1980 Nobel Prize in ChemistryFrederick Sanger (b. 1918), professor of biochemistry at Cambridge University who was a cowinner of the 1980 Nobel Prize in Chemistry
Summary of Event
Deoxyribonucleic acid (DNA) is often called an information-containing molecule. Genes, which are made of DNA, reside in the nucleus of a cell and directly control the functioning of that cell. To understand how cells function and how to manipulate genes, therefore, it is necessary to understand how the information is contained in DNA.
Every DNA molecule is built up by linking, in end-to-end fashion, a large number of smaller molecules into two chains. Only four different subunits, called nucleotides, are used to construct DNA. These nucleotides are called adenine (A), thymine (T), guanine (G), and cytosine (C). Every A in one chain is matched with a T in the other. Likewise, C’s are always matched with G’s. Thus the enormously complex human chromosome, which consists of a single long DNA molecule containing hundreds of millions of nucleotides, can be viewed as nothing more than two chains that consist of a linear sequence of A’s, C’s, T’s, and G’s.
In the 1960’s and 1970’s, techniques were developed that made it possible to determine, laboriously, the nucleotide sequence of a very small piece of ribonucleic acid (RNA), which is found in cells and is similar to DNA. Given that the DNA in a single human cell contains more than five billion nucleotides in forty-six long strands called chromosomes, however, it soon became clear that more powerful techniques for the analysis of DNA molecules would be needed if biologists were to make sense of this incredibly complex structure and gain the ability to manipulate genes directly.
Walter Gilbert and his colleague Allan Maxam had been using the techniques of bacterial genetics to try to understand how the bacterium Escherichia coli was able to turn on its ability to use milk sugar as a source of energy. Gilbert realized that he would have to know the DNA sequence—that is, the linear sequence of nucleotides—of the genes that controlled this process in order to understand fully how the bacterium was able to produce this switch in metabolism.
In early 1975, Gilbert, Maxam, and visiting scientist Andrei Mirzabekov began to experiment with various chemical treatments that could cut DNA molecules after specific nucleotides as a way to sequence the molecule. DNA to be sequenced was divided into four portions. Each portion was treated with a different set of chemical reagents; in one tube, the DNA was cut after adenine, while in the other tubes the DNA was cut after thymine, cytosine, or guanine.
The first nucleotide in a DNA molecule produces a fragment one nucleotide long, which appears in only one of the four chemical treatments and thus is sensitive to that particular chemical cleavage. The second nucleotide in the sequence can be determined by observing which chemical treatment made a radioactive fragment two nucleotides long. This analysis continues until the full sequence is determined. Approximately 250 to 300 nucleotides can be determined from a single set of reactions, and very long stretches of a DNA sequence can be obtained by linking the sequence of overlapping fragments.
By 1980, Frederick Sanger had already made major contributions to an understanding of the mechanisms by which genes control the functions of a cell. He had won the 1958 Nobel Prize in Chemistry for work leading to a practical method for determining the amino acid sequence of proteins. Like Gilbert, however, Sanger realized that a full understanding of the function of a gene would require an easy technique for sequencing DNA.
While Gilbert had used a chemical cleavage technique for sequencing DNA, Sanger chose to use a biological technique. Sanger took advantage of a discovery by Joachim Messing and his coworkers that DNA from any source could be linked end-to-end with the DNA of a small virus called M13. This technique was called cloning.
When DNA polymerase, the enzyme responsible for assembling nucleotides into long strands, was added, new DNA was made. DNA polymerase always pairs A’s in the template with T’s in the newly made DNA and vice versa. Likewise, DNA polymerase always paired C’s with G’s. The newly made DNA was complementary to the cloned template DNA, and the nucleotide sequence of this new DNA could be used to determine the nucleotide sequence of the cloned DNA. Again, nearly three hundred bases could be sequenced at one time, but Sanger’s technique proved to be both simpler and faster than Gilbert’s.
Although the techniques devised by Sanger and Gilbert were designed to describe the nucleotide sequence of any piece of DNA, other techniques for manipulating DNA would be required for genetic engineering to be possible. Paul Berg was one of the founders of the technique of cloning genes from two different organisms. These hybrid DNA molecules could then be produced in sufficient amounts to sequence easily. The genes also could be mutated and put back into the cells from which they were obtained, allowing researchers to determine the effects these specific changes had on gene function. Through the same techniques, genes from one organism could easily be introduced into the cells of another, thus adding new functions to organisms. These techniques were termed “genetic engineering.”
Significance
The information obtained from the techniques of cloning and DNA sequencing has revolutionized the understanding of how genes, cells, and organisms function. Incredibly complex processes such as the functioning of the nervous system and the brain, the development of embryos, the functioning of the immune system, and the genetic contribution to cancer can now be understood at a molecular level. The science of genetic engineering has become routine.
As a result of the ability to clone and manipulate genes, bacteria can make human proteins such as insulin or growth hormone, and plants can be produced that are resistant to herbicides or viral infections. These same techniques have made it possible for researchers to diagnose and treat genetic diseases in animals by replacing a defective gene in a cell with a normal one. These techniques will ultimately prove useful in the diagnosis and cure of common human genetic diseases such as cystic fibrosis and muscular dystrophy.
The DNA sequencing techniques developed by Gilbert and Sanger came to be used routinely in laboratories throughout the world. The ultimate achievement of the sequencing, however, was that it laid the foundation for scientists to determine the sequencing of the human genome, a task that was accomplished in the year 2000. This information provided a bonanza for the diagnosis and treatment of other genetic diseases and aided efforts to understand the the genetic capabilities of human beings.
Bibliography
Alberts, Bruce, et al. Molecular Biology of the Cell. 4th ed. New York: Garland, 2002. Clearly written introductory molecular biology textbook for undergraduate biology majors provides complete coverage of the major areas of knowledge that resulted directly from recombinant DNA technology. Includes excellent photographs and diagrams as well as valuable reference lists.
Lewin, Benjamin. Genes VI. New York: Oxford University Press, 2005. Molecular biology textbook for advanced undergraduate and graduate students provides a thorough, detailed survey of the science behind biotechnology and the techniques and discoveries in DNA technology and DNA sequencing. Includes excellent illustrations, glossary, and bibliographic references.
Suzuki, David, and Peter Knudtson. Genethics: The Clash Between the New Genetics and Human Values. Rev. ed. Cambridge, Mass.: Harvard University Press, 1990. Addresses the ethical issues that arise from the possibilities presented by DNA technology. Includes discussion of genetic engineering.
Watson, James D., et al. Molecular Biology of the Gene. 5th ed. San Francisco: Benjamin Cummings, 2004. Excellent, readable account of the discoveries that came from the basic science of recombinant DNA technology. Amply illustrated, with good references.
Watson, James D., and John Tooze. The DNA Story. San Francisco: W. H. Freeman, 1981. Highly readable historical account of the major discoveries in genetic engineering.