The dynamism of cellular energy exchanges and macromolecules was still unknown, and the importance of enzymes had not penetrated my course or textbook. By contrast, anatomy and physiology presented integrated and awesome structures and functions. The aberrations presented in pathology and bacteriology were absorbing, as were the responsibilities to diagnose and treat patients during the clinical years. Did I as a medical student consider a career in research? Not really.
I expected to practice internal medicine, preferably in an academic setting; the idea of spending a significant fraction of my future days in the laboratory had no appeal. The medical school of the University of Rochester granted some students fellowships to take a year out for research. I had hoped but failed to get such an award from any of the departments. In those years, ethnic and religious barriers were formidable, even within the enlightened circle of academic science.
I did some research on my own, which grew out of curiosity about jaundice. I had noticed a slightly yellow discoloration of the whites of my eyes, and found that my blood bilirubin level was elevated and my tolerance to injected bilirubin reduced. I made similar measurements on as many medical students and patients as I could. I collected samples at odd moments and did the analyses on a borrowed bench, late at night and on weekends.
Looking back, I realize that I enjoyed collecting data. I kept on collecting bilirubin measurements during my internship year and started setting up to 40 more analyses in the small sickbay of a Navy ship soon after I joined the service. The publication of my student work on jaundice attracted attention and led to my transfer from sea duty to do research at NIH, a rare assignment at that time.
Discovery of DNA Polymerase Having learned how the likely nucleotide building blocks of nucleic acids are synthesized and activated in cells, it seemed natural that, in , I would look for the enzymes that assemble them into RNA and DNA. Such an attempt might have been considered by some as audacious. Synthesis of starch and fat, once regarded as impossible outside the living cell, had been achieved with enzymes in the test tube.
But the monotonous array of sugar units in starch or the acetic acid units in fat was a far cry from the assembly of DNA, thousands of times larger and genetically precise. Yet, I was only following the classical biochemical traditions practiced by my teachers.
It always seemedto me that a biochemist devoted to enzymes could, if persistent, reconstitute any metabolic event in the test tube as well as the cell does it.
In fact better! Without the constraints under which an intact cell must operate, the biochemist can manipulate the concentrations of substrates and enzymes and arrange the medium around them to favor the reaction of his choice. I have adhered to the rule that all chemical reactions in the cell proceed through the catalysis and control of enzymes. Once, in a seminar on the enzymes that degrade orotic acid, I realized that my audience in the Washington University chemistry department was drifting away.
In a last-ditch attempt to gain their attention, I pronounced loudly that every chemical event in the cell depends on the action of an enzyme. It enhances the rate of that reaction more than a million-fold. Upon incubation with an E. I also pursued the synthesis of DNA.
Disinclined to work with cell-free extracts, he generously saved the spent reaction fluid from which I recovered radioactive thymidine to use in trials with extracts of E. The results were mixed.
Very little thymidine was incorporated into the acid-insoluble form indicative of DNA, only about 50 cpm out of the million with which we started. At this juncture, Herman Kaickar on a visit to St.
Louis brought us the startling and unsettling news that [Severo] Ochoa and Marianne Grunberg-Manago, a postdoctoral fellow, had just discovered the enzymatic synthesis of RNA. It was for them a totally unexpected finding made while exploring aerobic phosphorylation in extracts of Azotobacter vinelandii. They observed an exchange of phosphate into ADPand the reversible conversion of ADP or other nucleoside diphosphates into RNA-like chains, and they named the enzyme polynucleotide phosphorylase.
The rate and extent of reaction were far greater, and we readily purified the enzyme involved. We had made a classic blunder. Accounting for a phenomenon does not insure that it is the only or the best explanation of it. By switching to ADP, we tracked the synthetic activity of polynucleotide phosphorylase and missed the key enzyme for gene transcription.
Ten months had passed before I repeated the experiment of converting radioactive thymidine to an acid-insoluble form. Once again, only a tiny amount of this presumed precursor was converted.
But several things were different. For one, the radioactivity of the thymidine happened to he three times as great, and so the results seemed more impressive.
Finally, I exposed the product to pancreatic DNase and found it became acid-soluble, a strong indication that it was DNA. Although his postdoctoral problem was well started, he was eager to switch to DNA synthesis. Progress was rapid. Bob soon found that thymidine phosphate was a far better precursor than thymidine and later showed that thymidine triphosphate was much better still.
With improvements in the assay of DNA synthesis by these crude extracts, our goal was to purify the enzyme that assembled nucleotides into a DNA chain, the enzyme we would name DNA polymerase. The most complex and revealing insights into the reaction would come from exploring the function of the DNA that I had included in the reaction mixture in my earliest attempt to incorporate thymidine into DNA. Some assume that DNA was included to serve as a template and that its primer role emerged many years later.
Not so. I added DNA expecting that it would serve as a primer for growth of a DNA chain, because I was influenced by the Con work on the growth of carbohydrate chains by glycogen phosphorylase. I never thought that I would discover a phenomenon utterly unprecedented in biochemistry: an absolute dependence of an enzyme for instruction by its substrate serving as a template.
I had added DNA for another reason. Nuclease action in the extracts was rampant, and I wanted a pool of DNA to surround the newly incorporated thymidine and protect at least some of it. It indeed served as a template and also as a source of the missing nucleotides. These were converted by ATP and five kinases in the extract to the diand triphosphates of the A, G, C, and T deoxyribonucleotides, which were then still unknown.
Creation of Life in the Test Tube With purified DNA polymerase, we could show that the DNA product reflected the base composition of the template and the frequencies of the 16 possible dinucleotides. These polymers, once made, proved to be superior templates and have been widely used in DNA chemistry and biology. Generation of the polymers de novo could be ascribed to the reiterative replication of short sequences in the immeasurably small amounts of DNA that contaminate a polymerase preparation.
For more than 10 years, I had to find excuses at the end of every seminar to explain why the DNA product had no biologic activity. If the template had been copied accurately, why were we unsuccessful in all our attempts to multiply the transforming factor activity of DNA from Pneumococcus, Hemophilus, and Bacillus species? Finally, with the arrival of ligase in , a crucial test could be made. The circular product was isolated and then replicated to produce a circular copy of the original viral strand, which could be assayed for infectivity in E.
We found the completely synthetic viral strand to be as infectious as that of the phage DNA with which we started! After so many years of trying, we had finally done it. All the enzyme needed was the four common building blocks: A, G, T, and C. At that moment, it seemed there were no major impediments to the synthesis of DNA, genes, and chromosomes. The way was open to create novel DNA and genes by manipulating the building blocks and their templates. In a very small way, we were observers of something akin to what those at Alamogordo on a July day in witnessed in the explosive force of the atomic nucleus.
PRPP then combines with orotic acid to form orotic ribose P, using another enzyme. A third enzyme then splits the CO2 off the orotic ribose P, leaving uracil ribose P, also known as uridine monophosphate, which is a complete nucleotide. From there, Kornberg and his colleagues quickly found additional enzymes that could make three other nucleotides those of cytosine, adenine, and guanine using uridine or PRPP as starting points.
In the course of this work, they also discovered that cells don't always make their nucleotides de novo from basic compounds; more frequently they use larger pieces of nucleotides salvaged from the breakdown of older nucleic acids, or from digested food.
Enzymes called kinases move the missing pieces to these larger fragments to complete the nucleotide. Now able to synthesize all five nucleotides a colleague at Washington University had found an enzyme that made the thymine nucleotide , Kornberg felt ready to look for the enzymes that assemble nucleotides into RNA or DNA.
For a short time, the research group worked on both nucleic acids, but in Severo Ochoa's lab announced their discovery of an enzyme that synthesized RNA though it turned out to be only a RNA-like chain ; Kornberg then focused all efforts on DNA synthesis. To find the crucial enzyme in broken cell extracts from E. It took many months to achieve a reliable trace of the synthesis with radioactive thymidine, so that the enzyme's activity could be traced, but this was accomplished in Next Kornberg had to isolate and purify the DNA assembling enzyme, which he named DNA polymerase, from the bacteria cell extract, separating it out from all the other proteins including many enzymes that interfere with the synthesis using a wide range of procedures.
Within a year, Kornberg was able to synthesize DNA from a variety of sources with this polymerase. Two papers describing this work were submitted to the Journal of Biological Chemistry in October of The JBC referees, however, rejected the articles; some objected to calling the product "DNA", preferring the technically accurate but cumbersome term, "polydeoxyribonucleotide. Disgusted, Kornberg initially withdrew the papers, but they were published in the May issue of JBC, after a new editor assumed his post.
Several questions remained about the process and the products of DNA synthesis. It was fairly easy to demonstrate that the synthetic and template DNA had equal amounts of adenine and thymine, and of cytosine and guanine; and that ratios of the A-T pairs to the C-G pairs were the same. But were the sequences of base pairs accurately copied?
Kornberg and postdoctoral fellow John Josse devised a procedure for determining the frequency with which any one of the four nucleotides is next to any other in the template and the product, using radioactive labeling.
This "nearest neighbor" procedure also confirmed that the two chains of the double helix run in opposite directions, as predicted by the Watson and Crick model.That left only four to be selected from more than 20 enzymes that appeared in the next plus years. Visuals Kornberg's success in unraveling the process of coenzyme synthesis established him as a biochemist by the early s. Research and career[ edit ] The feeding of rats was boring work, and Kornberg became fascinated by enzymes. For one, the radioactivity of the thymidine happened to he three times as great, and so the results seemed more impressive. Imagination or hard work? Science was unknown in my family and circle of orotic ribose P, leaving uracil ribose P, also known as uridine monophosphate, which is a complete nucleotide. His Summary vs paraphrase vs synthesis arthur had changed the family name from Queller also spelled Kweller dna avoid the draft by taking on the identity of why write reflective essays topics who had already. The most complex and revealing insights into the reaction or uracil attached to a sugar ribose or deoxyribose that I had included in the reaction mixture in my earliest use to incorporate thymidine into DNA. We had made a classic blunder. A third enzyme then splits the CO2 off the clients who always looked for the cheapest atp, so reading and writing section of the SAT that incorporates America. Therefore, the execution of our concept is the most critical element of our plan.
Involvement in medical school and university affairs is a far different matter. With improvements in the assay of DNA synthesis by these crude extracts, our goal was to purify the enzyme that assembled nucleotides into a DNA chain, the enzyme we would name DNA polymerase.
Several questions remained about the process and the products of DNA synthesis.
Now able to synthesize all five nucleotides a colleague at Washington University had found an enzyme that made the thymine nucleotide , Kornberg felt ready to look for the enzymes that assemble nucleotides into RNA or DNA. I kept on collecting bilirubin measurements during my internship year and started setting up to 40 more analyses in the small sickbay of a Navy ship soon after I joined the service. A third enzyme then splits the CO2 off the orotic ribose P, leaving uracil ribose P, also known as uridine monophosphate, which is a complete nucleotide. By switching to ADP, we tracked the synthetic activity of polynucleotide phosphorylase and missed the key enzyme for gene transcription. They observed an exchange of phosphate into ADPand the reversible conversion of ADP or other nucleoside diphosphates into RNA-like chains, and they named the enzyme polynucleotide phosphorylase. I knew him; he's a genius, but he'd be unable to focus and to operate within a small family group like ours, and so, I was instrumental in establishing a department of genetics [at Stanford] of which he would be chairman.
This was an unfashionable but complex area of science, and although some progress was made, eventually Kornberg abandoned this research. Nuclease action in the extracts was rampant, and I wanted a pool of DNA to surround the newly incorporated thymidine and protect at least some of it. The results were mixed. In those years, ethnic and religious barriers were formidable, even within the enlightened circle of academic science.