Читать книгу Welcome to the Genome - Michael Yudell - Страница 24

Since the early part of the twentieth century, scientists had been aware of the vital connection between genes and enzymes, a type of protein that usually accelerates chemical reactions in an organism. As early as 1901, Archibald Garrod, a London physician studying metabolic disorders, recognized that patients with the disease alkaptonuria were lacking what he called a “special enzyme” that results in the body’s inability to break down a substance called alkapton (today we know that alkaptonuria is caused by a mutation in the HGD gene on chromosome 3, which impairs the body’s ability to break down the amino acids phenylalanine and tyrosin). By studying familial patterns of this disease, Garrod came to infer that the missing enzyme was a problem of inheritance; most of the children with the defect were born to parents who were first cousins. (20) This “shallow” gene pool made the emergence of this recessive trait more likely.

Four decades later at Stanford University, biochemist Edward Tatum and geneticist George Beadle refined Garrod’s observations, suggesting in 1941 that one gene codes for one enzyme, a theory that was a cornerstone of molecular biology for more than five decades. They were awarded a Nobel Prize for their discovery in 1958. (21) Although DNA itself was coming to be known to be the stuff of heredity, enzymes and other proteins, it was turning out, were essential to the successful operation of the cell and therefore of the organism. If hereditary information was carried on DNA, then the different classes of proteins are, in large part, heredity’s workhorses, delivering instructions for many of life’s intricacies at the beck and call of the DNA molecule itself.

Work at the cellular level, with its varied goals, was less directed, for example, than the search for the structure of DNA. Some scientists were busy taking the cell apart to determine how DNA replicated, others learning how proteins were synthesized, and still others inquiring about the nature and function of proteins. In fact, Arthur Kornberg carried out his Nobel Prize‐winning discovery of the protein in bacteria that controls DNA replication without Watson and Crick’s work in mind. Perhaps what Kornberg himself called his “many love affairs with enzymes” distracted him from the broader goings‐on in molecular biology. “The significance of the double helix did not intrude into my work until 1956,” Kornberg wrote, “after the enzyme that assembles the nucleotide building blocks into a DNA chain was already in hand.” (22)

Kornberg’s discovery, once known as DNA polymerase or Kornberg’s enzyme and now known as DNA polymerase I, catalyzes the addition of nucleotides to a chain of DNA (other DNA polymerases were discovered later, and were in turn known as polymerases II, III, etc.). In other words, DNA polymerase is the mechanism by which DNA clones or copies itself. Working with the bacteria E. coli, a bacteria that is usually beneficial to the function of the human digestive tract, Kornberg showed that the enzyme DNA polymerase was able to synthesize a copy of one strand of DNA. With a single strand of DNA in a test tube, the presence of DNA polymerase served as the catalyst (or initiator) for DNA replication. These experiments revealed only that the synthesized DNA was true to Chargaff’s rules, having the correct ratio of As to Ts and Cs to Gs. (23) Kornberg’s results did not, however, reveal the sequential arrangement of nucleotides, nor was it known at this time whether this laboratory model was what actually happened in living organisms. (24)

It later turned out that Kornberg’s polymerase was not the key polymerase in DNA replication; DNA polymerase III was. Scientists who questioned the function of Kornberg’s polymerase in live organisms were only partially correct; polymerase I’s role was still found to be vital, playing a key role in chromosome replication and DNA repair. (25) Over the next two decades the approaches pioneered by Kornberg and his associates resulted in the discovery of a broad array of enzymes and other proteins important in the replication of DNA and the translation of proteins. An intriguing aspect of these discoveries is that polymerase enzymes do not need to be in cells to work. Biochemists used this feature of polymerase to develop methods to take proteins out of cells and coax them to activate in test tubes. The other enormously important result of Kornberg’s work was that scientists now had a laboratory reagent—the DNA polymerase itself—that could be used in a test tube to replicate DNA.