2 Nucleic Acids Convey Genetic Information hat special molecules might carry genetic information was appreciated by geneticists long before the problem claimed the attention of chemists. By the 1930s, geneticists began speculating as to what sort of molecules could have the kind of stability that the gene demanded, yet be capable of permanent, sudden change to the mutant forms that must provide the basis of evolution. Until the mid-1940s, there appeared to be no direct way to attack the chemical essence of Lhe gene. It was known that chromosomes possessed a unique molecular constituent, deoxyribonucleic acid (DNA), but there was no way to show that this constituent carried genetic information, as opposed to serving merely as a molecular scaffold for a still undiscovered class of proteins especially tailored to carry genetic information. It was generally assumed that genes would be composed of amino acids because, at that time, they appeared to be the only biomolecules with sufficient complexity.
It therefore made sense to approach the nature of the gene by asking how genes function within cells. In the early 1940s, research an the mold Neuraspora, spearheaded by George W, Beadle and Edward Tatum. was generating increasingly strong evidence supporting the 30-year-old hypothesis of Archibald £. Garrod that genes work by controlling the synthesis of specific enzymes (the one gene—one enzyme hypothesis}. Thus, given that all known enzymes had, by this time, been shown to be proteins, the key problem was the way genes participate in the synthesis of proteins. From the very start of serious speculation, the simplest hypothesis was that genetic information within genes determines the order of the 20 different amino acids within lhe polypeptide chains of proteins.
In attempting to test this proposal, intuition was of little help even to the best biochemists, since there is no logical way to use enzymes as tools to deter mine the order of each amino arid added to a polypeptide chain. Such schemes would require, for the synthesis of a single type of protein, as many ordering enzymes as there are amino acids in (he respective protein. But since all enzymes known at that time were themselves proteins (we now know thai RNA can also act as an enzyme in a few instances), still additional ordering enzymes would be necessary to synthesize the ordering enzymes. This situation dearly poses a paradox, unless we Eissurne a fantastically interrelated series of syntheses in which a given protein has many different enzymatic specificities. With such an assumption, it might be possible (and then only with great difficulty) to visualize a workable cell. It did not seem likely, however, that most proteins would be found to carry out multiple tasks. In fact, all the current knowledge pointed to the opposite conclusion of one protein, one function.
Avery's Bombshell; DNA Can Carry
The Genetic information within DNA Is Conveyed by the Sequence of Its Four
Nucleotide Building Blocks (p ?8) •
Establishing the Direction of Protein Synthesis (p. !>7)
The Eta of Genomics (p 3B)
20 Nucleic Acids Convey Genetic Information
AVERY'S BOMBSHELL: DNA CAN CARRY GENETIC SPECIFICITY
That DNA might br> the key genetic molecule emerged most unexpectedly from studies on pneumonia-causing bacteria. In 1928 English microbiologist Frederick Griffith made the startling observation that nonvirulent strains of the bacteria became virulent when mixed with their heat-killed pathogenic counterparts. That such transformations from nonvirulence to virulence represented hereditary changes was shown by using descendants of the newly pathogenic strains to transform still other nonpathogenic bacteria. This raised the possibility that when pathogenic cells are killed by heat, their genetic components remain undamaged. Moreover, once liberated from the heat-killed cells, these components can pass through the cell wall of the living recipient cells and undergo subsequent genetic: recombination with the recipient's genetic apparatus (Figure 2-1). Subsequent research has confirmed this genetic interpretation. Pathogenicity reflects the action of the capsule gene, which codes for a key enzyme involved in the synthesis of the carbohydrate-containing capsule that surrounds most pneumonia-causing bacteria. When the S (smooth) allele of the capsule gene is present, then a capsule is formed around the cell that is nncnssary for pathogenesis (the formation of a capsule also gives a smooth appearance to the colonies formed from these cells). When the H (rough) allele of this gene is present, no capsule is formed and the respective cells are not pathogenic.
Within several years after Griffith's original observation, extracts of the killed bacteria were found capable of inducing hereditary transformations, and a search began for the chemical identity of the transforming agent. At that time, the vast majority of biochemists still believed that genes were proteins. It therefore came as a great surprise when in 1944, after some ten years of research, U.S. microbiologist Oswald T. Avery and his colleagues at the Rockefeller Institute caps caps
capE into capw cell
FIGURE 2-1 Transformation of a genetic characteristic of a bacteria! cell (Streptococcus pneumoniae) by addition of heat-killed cells of a genetically different strain. Here we show an R cell receiving a chromosomal fragment containing the rapsute gene from a heat-treated S cell. Since most K cells receive other chromosomal fragments, the efficiency of transformation for a given gene is usually tess than capE into capw cell
FIGURE 2-1 Transformation of a genetic characteristic of a bacteria! cell (Streptococcus pneumoniae) by addition of heat-killed cells of a genetically different strain. Here we show an R cell receiving a chromosomal fragment containing the rapsute gene from a heat-treated S cell. Since most K cells receive other chromosomal fragments, the efficiency of transformation for a given gene is usually tess than in New York, Colin M. MacLeod and Maclyn McCarty, made the momentous announcement that the active genetic principle was DNA (Figure 2-2). Supporting their conclusion were key experiments showing that the transforming activity of their highly purified active fractions was destroyed by pancreatic deoxyribonuclease, a recently purified enzyme that specifically degrades DNA molecules to their nucleotide building blocks and has no effect on the integrity of protein molecules or RI\A. The addition of either pancreatic ribonu-ctease [which degrades RIVA) or various proteolytic enzymes (protein-destroying) had no influence on the transforming activity.
Even more important confirmatory evidence csme from chemical studies with viruses and virus-infected cells. By 1950 it was possible to obtain a number of essentially pure viruses and io determine which types of molecules were present in them. This work led to the very important generalization that all viruses contain nucleic acid. Since there was at that time a growing realization that viruses contain genetic material, the question immediately arose as to whether the nucleic acid component was the cairicr of viral genes. A crucial test of the question came from isotopic study of the multiplication of T2, a bacterial virus (bacteriophage, or phage) containing a DNA core and a protective shell built up by the aggregation of a number of different protein molecules. In these experiments, performed in 1952 by Alfred D. Hershey and Martha Chase working at Cold Spring Harbor Laboratory on long Island, (he protein coat was labeled with the radioactive isotope ar,S and the DNA core with the radioactive isotope nP. The labeled virus was then used to follow the fates of the phage protein and nucleic acid as phage multiplication proceeded, particularly to see which labeled atoms from the parental virus entered the host cell and later appeared in the progeny phage.
Clear-cut results emerged Irani these experiments; much of the parental nucleic acid and none of the parental protein was detected in the progeny phage (Figure 2-3). Moreover, it was possible to show that little of the parental protein even enters the bacteria; instead, it stays attached to the outside of the bacterial cell, performing no function after the DNA component has passed inside. This point was neatly shown by violently agitating infected bacteria after the entrance of the DNA; the protein coats were shaken off without affecting the ability of the bacteria to form new phage particles.
With some viruses it is now possible to do an even more convincing experiment. For example, purified DNA from the mouse virus polyoma can enter mouse cells and initiate a cycle of viral multiplication producing many thousands of new polyoma particles. The primary function of viral protein is thus to protect its genetic nucleic acid component in its movement from one cell to another. Thus no reason exists for proteins to play any part in the structure of a gene.
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