7.3: Identifying DNA as the genetic material
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The exact location, and the molecular level mechanisms behind the storage and transmission of the genetic information, still needed to be determined. Two kinds of experiment led to the realization that genetic information was stored in a chemically stable form. In one set of studies, H.J. Muller (1890–1967) found that exposing fruit flies to X-rays (a highly energetic form of light) generated mutations that could be passed from generation to generation. This suggested that genetic information was stored in a chemical form and that that information could be altered through interactions with radiation. Once altered, the information was again stable.
The second piece of experimental evidence supporting the idea that genetic information was encoded in a stable chemical form came from a series of experiments initiated in the 1920s by Fred Griffith (1879–1941). He was studying two strains of the bacterium Streptococcus pneumoniae. These bacteria cause bacterial pneumonia and, when introduced, killed infected mice. Griffith grew these bacteria in the laboratory. This is known as culturing the bacteria. We say that bacteria grown in culture have been grown in vitro or in glass (although in modern labs, they are often grown in plastic), as opposed to in vivo or within a living animal. Following common methods, he grew bacteria on plates covered with solidified agar (a jello-like substance derived from salt water algae) containing various nutrients. Typically, a liquid culture of bacteria is diluted and spread on these plates, with individual and isolated bacteria coming to rest on the agar surface. Individual bacteria bind to the plate independently of, and separated from, one another. Bacteria are asexual and so each bacterium can grow up into a colony, a clone of the original bacterium that landed on the plate. The disease-causing strain of S. pneumoniae grew up into smooth or S-type colonies, due to the fact that the bacteria secrete a slimymucus-like substance. Griffith found that mice injected with S strain S. pneumoniae quickly sickened and died. However, if he killed the bacteria with heat before injection the mice did not get sick, indicating that it was the living bacteria that produced (or evoked) the disease symptoms rather than some stable chemical toxin.
During extended cultivation in vitro, however, cultures of S strain bacteria sometimes gave rise to rough (R) colonies; R colonies were not smooth and shiny but rather rough in appearance. This appeared to be a genetic change since once isolated, R-type strains produced R-type colonies, a process that could be repeated many, many times. More importantly, mice injected with R strain S. pneumoniae did not get sick. A confusing complexity emerged however; mice co-injected with the living R strain of S. pneumoniae (which did not cause disease) and dead S strain S. pneumoniae (which also did not cause the disease) did, in fact, get sick and died! Griffith was able to isolate and culture S.pneumoniae from these dying mice and found that, when grown in vitro, they produced smooth colonies. He termed these S-II (smooth) strains. His hypothesis was that a stable chemical (that is, non-living) component derived from the dead S bacteria had "transformed" the avirulent (benign) R strain to produce a new virulent S-II strain204. Unfortunately Fred Griffith died in 1941 during the bombing of London, which put an abrupt end to his studies.
In 1944, Griffith's studies were continued and extended by Oswald Avery, Colin McLeod and Maclyn McCarty. They set out to use Griffith's assay to isolate what they termed the “transforming principle” responsible for turning R strains of S. pneumoniae into S strains. Their approach was to grind up cells and isolate their various components, such as proteins, nucleic acids, carbohydrates, and lipids. They then digested these extracts with various enzymes (reaction specific catalysis) and ask whether the transforming principle remained intact.
Treating cellular extracts with proteases (which degrade proteins), lipases (which degrade lipids), or RNAases (which degrade RNAs) had no effect on transformation. In contrast, treatment of the extracts with DNAases, enzymes that degrade DNA, destroyed the activity. Further support for the idea that the “transforming substance” was DNA was suggested by the fact that it had the physical properties of DNA; for example it absorbed light like DNA rather than protein (absorption spectra of DNA versus protein. Subsequent studies confirmed this conclusion. Furthermore DNA isolated from R strain bacteria was not able to produce S-strain from R strain bacteria, whereas DNA from S strain bacteria could transform R strains into S strains. They concluded that DNA derived from S cells contains the information required for the conversion–it is, or rather contains, a gene required for the S strain phenotype. This information had, presumably, been lost by mutation during the formation of R strains. The phenomena exploited by Griffiths and Avery et al., known as transformation, is an example of horizontal gene transfer, which we will discuss in greater detail later on. It is the movement of genetic information from one organism to another. This is a distinctly different process than the movement of genetic information from a parent to an off-spring, which is known as vertical gene transfer. Various forms of horizontal gene transfer occur within the microbial world and allow genetic information to move between species. For example horizontal gene transfer is responsible for the rapid expansion of populations of antibiotic-resistant bacteria. Viruses use a highly specialized (and optimized) form of horizontal gene transfer205. The question is, why is this even possible? While we might readily accept that genetic information must be transferred from parent to offspring (we can see the evidence for this process with our eyes), the idea that genetic information can be transferred between different organisms that are not (apparently) related is quite a bit more difficult to swallow. As we will see, horizontal gene transfer is possible primarily because all organisms share the same basic system for encoding, reading and replicating genetic information. The hereditary machinery is homologous among existing organisms.
Contributors and Attributions
Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.