“On The Central Dogma”

Introduction Since Francis Crick and James Watson’s 1953 discovery of DNA’s double helix structure at Cambridge, several milestones in molecular biology have moved research in the field forward. Among these, Crick’s landmark 1957 “Central Dogma” lecture (published in 1958 in Nature as “On Protein Synthesis”) was so influential that it “permanently altered the logic of ​ biology” in just a few paragraphs (Judson, 1979). Crick’s central dogma not only “defined the field” of molecular biology over the next half-century, but also built important connections with other areas of study, from genetics and medicine to information theory and evolutionary biology (Ridley, 2009). Yet while its advocates argue today that Crick’s central dogma (and the related “sequence hypothesis”), combined with Darwin’s natural selection, can “provide the underpinning to all biology”, others claim that Crick’s framework has been dead for decades, due to the discovery of prions and “molecular chaperones,” among other phenomena (Morange, 2008; Koonin, 2012). These strongly differing opinions on the central dogma’s place in molecular biology only prove its ongoing relevance as both a guideline for research and focus of debate. The Central Dogma In his 1957 lecture at University College London and subsequent 1958 article, Crick announced several predictions regarding nucleic acids as the sole bearers of genetic information, and their connection to the making of proteins. He argued that “the main function of the genetic material is to control (not necessarily directly) the synthesis of proteins” (Crick, 1958). While this statement may seem commonplace to modern-day readers, it was indeed an unsettling idea to the many biologists who did not believe that genes were made exclusively of DNA (Cobb, 2016). The previous discovery of DNA as the primary source of genetic information in organisms provided an essential foundation for the central dogma. The Avery-MacLeod-McCarty experiment on bacteria transformation disproved the widely believed idea that protein carries hereditary information, suggesting that DNA did this instead (although technological limitations prevented the researchers from completely ruling out the role of protein contaminants) (Avery et al., 1944; Cobb, 2016). Because this DNA was understood as part of the chromosomes in the nucleus, while protein synthesis was known to somehow happen in the cell cytoplasm, Crick hypothesised the most probable scenario: a DNA-to-RNA-to-Protein path of information flow leading to protein synthesis (Morange, 2009). In the absence of any previous scientific evidence that this information might travel in the reverse direction, or any known mechanism that could allow this to happen, Crick (1958) stated that “once ‘information’ has passed into protein it cannot get out again”, the key principle of his “central dogma.” Crick’s lecture also included the related “Sequence Hypothesis,” which stated that the specific sequence of nucleic acids’ bases served as a code for the creation of a protein’s amino acid sequence. Furthermore, he suggested that a protein’s folding mechanism was “a function of the order of amino acids, provided it takes place as the newly formed chain comes off the template,” with the protein’s final three-dimensional structure coded into its one-dimensional structure of amino acids (Crick, 1958). This one-dimensional sequence was the “information” that traveled on the paths allowed by the central dogma. A “Grand Hypothesis” It is most impressive that Crick could make such accurate speculations about protein synthesis based on the little experimental data was available at time. Researchers were only just beginning to understand the role of ribosomes in the process, and the existence of tRNA “adaptors” was another one of Crick’s predictions that was only later confirmed (Cobb, 2016). Still, it could be said that one mistake Crick made was not in his 1957-58 predictions, which all proved to be accurate, but in his decision to use the term “dogma” to describe them. Crick (1988) later confessed that he had only meant “basic assumption,” writing, “I used the word in the way I myself thought about it...and simply applied it to a grand hypothesis that, however plausible, had little direct experimental support”. Indeed, Crick’s “grand hypothesis” has guided researchers’ fundamental understanding of molecular biology for decades. In his memoir, Crick (1988) reflected on how a hypothesis such as the central dogma can provide “more positive and explicit hypotheses” for molecular biologists, either successfully guiding research or being abandoned and replaced by another theory. While these types of theories may be more commonly associated with physics, Crick (ibid.) emphasised their significance for biology as well, writing that because biological problems “have evolved by natural selection, the mechanisms involved are usually too accidental and too intricate” for one researcher to solve. With this in mind, Crick put forward the central dogma as a valuable tool for generations of scientists as they pursue their own research in not only the field of molecular biology, but also genetics, evolutionary biology, and medicine. Early Modifications of the Central Dogma The first major challenge to the central dogma came from Howard Temin and David Baltimore’s separate discoveries of reverse transcriptase, an enzyme capable of catalyzing the synthesis of DNA from an RNA template. This enzyme allows specific viral RNA sequences to fuse securely into host genomes, enacting chronic infection and the production of new copies of the virus. Although Baltimore’s 1970 paper regarding reverse transcriptase attempted to undermine the central dogma, arguing that reverse transcriptase was an example of RNA leading to changes in DNA, this claim was based on a misunderstanding of Crick’s hypothesis. While Jim Watson’s 1965 textbook Molecular Biology of the Gene did define the central ​ ​ dogma as simply “DNA→ RNA→ Protein”, Crick’s 1958 paper had actually argued that there simply was not yet evidence of RNA-to-DNA interaction, and he did not completely rule out that possibility (Watson et al., 2014; Cobb, 2017). In his reply to Temin and Baltimore, Crick (1970) accepted the idea that RNA could be converted into DNA, but added that he considered it a “rare” occurrence. Revisiting his theory, Crick (1970) emphasized that DNA and RNA macromolecules could replicate themselves, and that there could be “possible transfers” of information from RNA to DNA and DNA to protein. Other transfers from protein to protein, protein to RNA, and protein to DNA were still marked as “most unlikely” (ibid.). While the discovery of reverse transcriptase did not disprove or “reverse” Crick’s hypothesis, it did call for the central dogma to be reevaluated and clarified in relation to these new findings (Anonymous, 1970). The Pathway of Sickle Cell Anemia Sickle cell anemia, an autosomal-recessive blood disease, is one of the many genetic conditions best understood through the pathway for protein synthesis outlined in the central dogma. A single point mutation at position 6 in the beta-chain gene for hemoglobin, replacing an adenine with a thymine nucleotide, results in a mutant hemoglobin S protein with a hydrophobic valine (triplet GTG), instead of a normal hemoglobin A protein with a hydrophilic glutamic acid (triplet GAG) (Cox et al., 2015; Watson et al., 2014). By gathering to make insoluble fibers in the erythrocytes, the hemoglobin S proteins ultimately cause these blood cells to become misshapen after deoxygenation in the blood vessels. The sickle-shaped blood cells are likely to burst, resulting in anemia, organ failure, and often death. The devastating effect of a single base change in the genetic code can be traced along each step of the central dogma’s information flow. Just as the central dogma can clarify the cellular process that produces sickle cells, it can also provide clues about how this condition could be cured through new gene therapies. One treatment currently being researched involves reactivating the production of fetal hemoglobin (HbF), normally only produced during fetal development, by knocking out the BCL11A gene which produces a repressor protein to prevent production of HbF in adults (Cools, 2012; Dana Farber, 2016). Another possible therapy involves actually replacing the thymine base with an adenine in stem cells at the site of red blood cell production in the bone marrow using CRISPR “gene editing” technology (Ledford, 2016; Doudna, 2018). Both of these treatments are approaching the clinical trial stage, and represent a new field of genetic medicine that can intervene at various stages of the central dogma’s information flow in order to treat genetic conditions (Howard Hughes Medical Institute, 2017). Neo-Darwinism at the Molecular Level The central dogma also made a significant contribution to the “Modern Synthesis” of evolutionary biology, developing an understanding of evolution that had its beginnings in the work of Charles Darwin more than a century earlier. Darwin’s theory of natural selection depended upon three principles: that because there are limited resources, organisms have to compete to survive, that variation occurs within a species, and that some of these variations can be passed along to the next generation (Gould, 2001). While Jean-Baptiste Lamarck also agreed that species could change over time, he argued that this evolution happened by the “will of the organism”, an idea that Darwin opposed (Ridley, 2004). Lamarck also believed that as organisms developed, they obtained individual characteristics due to “accidents, diseases, and muscular exercise”, and that these modifications could be inherited by organisms’ offspring (ibid). Lacking Mendel’s understanding of genetic heredity, Darwin could not completely disregard the role of acquired characteristics in evolutionary change, but many of his 20th century followers strongly opposed this Lamarckian theory. In the first half of the 20th century, “Neo-Darwinian” evolutionary biologists attempted to combine research on small variations across large populations of organisms with Mendel’s ideas about heredity in order to show that Darwinian natural selection could operate without Lamarck’s acquired characteristics (Zimmer, 2006; Ridley, 2004). Importantly, August Weismann argued that acquired characteristics are not passed to offspring by showing that only changes to the germ line, and not the soma line, could be inherited in an organism. This research on the separation of germ and soma lines, although limited to some animals, raised the question of “whether Lamarckian inheritance had any influence in evolution at all” (Ridley, 2004). Crick’s central dogma provided strong support for the Neo-Darwinians’ view of evolutionary biology at the molecular level, arguing that there was simply no route for “acquired characteristics” to become hereditary. Because “once ‘information’ has passed into protein it cannot get out again,” any information about changes to proteins during a cell’s lifetime could not possibly transfer back to the DNA (Morange, 2008; Crick, 1958). Furthermore, while Weismann’s theory of germ-soma separation had applied only to animals, Crick’s hypothesis applied to all organisms (Cobb, 2016). Information Flows Unlike many other biologists before him, Crick’s 1958 article referred to protein synthesis as a flow of information rather than a flow of energy or matter. By “information,” Crick (1958) meant “the specification of the amino acid sequence of the protein”. Crick used the term “information” only a metaphor, which may have seemed vague to researchers in developing fields such as cybernetics and information theory, who did not take Crick’s groundbreaking paper as an opportunity to begin collaborating with biologists (Cobb, 2016; Gleick, 2012). Nevertheless, it was a metaphor of great importance within biology, as it helped lay the foundation of a new way of seeing genes, evolution, and life. The leading evolutionary biologist Richard Dawkins was one scientist who adopted this metaphor in his own work. For Dawkins, genes are “pure information” that can be “encoded, recoded, and decoded, without any degradation or change of meaning” (1996). Importantly, these genes are “digital” information made of base sequences, which can be copied over and over again down the generations with great accuracy, with just the “occasional errors to introduce variety” (ibid). In Dawkins’s (2016) influential “gene-centered” view of evolution, all living things, from humans to chipmunks to chimpanzees, are just “survival machines” used by genes to replicate themselves in the form of information. Recent Challenges to the Central Dogma A more recent challenge to the validity of the central dogma was the discovery of prions, cellular proteins that are capable of changing their structure, as evident in the neuro-degenerative conditions scrapie and Creutzfeldt-Jakob Disease (vCJD) (Koonin, 2012). Prions can transfer the pathogenic form of their 3D structure (“conformation”) to a normal protein. This, in some measure, might be seen as contradicting Crick’s view that information transfer from protein to protein is not possible. Crick’s defenders argue that the central dogma only referred to the sequence of amino acids, which remained the same, and not their 3D structure (Cobb, 2016). Others argue that prions do represent a form of information transfer among proteins, greatly undermining the central dogma as a result (Koonin, 2012). In the special case of scrapie and vCJD disease, the central dogma may have not helped, but hindered important medical research. This came down to the fact that the central dogma was such an influential guide to researchers that it was difficult for many to imagine that the disease under investigation might be caused by the transfer of information (in the form of folding) from one protein to another (Ridley, 1999). It was only in 1982 that the geneticist Stanley Prusiner was able to demonstrate that this was the case, however unlikely, making a breakthrough in research on this deadly set of diseases. A further challenge came with the discovery in the mid-1980s that proteins, specifically “molecular chaperones”, could catalyse the folding of other proteins, representing another type of direct information transfer between proteins, in this case in the form of a protein’s 3D shape. While Crick (1958) had described his idea that “protein folding is simply a function of the order of amino acids” as a “more likely hypothesis”, he did not deny that some other mechanism might also be involved. In any case, it has been found that the molecular chaperones’ function is mainly limited to avoiding “accidents” in protein folding, in some cases simply providing suitable conditions for folding, as in the case of chaperonins (Morange, 2009; Cobb, 2016). Nevertheless, researchers remain open to the possibilities that the central dogma might need to be revised to take new evidence of any additional roles played by chaperones into account. Conclusion 2018 sees the 60th year anniversary of Crick’s landmark publication “On Protein Synthesis.” As discussed in this essay, Crick’s central dogma not only has provided a prominent outline for the information flow of nucleic acids and proteins in the cell, but also has played a significant role in offering support to Neo-Darwinian evolutionary biology, encouraging biologists to think in terms of “information”, and guiding the search for cures in genetic diseases such as sickle-cell anemia. Perhaps what is even more important is the fact that the central dogma has been the centre of a 60-year dispute over the most fundamental aspects of biology. At the end of the day, what I believe is so important about the central dogma is that it is not a “dogma” at all, but rather a group of bold predictions based on the best available evidence. Because these predictions can be supported, modified, or challenged by future generations of dedicated scientists, the central dogma serves as a reminder of the spirit of daring curiosity that makes science so remarkable.

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