What’s going on? The Central Dogma sounds like the backbone of an entire discipline. If it’s really a “dogma” how come it gets refuted on a regular basis? If it’s really so “central” to the field of molecular biology then why hasn’t the field collapsed?
In order to answer these questions we need to understand what the Central Dogma actually means. It was first proposed by Francis Crick in a talk given in 1957 and published in1958 (Crick, 1958). In the original paper he described all possible directions of information flow between DNA, RNA, and protein. Crick concluded that once information was transferred from nucleic acid (DNA or RNA) to protein it could not flow back to nucleic acids. In other words, the final step in the flow of information from nucleic acids to proteins is irreversible.
Fig. 1. Information flow and the sequence hypothesis. These diagrams of potential information flow were used by Crick (1958) to illustrate all possible transfers of information (left) and those that are permitted (right). The sequence hypothesis refers to the idea that information encoded in the sequence of nucleotides specifies the sequence of amino acids in the protein.Crick restated the Central Dogma of Molecular Biology in a famous paper published in 1970 at a time when the premature slaying of the Central Dogma by reverse transcriptase was being announced (Crick, 1970). According to Crick, the correct, concise version of the Central Dogma is ...
... once (sequential) information has passed into protein it cannot get out again (F.H.C. Crick, 1958)
The central dogma of molecular biology deals with the detailed residue-by-residue transfer of sequential information. It states that such information cannot be transferred from protein to either protein or nucleic acid. (F.H.C. Crick, 1970)
Unfortunately, there’s second version of the Central Dogma that’s very popular even though it’s historically incorrect. This version is the simplistic DNA → RNA → protein pathway that was published by Jim Watson in the first edition of The Molecular Biology of the Gene (Watson, 1965). Watson’s version differs from Crick’s because Watson describes the two-step (DNA → RNA and RNA → protein) pathway as the Central Dogma. It has long been known that these conflicting versions have caused confusion among students and scientists (Darden and Tabery, 2005; Thieffry, 1998). I argue that as teachers we should teach the correct version, or, at the very least, acknowledge that there are conflicting versions of the Central Dogma of Molecular Biology.Announcing the (Premature) Death of the Central Dogma
The central dogma of biology holds that genetic information normally flows from DNA to RNA to protein. As a consequence it has been generally assumed that genes generally code for proteins, and that proteins fulfil not only most structural and catalytic but also most regulatory functions, in all cells, from microbes to mammals. However, the latter may not be the case in complex organisms. A number of startling observations about the extent of non-protein coding RNA (ncRNA) transcription in the higher eukaryotes and the range of genetic and epigenetic phenomena that are RNA-directed suggests that the traditional view of genetic regulatory systems in animals and plants may be incorrect.
Mattick, J.S. (2003) Challenging the dogma: the hidden layer of non-protein-coding RNAs in complex organisms. BioEssays 25:930-939.
The central dogma, DNA makes RNA makes protein, has long been a staple of biology textbooks.... Technologies based on textbook biology will continue to generate opportunities in bioinformatics. However, more exciting prospects may come from new discoveries that extend or even violate the central dogma. Consider developmental biology. The central dogma says nothing about the differences between the cells in a human body, as each one has the same DNA. However, recent findings have begun to shed light on how these differences arise and are maintained, and the biochemical rules that govern these differences are only being worked out now. The emerging understanding of developmental inheritance follows a series of fundamental discoveries that have led to a realization that there is more to life than the central dogma.
Henikoff, S. (2002) Beyond the central dogma. Bioinformatics 18:223-225.
It will take years, perhaps decades, to construct a detailed theory that explains how DNA, RNA and the epigenetic machinery all fit into an interlocking, self- regulating system. But there is no longer any doubt that a new theory is needed to replace the central dogma that has been the foundation of molecular genetics and biotechnology since the 1950s.
The central dogma, as usually stated, is quite simple: DNA makes RNA, RNA makes protein, and proteins do almost all of the work of biology.
Gibbs. W.W. (2003) The unseen genome: gems among the junk. Sci. Am. 289:26-33.
The pathway version of the Central Dogma is the one that continues to get all the attention. It’s the version that is copied by almost all textbooks of biochemistry and molecular biology. For example, the 2004 edition of the Voet & Voet biochemistry textbook says,
In 1958, Crick neatly encapsulated the broad outlines of this process in a flow scheme he called the central dogma of molecular biology: DNA directs its own replication and its transcription to yield RNA, which, in turn, directs its translation to form proteins. (Voet and Voet, 2004)If the Watson pathway version of the Central Dogma really was the one true version then it would have been discarded or modified long ago. In his original description, Watson drew single arrows from DNA to RNA and from RNA to protein and stated ....
The arrow encircling DNA signifies that it is the template for its self-replication; the arrow between DNA and RNA indicates that all cellular RNA molecules are made on DNA templates. Most importantly, both these latter arrows are unidirectional, that is, RNA sequences are never copied on protein templates; likewise, RNA never acts as a template for DNA.
Fig. 2. Watson’s version of the Central Dogma. This figure is taken from the first edition of The Molecular Biology of the Gene (p. 298).Watson's statement is clearly untrue, as the discovery of reverse transcriptase demonstrated only a few years after his book was published. Furthermore, there are now dozens of examples of information flow pathways that are more complex than the simple scheme shown in Watson’s 1965 book. (Not to mention the fact that many information flow pathways terminate with functional RNA’s and never produce protein.)
Watson’s version of the Central Dogma is the one scientists most often refer to when they claim that the Central Dogma is dead. The reason it refuses to die is because it is not the correct Central Dogma. The correct version has not been refuted.
Crick was well aware of the difference between his (correct) version and the Watson version. In his original 1958 paper, Crick referred to the standard information flow pathway as the sequence hypothesis. In his 1970 paper he listed several common misunderstandings of the Central Dogma including ....
It is not the same, as is commonly assumed, as the sequence hypothesis, which was clearly distinguished from it in the same article (Crick, 1958). In particular, the sequence hypothesis was a positive statement, saying that the (overall) transfer nucleic acid → protein did exist, whereas the central dogma was a negative statement saying that transfers from protein did not exist.
So, how do we explain the current state of the Central Dogma? The Watson version is the one presented in almost every textbook, even though it is not the correct version according to Francis Crick. The Watson version has become the favorite whipping boy of any scientist who lays claim to a revolutionary discovery, even though a tiny bit of research would uncover the real meaning of the Central Dogma of Molecular Biology. The Watson version has been repeatedly refuted or shown to be incomplete, and yet it continues to be promoted as the true Central Dogma. This is very strange.The Sequence Hypothesis and the Central Dogma in 1957
My own thinking (and that of many of my colleagues) is based on two general principles, which I shall call the Sequence Hypothesis and the Central Dogma. The direct evidence for both of them is negligible, but I have found them to be of great help in getting to grips with these very complex problems. I present them here in the hope that others can make similar use of them. Their speculative nature is emphasized by their names. It is an instructive exercise to attempt to build a useful theory without using them. One generally ends in the wilderness.
The Sequence Hypothesis. This has already been referred to a number of times. In its simplest form it assumes that the specificity of a piece of nucleic acid is expressed solely by the sequence of its bases, and that this sequence is a (simple) code for the amino acid sequence of a particular protein.
This hypothesis appears to be rather widely held. Its virtue is that it unites several remarkable pairs of generalizations: the central biochemical importance of proteins and the dominating role of genes, and in particular of their nucleic acid; the linearity of protein molecules (considered covalently) and the genetic linearity within the functional gene, as shown by the work of Benzer and Pontecorvo; the simplicity of the composition of protein molecules and the simplicity of nucleic acids. Work is actively proceeding in several laboratories, including our own, in an attempt to provide more direct evidence for this hypothesis.
The Central Dogma. This states that once “information” has passed into protein it cannot get out again. In more detail, the transfer of information from nucleic acid to nucleic acid, or from nucleic acid to protein may be possible, but transfer from protein to protein, or from protein to nucleic acid is impossible. Information means here the precise determination of sequence, either of bases in the nucleic acid or of amino acid residues in the protein.
Crick, F.H.C. (1958) On protein synthesis. Symp. Soc. Exp. Biol. XII:138-163 quoted in Judson, H.F. The Eight Day of Creation, Expanded Edition (1979, 1996) p. 332.
The Crick version is correct—it has never been seriously challenged—but few textbooks refer to it. One exception is Lewin’s GENES VIII (Lewin, 2004) (and earlier editions). Lewin defines the Central Dogma of Molecular Biology as,
The central dogma states that information in nucleic acid can be perpetuated or transferred but the transfer of information into protein is irreversible. (B. Lewin, 2004)I recommend that all biochemistry and molecular biology teachers adopt this definition—or something very similar—and teach it in their classrooms.
Crick, F.H.C. (1958) On protein synthesis. Symp. Soc. Exp. Biol. XII:138-163
Crick, F. (1970) Central Dogma of Molecular Biology. Nature 227, 561-563. [PDF file]
Darden, L. and Tabery, J. (2005) Molecular Biology
Lewin, B. (2004) GENES VIII Pearson/Prentice Hall
Mattick, J.S. (2003) Challenging the dogma: the hidden layer of non-protein-coding RNAs in complex organisms. BioEssays 25:930-939
Mattick, J.S. (2004) The hidden genetic program of complex organisms. Sci. Am. 291:60-67.
Thieffry, D. (1998) Forty years under the central dogma. Trends Biochem. 23:312-316.
Watson, J.D. (1965) The Molecular Biology of the Gene. W.A. Benjamin. Inc. New York
No comments:
Post a Comment