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Central Dogma3 Conversation on Central Dogma Page 1 of 11 A Conversation about Central Dogma sequences code for proteins of differing of Molecular Biology sequences. Student: OK. Now, what does the Central Dogma Student: What is the Central Dogma of Molecular state? Biology? Teacher: Again, in Crick’s words, the Central Teacher: The Central Dogma of Molecular Dogma states that “ that once ‘information’ has Biology was proposed by Sir Francis Crick in passed into protein it cannot get out again .” The paper he published in 1958 [1]. In this paper, Crick ‘information’ in a nucleic acid is its sequence of discussed a theoretical framework for the nucleotides. The ‘information’ in a protein is its mechanisms leading to protein synthesis; for this sequence of amino acids. This statement by Crick Crick presented two general principles, which he means that a sequence of amino acids cannot be termed the Sequence Hypothesis and the Central used to provide instruction for synthesis of either a Dogma, though there was scant experimental nucleic acid or a protein. Crick elaborated on this evidence for either. point as follows “the transfer of information from nucleic acid to nucleic acid, or from nucleic acid Student: OK. So what does the Sequence to protein may be possible, but transfer from Hypothesis state? protein to protein, or from protein to nucleic acid is impossible .” Teacher: In Crick’s own words, the sequence hypothesis “ assumes that the specificity of a piece Student: This sounds clear enough. But I of nucleic acid is expressed solely by the sequence 2 remember reading on Wikipedia that the Central of its bases, and that this sequence is a simple Dogma relates to the ‘residue by residue transfer code for the amino acid sequence of a particular of sequential information ’, which sounded quite protein .” complicated. Can you explain this in simpler terms? Also, is this different from what Crick had Student: Can you explain it in simpler words? said? Teacher: There are two parts to this statement. Teacher: Let us first consider the phrase ‘residue One: the unique identity, or specificity, of a by residue ’. Residues refer to monomeric units in segment of nucleic acids it its sequence. For a polymeric molecule. The monomeric units are example, a dinucleotide AT is different from a deoxynucleotides in DNA, ribonucleotides in dinucleotide TA (remember that the sequence of RNA, and amino acids in proteins. nucleic acids is written in the 5’ 3’ direction). Two: the sequence of a nucleic acid forms the Now consider ‘transfer of sequential information ’. instruction for a sequence of a protein. At the time We know that in this context, ‘information’ is the when Crick made this statement, rules governing sequence of nucleotides or amino acids; to clarify the correspondence between nucleotides in the this, Wikipedia has termed it as “ sequential nucleic acid and amino acids in the protein were information ”. I see how the word ‘ sequential ’ can not known, but this statement provided a stimulus be confusing, since it might suggest a certain order to uncover these rules – this correspondence (sequence) of events rather than the actual between nucleotides and amino acids is now called composition (sequence) of the polymer. the Genetic Code. Combining the two parts of this “Sequence-based information” might be a clearer statement, we see that the sequence hypothesis term in this context. states that DNA/RNA segments of differing Now comes the most interesting term – “transfer ”. It means that the information (sequence) in one 1 Crick, F.H.C. (1958): On Protein Synthesis. Symp. Soc. Exp. Biol. XII, 139-163. 2http://en.wikipedia.org/wiki/Central_dogma_of_molecular_ biology Conversation on Central Dogma Page 2 of 11 molecule can be copied to create another molecule First, the purpose of replication is to produce an with the same information (sequence). In other identical copy of a molecule. Therefore, each words, one molecule (‘mother’) acts as a template residue in one parent molecule is copied to or guide for the synthesis of another molecule produce one daughter molecule of the same type. (‘daughter’) with the same information. This Replication is complete when there are two property of reproduction – creation of units of identical molecules – that is, molecules with the similar type - is characteristic of living beings. same sequence information. Student: So, how does the ‘ residue by residue On the other hand, the purpose of gene replication transfer of sequential information ’ occur? is to meet the immediate needs of a cell. Hence only that segment of a molecule which carries Teacher: Two types of activities are central to information relevant for a particular requirement is living beings: reproduction and survival. copied. Instructions are required for both these types of activities, and hence there is a requirement for two Second, only one complete copy of a molecule is types of ‘transfer of information ’. The transfer made at the end of a replication cycle. However, required for reproduction is called replication (also during gene expression, the need might be for called ‘vertical transfer’), wherein information is some abundant substance, so multiple copies of a transferred from a parent generation to a progeny particular segment of a molecule may be made. In generation. The transfer required for survival can technical terms, we can say that the stoichiometry be called ‘gene expression’, wherein information or ratio of template: product is 1:1 in replication present within a cell is used within the same cell but is often 1:many during gene expression. for carrying out various processes such as respiration that are necessary to meet its Finally, the end-product of replication is a immediate requirements. molecule of the same type as the template molecule, though there might be intermediate Student: I understand replication. But what is a molecules of different types. For example, the “gene” and how is it “expressed”? end-product of replication of DNA is DNA, thought it might occur via an RNA intermediate. Teacher: It is not easy to define a gene since new But the end-product of gene replication is often a discoveries are continually changing our concept molecule of a type different from the template. In of a gene [ 3,4]. For the purpose of our discussion of most cells, the template for gene expression is the Central Dogma of Molecular Biology, we can DNA, but the end-product is protein. consider a gene to comprise one or more segments of one or more molecules that need to be copied to Student: OK. Can you tell me now about transfer carry out a particular function. Continuing with of sequence-based information during replication? this simple definition of a gene, we can say that gene expression comprises processes involving Teacher: In all cellular organisms, the genetic transfer of sequence-based information present in material is double strand DNA, that is, ds DNA a gene. the molecule that determines the identity of a cell. For replication, one molecule of DNA has to be Student: Are there differences between transfer of copied to make two identical molecules of DNA. information in replication and gene expression? As you know, the two polynucleotide strands Teacher: Yes, there are three main differences in comprising a molecule of ds DNA are terms of transfer of information. complementary to each other. This means that an A residue on one strand is paired with a T residue on the other strand, and a G residue on one strand is paired with a C residue on the other strand. The 3 Pearson H (2006) Nature 441: 399-401 simplest mechanism to achieve replication is as 4 Gerstein et al. (2007) Genome Res . 17: 669-681 follows: the two strands comprising a ds DNA Conversation on Central Dogma Page 3 of 11 molecule are separated, and each strand acts as a template for the synthesis of the complementary strand. Synthesis of the complementary strand is done using the Watson-Crick base pairing: A pairs with T, and G pairs with C. In this way, two identical molecules of ds DNA are produced from one molecule of ds DNA. Some viruses (such as M13 and phiX174) have a single stranded DNA genome. To replicate a ss Student: Why are these steps called ‘transcription’ DNA genome, the DNA is first copied using and ‘reverse transcription’? complementary base pairing to produce a complementary strand, which is then copied to Teacher: Well, the four building blocks for DNA produce copies of genomic DNA. are dATP, dGTP, dCTP, and dTTP, whereas the four building blocks for RNA are rATP, rGTP, Replication is catalyzed rCTP, and rUTP. Historically, the use DNA as a by an enzyme called template to synthesize RNA was observed first; DNA polymerase this process was termed transcription to denote (DNAP). The transfer of that it involved a change of ‘script’ of the information during information from dNTPs to rNTPs. replication of DNA can be depicted as shown in Later, when the use of RNA as a template to figure 1. synthesize DNA was observed, the process was termed ‘reverse transcription’ to note that Student: This looks identities of template and intermediate molecules simple. Are there other in transcription was reversed in this process, that routes for transfer of sequence-based information is, the template was RNA and the intermediate for the purpose of replication? was DNA. Teacher: Yes, there is another route for replication Student: OK. So life forms with DNA as their of DNA genomes, as seen in cauliflower mosaic genetic material transfer sequence-based virus (a plant virus) and hepatitis B virus (an information during replication either by DNA animal viruses), where the DNA genome DNA or by DNA RNA DNA. replicates by a circuitous route.
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