Watson-Crick hypothesis

Today s understanding of information pathways has arisen from the convergence of genetics, physics, and chemistry in modern biochemistry. This was epitomized by the discovery of the double-helical structure of DNA, postulated by James Watson and Francis Crick in 1953 (see Fig. 8-15). Genetic theory contributed the concept of coding by genes. Physics permitted the determination of molecular structure by x-ray diffraction analysis. Chemistry revealed the composition of DNA. The profound impact of the Watson-Crick hypothesis arose from its ability to account for a wide range of observations derived from studies in these diverse disciplines. 

Soon after the Watson—Crick hypothesis was published, scientists began to extend it to yield what Crick called the central dogma of molecular genetics. This dogma stated that genetic information flows as follows ... 

The Watson and Crick hypothesis for DNA replication proposed that each strand of DNA is used as a template for the production of one of the daughter DNA molecules. Thus the result of replication would he that one strand of DNA is present in each daughter molecule of DNA. This is a semi-conservative mechanism of replication. A simplified diagram of replication is shown in Fig. S.AIO however, the replication patterns are different in bacteria and in eukaryotes. 

The 3 terminal redundancy of the genetic code and its mechanistic basis were first appreciated by Francis Crick in 1966. He proposed that codons and anticodons interact in an antiparallel manner on the ribosome in such a way as to require strict Watson-Crick pairing (that is, A-U and G-C) in the first two positions of the codon but to allow other pairings in its 3 terminal position. Nonstandard base pairing between the 3 terminal position of the codon and the 5 terminal position of the anticodon alters the geometry between the paired bases Crick s proposal, labeled the wobble hypothesis, is now viewed as correctly describing the codon-anticodon interactions that underlie the translation of the genetic code. 

What are the rules that govern the recognition of a codon by the anticodon of a tRNA A simple hypothesis is that each of the bases of the codon forms a Watson-Crick type of base pair with a complementary base on the anticodon. The codon and anticodon would then be lined up in an antiparallel fashion. In the diagram in the margin, the prime denotes... 

As discussed in the accompanying article [97], (poly)nucleotides, especially those building Watson-Crick pairs, are uniquely photostable (see also [33,120,121]). The Zn world concept explains this unique photostability by the role of the UV light not only as an energy source, but also as a selective factor during the first evolutionary steps. The unique photostability of (poly)nucleotides finds no explanation in any other hypothesis on the origin of life. 

Structural studies as well as sequence comparisons among polymerases strongly suggest the hypothesis that the phosphoryl transfer reaction of all polymerases is catalyzed by a two metal ion mechanism that was originally proposed by analogy to the well studied two metal mechanism in the 3 exonuclease reaction (14). It is perhaps of interest to note that such a mechanism, which involves only the properties of two correctly positioned divalent metal ions, could easily be used by an enzyme made entirely of RNA and thus could function in an all RNA world. The fidelity of DNA synthesis appears to arise from two sources. First, enforced Watson-Crick interactions at the polymerase active site increases the accuracy of the incorporation step (9,13). Second, there is a competitive editing at the 3 exonuclease active site that removes misincorporated nucleotide (3,5). When nucleotides are... 

Watson and Crick proposed the hypothesis of semiconservative replication soon after publication of their 1953 paper on the structure of DNA, and the hypothesis was proved by ingeniously designed experiments carried out by Matthew Meselson and Franklin Stahl in... 

An even more remarkable prediction was made by Crick and Watson (1953) of the structure of the vast molecules, such as that of deoxyribonucleic acid (DNA), which are concerned with the maintenance and transference of genetic information. These molecules contain very long chains of nucleotide units linked by covalency bonds. (A nucleotide consists of the residue of a sugar, often ribose or deoxyribose, one of a purine or pyrimidine base, and one of phosphoric acid, bonded together.) The Crick-Watson hypothesis was that the macromolecule consists of two such chains,... 

Inspection of a codon table shows that in most instances of multiple codons for a single amino acid, the variation occurs in the third base of the codon (see Table 15.1). Crick noted that the pairing between the 3 -base of the codon and the 5 -base of the anticodon does not always follow the strict base-pairing rules that he and Watson had previously discovered (i.e., A pairs with U, and G with C). This observation resulted in the wobble hypothesis. 

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