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He surmised therefore that at the heart of the translation process base pairing might also apply

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He surmised therefore that, at the heart of the translation process, base pairing might also apply. This made thinking about how genes might be encoded much easier: a DNA molecule, known to be very long, could have a unique base sequence which might code in some way for the linear sequence of amino acids in proteins. A one-to-one relationship might exist, and this was formulated by Crick in 1957 as the "Sequence Hypothesis": the sequence of bases in a chain of DNA encodes the sequence of amino acids in proteins.By now Crick was thinking about the genetic code: if the amino acid sequence of a protein is coded by the base sequence in DNA, what is the exact relationship? There seemed to be 20 different amino acids to be coded by just four bases. And, in 1961, Arthur Kornberg and his colleagues at Stanford provided evidence for base pairing and that the two strands run in opposite directions (a requirement of the model).Not only did their model indicate how genetic information might be multiplied, it also provided insight into the way genetic information might be encoded: as there was no informational content in DNA, other than the sequence of the bases in each chain, genetic information could only reside in that. In essence, each double helix contains the information twice, once in each strand. Separate the two strands and each can serve as a template for a new double helix.With publication of the two papers, there was great excitement amongst the very few who saw their significance. The seismic upheaval which followed took about eight years to be noticed by the wider scientific community, including most biochemists.

But during this interval powerful evidence in support of their model came from two sources: Matthew Meselson and Frank Stahl in 1957, then in Pasadena, showed that during replication the two strands of DNA are separated and each serves as a template for a new DNA molecule. This fitted the proposed structure exactly.At the end of one of the most profound scientific papers ever written, Watson and Crick added: It has not escaped our attention that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.This was added to pre-empt anyone else pointing out the obvious, and presaged their next paper, published five weeks later in Nature, which explained how the specific base pairing could provide a mechanism for gene - and information - duplication. This pairing also provided a basis for an earlier observation made by Erwin Chargaff at Columbia: by chemical analysis of DNA from different sources he found that the proportions of the bases varied from one organism to another, yet in each there seemed to be equal numbers of As and Ts, and of Gs and Cs. The specificity of the base pairing arises from the particular hydrogen bonds which each base can make only with its partner. It consists of two DNA chains running in opposite directions and twisted around each other with base pairs in the middle - A with T, and G with C: the now familiar double helix.

Above all, they were hungry, eager to solve what they perceived to be the outstanding question of their time. And also, to deprive Linus Pauling of Pasadena, the doyen of theoretical chemistry, of that success.Their model was published in Nature in April 1953. At the time, protein crystallography seemed an impossible task and, when in 1951 James Watson joined the unit enthused about trying to find the structure of DNA, Crick was ready to change his focus.The route by which they came to their celebrated model for the structure of DNA has been extensively analysed. The BBC later made a film, Life Story (1987, based in part on The Double Helix, Watson's best-selling "personal account" of 1968, with Tim Pigott-Smith as Crick and Jeff Goldblum as Watson), which provides an excellent history of the science and personalities surrounding their discovery. Suffice it to say that their success came from trying to build a model which would satisfy the many known chemical restraints, from a knowledge of unpublished X-ray studies of DNA by Rosalind Franklin, Raymond Gosling and Maurice Wilkins in London, from Crick's insight into his own helical diffraction theory and from Watson's discovery of how the bases could interact. (RNA molecules have almost the same four bases, A, C, G and U - U and T are very closely related - and a slightly modified backbone.)While Crick started out working with Perutz on haemoglobin, his first theoretical contribution, with William Cochran and Vladimir Vand, was to calculate the X-ray pattern given by a helical molecule. The Cambridge biochemist Fred Sanger showed that insulin (a protein) had a unique linear amino acid sequence, being constructed from a standard set of 20 different kinds of amino acid.

Alexander Todd and his colleagues, in Manchester and later in Cambridge, showed that DNA and its closely related RNA (ribonucleic acid) are linear molecules made of nucleotides, of which there are just four types: the DNA units all share the same backbone, but have one or other of the four bases A, C, G or T. To appreciate what came next, one must be aware of the emptiness in our understanding of inheritance at that time.Geneticists had defined the existence of genes, abstract units of inheritance. Genes were widely believed to be made of proteins, perhaps with some nucleic acid included, although Oswald Avery and his colleagues in the United States had, by 1944, provided firm evidence that they were made of deoxyribonucleic acid (DNA). How could such information be accurately duplicated during cell division? Biochemists had discovered that the catalysts in our bodies - enzymes - are proteins, made of amino acid units in some sort of assembly. These accelerate all the chemical reactions of which life consists.

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