After his return to the United States in the summer of 1953, Watson resumed his investigation of Tobacco Mosaic Virus, the simple, stable, highly infectious virus that became the standard experimental organism for the new science of virology in the 1880s. He was interested in the rod-like shape of TMV, whose regular protein subunits Watson in 1952 (while at Cambridge) had shown to be arranged in a helix. Drawing on the work of Rosalind Franklin (who herself had shifted from DNA to TMV in 1953), Watson suspected that the virus' hollow core was ringed by strands of RNA (as his drawing illustrated).. In his letter Watson reported that he and Don Caspar had determined the molecular weight of TMV RNA to be 240,000 (i.e. 240,000 times the weight of an atom of carbon 12). Molecular weight is a fundamental measure in physical chemistry. Watson also mentioned other developments in his laboratory at the California Institute of Technology, such as X-ray images of crystals of Southern Bean Mosaic virus, like TMV a plant virus but spherical in shape (as mentioned in the letter), as well as the acquisition of an advanced Spinco centrifuge.. Most interesting, Watson expressed his reaction to Crick's RNA Tie Club note "On Degenerate Templates and the Adaptor Hypothesis," which ended on a discouraging note regarding the complexities of the genetic code. Watson insisted that "[w]e must find RNA structure before we give up and return to viscosity and birdwatching," their respective fields when they first involved themselves in scientific study.
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