 On the basis of her pictures, Rosalind Franklin discovered the full structure of the virus in 1955.In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified tobacco mosaic virus RNA and its coat protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the means through which viruses were created within their host cells.By 1935, many plant diseases were thought to be caused by viruses.