Final answer:
The main scientific weakness in Watson and Crick's paper on the structure of DNA was lack of experimental evidence. Their model did not incorporate direct experimental evidence or data from DNA itself.
Step-by-step explanation:
The main scientific weakness in Watson and Crick's paper on the structure of DNA was lack of experimental evidence. They proposed the double helix structure of DNA based on the interpretation of existing data, such as X-ray crystallography images taken by Rosalind Franklin. However, their model did not incorporate direct experimental evidence or data from DNA itself. This lack of empirical evidence weakened their conclusions.