How did Watson and crick determine the three demensional shape of DNA?

Answer 1

When Francis Crick began his research, it was still unknown WHERE and HOW genetic codes for life were kept. In other words, nobody knew how a fertilized human egg "knew" to grow into a human while a fertilized rose seed "knew" to grow into a rose -- the location of this genetic information in terrestrial life was speculative at best. Some thought it was in the proteins within cells, others thought it might be in a cell's DNA -- a chemical found in all living cells, but for which nobody knew its function.

Linus Pauling of Caltech (along with other co-workers) analyzed x-ray diffraction patterns of proteins, and was able to surmise that each had a helical structure. He was thus on his way to discover the structure of DNA, and might have done so first except (1) he lacked high quality x-ray patterns and (2) he began with a mistaken idea that it was a triple helix.

Both he and Francis Crick surmised that any genetic code needed to be stable over many years, and thus the structure of this code had to involve very strong chemical bonds. They also both knew that DNA consisted of four amino acids, and that two of these were always found to be equal in volume to two others. The question then became HOW these amino acids combined.

When Crick and Watson observed the high quality "Photo 51," they observed a helical pattern for DNA, and were even able to make a rough estimate of its size. This was despite the fact that Dr. Franklin vehemently disagreed with their analysis.

The two of them made a model of DNA based on their hunches, but this model turned out to be embarassingly wrong -- something Franklin was able to note in a matter of minutes. They had placed molecules on the outside of DNA that, if the molecules had come in contact with water, the DNA structure would have collapsed.

Changing the basic structure so that these molecules were on the inside, C & W were able to make a model that fit all the requirements noted above. A few months later they speculated that the genetic code for all Earth life was "recorded" in the structure of DNA. Within a few years, they were shown to be correct.