Crick and Watson were downcast by the news from Peter Pauling that his father appeared to have solved DNA.

Alternating between bouts of despair and denial - trying to figure out how he could have beaten them and then deciding that he certainly could not have without seeing Wilkins and Franklin's x-ray work and then thinking, well, of course, he is Pauling, so anything is possible - they continued working on the problem themselves. If they could come up with something independently before Pauling's paper appeared, at least they might share credit.

This time they added a critical observation. Crick and Watson knew Erwin Chargaff, an acerbic and opinionated Austrian-born biochemist who used chromatography to analyze the chemical composition of nucleic acids. Chargaff had told them about a simple relationship he had found between the occurrence of different bases in DNA: adenine and thymine were present in roughly the same amounts and so were guanine and cytosine. One of each pair was a larger purine; the other, a smaller pyrimidine.

Chargaff had told Pauling the same thing when they shared a shipboard dining room during an Atlantic crossing in 1947, but Pauling found Chargaff annoying and ignored his tip.

It made all the difference to Crick and Watson. Franklin's criticisms had already pointed them toward putting the phosphates on the outside of the molecule; now they had the clue of a one-to-one relationship between the bases on the inside. They began thinking about helixes in which the purines and pyrimidines lined up somehow down the core of the molecule.