All we had to work on were certain fragmentary experimental results, themselves often rather uncertain and confused, and a boundless optimism that the basic concepts involved were rather simple and probably much the same in living things. In such a situation well constructed theories can play a really useful part in stating problems clearly and thus guiding experiment.
With roughly that desire in mind, Crick famously argued in 1958 that the 9 possible transformations could be divided into three groups.
The first group consisted of those for which there was positive evidence. At the time, this was limited to:
- DNA → DNA
- DNA → RNA
- RNA → Protein
- RNA → RNA
- RNA → DNA
- DNA → Protein
The last group were felt, on theoretical reasons (rather than on lack of experimental support), to be extremely unlikely to occur. They were:
- Protein → Protein
- Protein → RNA
- Protein → DNA
As you can see, Crick was very skeptical of the idea that information could be transferred from a protein to anything else. (Briefly, he thought that the complicated machinery for translating the DNA/RNA 'alphabet' into a completely different amino acid 'alphabet' couldn't work backwards, and there was no evidence of any alternative complicated machinery to perform this back-translation.) Proteins were thus dead ends for information, and this insight constituted his 'Central Dogma of Molecular Biology': "once (sequential) information has passed into protein it cannot get out again."
Just as Crick had hoped, this insight proved to be both valid and enormously useful. Incidentally, the first of the second group's constituents has now found experimental support, largely in the form of the retroviruses (like HIV), which initially use their own RNA to back-translate their information into DNA. Will we one day discover a "DNA → Protein" transformation?
Crick wrote an extremely accessible defense of his Central Dogma in the journal Nature in 1970. You can access the PDF by clicking here; I strongly recommend it for its wonderful clarity and its knack of hitting nails on heads. It's superb.