Well, it was the 70s. The "start" and "stop" codons were known. They could work out that transcription proteins would seek out those codons, produce a strip of mRNA of the DNA bases between those codons, zip it on over to the ribosome, and crank out a protein.

Then there's the rest of the genome. Vast stretches of DNA that don't have the signals needed to transcribe proteins. Why?

They didn't know. They had no idea. It would be decades before they even had a complete copy of the genome. It was years of grinding effort, of trying to work out the big picture by staring through a straw. DNA methylation, gene expression, histone stuff, the entire field of epigeneics-- non-coding DNA playing an active role in cellular operation without directly producing proteins-- was still in the future.