The "one gene-one enzyme" hypothesis proposed by Beadle
and Tatum in 1941 was a staple of the Central Dogma for more than 40
years. However, it is now known that the long strands of RNA found
in the nuclei of eukaryotic cells called heterogenous nuclear RNA (hnRNA)
is pre-translation mRNA which must be processed by an alternative pathway
prior to the initiation of protein translation.
Each strand of hnRNA is composed of not only usuable transcripts (exons),
but also sequences not represented in mature mRNA (introns),
demonstrated by DNA-hnRNA hybridization. This has led to the concept
of "split genes."
Introns must be excised from the mRNA transcript prior to translation.
One long strand can be processed to produce protein isoforms encoded by
the same gene.