Final answer:
The one gene-one polypeptide hypothesis had to be modified because it was found that enzymes can be composed of multiple polypeptides, and a single gene can produce various polypeptides through alternative splicing.
Step-by-step explanation:
The one gene-one polypeptide hypothesis has had to be modified because it was discovered that enzymes sometimes consist of more than one polypeptide, each coded for by its own gene. In addition, through alternative splicing, a single gene can give rise to multiple related polypeptides. The discoveries concerning mRNA processing, such as intron removal and exon junction, showed that a single gene can code for multiple proteins, depending on how the mRNA is spliced. Furthermore, not all genes code for polypeptides; some genes are responsible for producing tRNA and rRNA molecules that play crucial roles in protein synthesis.