As part of an early experiment on the secretory system, an in vitro translation system consisting only of mRNA and ribosomes resulted in secretory proteins that were larger than the identical protein when translated in a cell. Why?

Alternative splicing of secretory protein mRNAs requires additional proteins not included in the in vitro system.

Explanation:

Alternative splicing is a cellular process that produces messenger RNAs (mRNAs) with different sizes and therefore proteins with different lengths. This cellular process requires the presence of specific cofactors that enables to remove introns and select exons in the precursor mRNA (pre-mRNA)