Final answer:
Blocking the enzyme responsible for mRNA splicing results in mRNA that retains introns, which impedes proper protein synthesis and may lead to the generation of defective proteins or degradation of mRNA.
Step-by-step explanation:
If a scientist is investigating a new drug that blocks the action of the enzyme responsible for splicing mRNA molecules, the correct statement about the effectiveness of this drug would be that mRNA will lack introns. Splicing is the process that removes introns from pre-mRNA, which are regions that do not code for proteins and joins the remaining exons, which do code for proteins.
Blocking this enzyme would prevent the removal of introns, resulting in an mRNA molecule that is not properly processed and therefore defective when it comes to protein synthesis. This could lead to the production of defective proteins or the mRNA could be degraded if cellular mechanisms recognize it as aberrant.