Biologists have investigated how fast pre-mRNA splicing occurs by treating cells with a toxin that blocks the production of new pre-mRNAs, then following the rate of splicing of the pre-mRNAs that were transcribed before adding the toxin. Why is addition of a toxin important in this study?

It is very important to add the toxin for the blockage of new pre-mRNAs production because it would be much difficult and near to impossible to follow the speed of splicing of pre-mRNA, if new pre-mRNAs produces constantly during the study.

Addition of toxin will help the biologists to focus on the splicing rate of mRNA from start to end.