Final answer:
The process of modifying an mRNA molecule after it has been transcribed is known as post-transcriptional modification, which is essential for the proper expression and control of genes.
Step-by-step explanation:
The process by which the nucleotide sequence of an mRNA molecule is changed after it has been transcribed is called post-transcriptional modification. This enzymatic process includes various steps, such as RNA splicing, where introns are removed and exons are joined to produce a continuous coding sequence. The mature mRNA then exits the nucleus to be translated into a protein during the process of translation. The post-transcriptional modification plays a critical role in gene expression and control.