Final answer:
Introns are the non-coding regions of mRNA that are removed through splicing before the mature mRNA is sent to the ribosome for protein synthesis.
Step-by-step explanation:
The parts of the messenger RNA (mRNA) that are removed and not sent to the ribosome are known as introns. During mRNA processing in eukaryotic cells, these non-coding regions are excised through a process called splicing. Spliceosomes play a crucial role in this process, ensuring that introns are precisely removed, and the remaining coding regions, called exons, are rejoined to form the mature mRNA. This mature mRNA, which now only contains sequences that code for proteins, is then exported from the nucleus to the ribosome for translation into a protein.