Final answer:
Introns are non-coding regions of an RNA molecule that are removed from the pre-mRNA through splicing before it leaves the nucleus. The remaining coding regions, called exons, are spliced together to form the mature mRNA, which is then translated into protein.
Step-by-step explanation:
Portions of an RNA molecule that do not code for a protein and are removed before the RNA leaves the nucleus are known as introns. In the process of RNA splicing, introns are excised from the pre-mRNA, which is the RNA transcript before it is processed for translation. The remaining sequences, called exons, are then joined together to form the mature mRNA molecule that can leave the nucleus for protein synthesis. This editing process ensures that only the portions of the RNA molecule that should be translated into protein are expressed.
The excision of introns and the linking of exons is carried out by a complex known as the spliceosome. This process not only removes non-coding sequences but can also lead to the generation of multiple protein variants through a mechanism known as alternative splicing. The highly regulated editing of pre-mRNA is essential for the diversity of proteins that can be produced in eukaryotic cells.