Final answer:
When mRNA leaves the nucleus and enters the cytoplasm, it only has a complete set of exons, which are the regions that code for proteins. The introns, which do not encode functional proteins, are removed during splicing.
Step-by-step explanation:
When mRNA leaves the nucleus and enters the cytoplasm, it undergoes a process called splicing, in which the introns are removed and the exons are joined together. Therefore, when mRNA enters the cytoplasm, it only has a complete set of exons, which are the regions that code for proteins. The introns, which do not encode functional proteins, are removed during splicing while the pre-mRNA is still in the nucleus.