Final answer:
The excised piece of pre-mRNA is known as an intron, which is removed and then typically degraded during the splicing process by spliceosomes within the nucleus. The remaining exons are rejoined to produce the mature mRNA that is translated into protein.
Step-by-step explanation:
The excised piece of pre-mRNA is called an intron. After a pre-mRNA molecule is transcribed from DNA, it contains both exons, which encode for proteins, and introns, which do not. The process of removing the introns is known as splicing. During splicing, spliceosomes target the introns to be removed from the mRNA sequence. Once the introns are cut out, they are typically degraded within the nucleus. The remaining exons, which are segments that will be expressed as proteins, are rejoined to form the mature mRNA sequence that exits the nucleus to be translated into proteins. If splicing is not carried out properly and introns are not completely and precisely removed, the reading frame may be altered, resulting in a nonfunctional protein.