The protein-coding genes of a eukaryote typically contain regions of DNA that serve no coding function. Noncoding regions, called introns, interrupt the coding regions, called exons.
When the gene is transcribed into RNA, both the coding and noncoding regions are copied. However, a eukaryotic cell has a mechanism for removing the introns from RNA. In a process called RNA splicing, a newly transcribed RNA molecule is cut at the intron-exon boundaries, its introns are discarded, and its exons are joined together. RNA splicing occurs within the nucleus before the RNA migrates to the cytoplasm. In the cytoplasm, ribosomes translate the RNA—now containing uninterrupted coding information—into protein.
Before an RNA can be translated into a protein, its introns must be removed. A eukaryotic cell splices out these introns soon after the RNA is transcribed. If the introns are not removed, the RNA would be translated into a nonfunctional protein.
Although introns are discarded, they do contain important sequences. The cell's splicing machinery—the small nuclear ribonucleoprotein particles, or snRNPs—bind to essential sequences, called consensus sequences, within the introns. The snRNPs use these sequences as markers to direct them to the correct splice sites.
A mutation in a consensus sequence can cause serious problems for a cell. For example, this type of mutation is the cause of one form of the genetic disease β-thalassemia, which results in severe, chronic anemia. People with β-thalassemia make defective red blood cells because they cannot properly produce β-globin, a polypeptide component of hemoglobin. In this disease, the pre-mRNA for β-globin is incorrectly spliced, and the resulting mRNA codes for a defective polypeptide.
Textbook Reference: Concept 10.2 DNA Expression Begins with Its Transcription to RNA