Final answer:
An mRNA with an unspliced intron is retained in the nucleus, where it is usually degraded or involved in gene regulation. Spliceosomes meticulously remove introns and join exons to ensure accurate protein synthesis. Additionally, pre-mRNA undergoes 5' capping and addition of a poly-A tail which are essential for its stability.
Step-by-step explanation:
Retention of mRNA with Unspliced Introns
An mRNA with an unspliced intron is retained within the nucleus of a eukaryotic cell. The processing of pre-mRNA involves several critical steps, including splicing, which is the removal of non-coding sequences known as introns from the pre-mRNA transcript. The segments that code for proteins, called exons, are then rejoined to form a mature mRNA molecule. This mature mRNA molecule later exits the nucleus to be translated into a protein in the cytoplasm.
During the splicing process, complexes known as spliceosomes meticulously remove introns and join exons together with the accuracy and precision of a single nucleotide. If an intron is incorrectly or incompletely spliced, it can lead to the production of a dysfunctional protein, as it disrupts the correct reading frame of the exons. This precise sequence-specific mechanism of splicing ensures that the mature mRNA will be properly formatted for protein synthesis. Moreover, intron retention typically occurs when these mechanisms fail or are intentionally part of a gene regulation strategy, where the transcript may be retained and eventually degraded within the nucleus, potentially regulating the amount of protein produced.
Each pre-mRNA can contain more than 70 individual introns that must be spliced out. In addition to splicing, pre-mRNA also undergoes 5' capping and the addition of a poly-A tail at the 3' end, which are crucial modifications for the stability and export of the mature mRNA from the nucleus.