Final answer:
Nuclear splice sites are specific GU at the 5' end and AG at the 3' end sequences in pre-mRNA that are essential for the splicing process, which involves the precise removal of introns by the spliceosome and the rejoining of exons to generate a functional mRNA for protein synthesis.
Step-by-step explanation:
Nuclear Splice Sites in Pre-mRNA Processing
Within eukaryotic genes, nuclear splice sites are short sequences crucial for removing non-coding sections known as introns from pre-messenger RNA (pre-mRNA). The splicing mechanism is highly precise, requiring perfect accuracy to avoid a shift in the codon reading frame which would lead to the production of dysfunctional proteins. In particular, at the 5' and 3' ends of the introns, specific nucleotide sequences are present: GU (at the 5' splice site) and AG (at the 3' splice site), which are hallmark features of GU-AG introns. These sequences are recognized by the spliceosome, a complex composed of proteins and RNA, which facilitates the splicing process, excising introns and rejoining the remaining sequences called exons, which actually code for proteins.
Spliceosomal introns form a unique structure, and during splicing, they can form a 'lariat' structure at an A residue branch site, a configuration essential to the splicing process. Introns vary in length and frequency across different genes and species, with some genes containing multiple introns that can account for a significant portion of the gene's length. The precise removal of these sequences is crucial for generating a functional mRNA strand that will correctly translate into a protein.