Final answer:
Nonsense-mediated decay (NMD) is facilitated by the Exon Junction Complex (EJC) and is governed by the '-55 boundary rule', which requires premature stop codons to be over 55 nucleotides upstream of the exon-exon junction to trigger NMD. Although Rule 1 is important for normal splicing, Rule 2 is critical for activating NMD. The correct multiple-choice option is (c).
Step-by-step explanation:
Nonsense-mediated decay (NMD) is a surveillance pathway that detects and degrades mRNAs containing premature stop codons (nonsense codons), which can arise from mutations or errors in splicing. The presence of an Exon Junction Complex (EJC) downstream of a nonsense codon is a signal for NMD. According to Rule 2, also known as the "-55 boundary rule", only premature termination codons that are located more than 55 nucleotides upstream of an exon-exon junction can trigger NMD. This rule underscores the importance of the positioning of the EJC relative to the premature termination codon. However, the specific distance of 20-24 nucleotides in Rule 1 is less about NMD and relates more to the normal positioning of the EJC during splicing, which is typically 20-24 nucleotides upstream of the exon-exon junction.
The concept of mRNA processing is critical, which involves the removal of introns and the connection of exons. NMD is one of the mechanisms that ensures quality control of mRNA before translation. For NMD to be triggered, Rule 2 must be satisfied, meaning the premature stop codon must be sufficiently upstream of the final exon-exon junction so the EJC can signal NMD, leading to the mRNA's degradation and preventing the synthesis of potentially harmful truncated proteins. The correct multiple-choice option is (c).