Correct option is D
Explanation-
RNA interference is a gene-silencing mechanism where double-stranded RNA (dsRNA) is introduced. The dsRNA is processed into small interfering RNAs (siRNAs). These siRNAs bind to complementary sequences in the mature mRNA of the target gene. The RNA-induced silencing complex (RISC) degrades the bound mRNA → gene expression is knocked down. RNAi targets mature mRNA in the cytoplasm. Mature mRNA consists of exons (coding sequences), 5' UTR (untranslated but part of mature mRNA) and 3' UTR (untranslated but part of mature mRNA) . Introns are NOT present in mature mRNA because they are spliced out during mRNA processing inside the nucleus.
Introns exist only in pre-mRNA (the unspliced form). By the time the mRNA reaches the cytoplasm (where RNAi machinery works), introns have already been removed. If you design an RNAi construct against an intronic region there will be no matching sequence in mature mRNA.
Thus, RNAi will fail to find its target and no silencing will occur. Therefore, intronic regions are useless for RNAi constructs.
Incorrect options:
Option a - 5′ UTR of the mature transcript — used sometimes, especially if it contains regulatory elements.
Option b - 3′ UTR of the mature transcript — also sometimes used, especially in post-transcriptional regulation studies.
Option c - Exonic region — most commonly used, as it ensures transcript degradation.
Final Correct Answer: Option d — Intronic region
