The process of RNA splicing involves the removal of

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

RNA splicing is a tightly orchestrated post-transcriptional modification performed by the spliceosome, a megacomplex of five small nuclear ribonucleoproteins (snRNPs: U1, U2, U4, U5, and U6) and dozens of auxiliary proteins. The spliceosome assembles de novo on each pre-mRNA transcript at specific consensus sequences: the 5′ splice site (consensus GU), the branch point adenine, the polypyrimidine tract, and the 3′ splice site (consensus AG). U1 snRNP recognizes and base-pairs with the 5′ splice site, while U2 snRNP binds the branch point sequence, inducing a conformational distortion that bulges out a specific adenine residue. This bulged adenine carries a 2′-hydroxyl group that performs a nucleophilic attack on the phosphodiester backbone at the 5′ splice site — a transesterification reaction requiring no external energy input, merely the rearrangement of covalent bonds. The result is formation of a lariat intermediate, a lasso-shaped RNA loop held by an unusual 2′–5′ phosphodiester bond at the branch point. A second transesterification then ligates the upstream and downstream exons together while releasing the excised lariat for subsequent debranching and degradation.

Why Other Options Are Wrong

The critical distinction between introns and exons rests on the directionality of spliceosomal catalysis: the spliceosome is directed to remove non-coding intervening sequences (introns) while deliberately preserving coding sequences (exons) in mature mRNA. Exon–intron boundaries are defined not solely by primary sequence but also by cooperative binding of serine/arginine-rich (SR) proteins to exonic splicing enhancers (ESEs) and heterogeneous nuclear ribonucleoproteins (hnRNPs) to exonic splicing silencers (ESSs). This combinatorial regulation determines which segments are retained versus excised, and in alternative splicing — affecting over 90% of human genes — differential splice site selection generates multiple protein isoforms from a single gene.

PILLAR 2 — STEP-BY-STEP LOGIC

The question asks directly what is removed during RNA splicing. The term "removal" refers unambiguously to the catalytic excision step in which the spliceosome cleaves at both splice sites and discards the intervening segment. By definition in molecular biology, introns (from "intragenic regions") are the sequences that are spliced out and degraded in the nucleus. Exons (from "expressed regions") are the sequences retained in the mature mRNA that will ultimately be read by ribosomes during translation. The transesterification chemistry described in Pillar 1 makes clear that the lariat structure — formed from the intron — is the product released from the spliceosome. Therefore, Option A (Introns) is correct.

Consider the fate of each molecular species: the ligated exons are exported through the nuclear pore complex to the cytoplasm as functional mRNA; the excised intron lariat is debranched by the enzyme RNA debranching enzyme (DBR1), then degraded by nuclear exonucleases such as the exosome complex. This asymmetry — one product conserved, one destroyed — is the mechanistic basis for answering this question correctly.

PILLAR 3 — DISTRACTOR ANALYSIS

Option B (Exons) is the most dangerous distractor because it represents a direct inversion of the correct terminology. Students who have memorized the terms "intron" and "exon" but failed to encode which is removed versus retained will select this option. The confusion often stems from mnemonic failure or from conflating "exiting" with "exon." In reality, exons are the retained, expressed sequences — the word derives from "expressed," not "excised."

Option C (UTRs) traps students who recognize that untranslated regions (5′ UTR and 3′ UTR) are non-coding and therefore assume they must be removed. However, UTRs flank the coding sequence within mature mRNA and remain present after splicing. The 5′ UTR contains regulatory elements such as the Kozak consensus sequence that modulates translation initiation, while the 3′ UTR harbors binding sites for regulatory miRNAs and AU-rich elements that control mRNA stability. Their removal would abolish critical post-transcriptional regulation.

Option D (Codons) reflects confusion between the levels of molecular organization — gene structure versus the genetic code. Codons are triplets of nucleotides within exons that specify amino acids during translation. They are read by tRNA anticodons in the ribosome's A site; they are not structural features of pre-mRNA that could be targeted by the spliceosome. Selecting this option indicates a fundamental conflation of RNA processing (splicing) with the translation machinery, two processes separated by both mechanism and cellular compartmentalization.