Eukaryotic genes – coding segments: In eukaryotic nuclear genes, which parts of the DNA sequence actually encode the amino-acid primary structure of a protein after RNA processing?

Introduction / Context:
Eukaryotic genes are typically split by introns and require post-transcriptional processing. Distinguishing exons from introns—and recognizing the roles of promoters and enhancers—is fundamental to molecular genetics and biotechnology (e.g., cDNA cloning, splicing analysis).

Given Data / Assumptions:

• Primary transcripts (pre-mRNAs) contain both exons and introns.
• RNA splicing removes introns and joins exons to form mature mRNA.
• Promoters and enhancers regulate transcription initiation but are not translated.

Concept / Approach:
Only sequences that remain in the mature mRNA and are read by ribosomes specify the amino acid sequence. These are exons. Introns are excised by the spliceosome; promoters are DNA elements required for transcription start but do not code for protein; enhancers modulate expression levels at a distance, also non-coding. Therefore, the protein’s primary structure derives from exon sequences that are in-frame and not part of UTRs if strictly counting codons, though 5′ and 3′ UTR exonic regions are transcribed but untranslated.

Step-by-Step Solution:
Identify processing steps: capping, splicing, polyadenylation. Recognize that mature mRNA = exons (including UTRs) joined together. Only coding exonic ORFs translate into amino acids. Select exons as the sequences that code for the primary structure.

Verification / Alternative check:
cDNA synthesized from mature mRNA lacks introns, matching the coding exons used by ribosomes in translation.

Why Other Options Are Wrong:
Entire gene includes non-coding regions; introns are removed; promoters/enhancers are regulatory DNA and not translated.

Common Pitfalls:
Equating “gene” with “coding”; many gene regions control expression but are not themselves translated.

Final Answer:
Exons (expressed, retained sequences).