What is heterogeneous nuclear RNA primarily composed of?

Heterogeneous nuclear RNA (hnRNA) is primarily composed of both introns and exons. This form of RNA is found in the nucleus of eukaryotic cells and represents the initial transcripts synthesized from DNA before they undergo processing. During this processing, introns—which are non-coding sequences—are removed, and exons— the coding sequences—are spliced together to form mature messenger RNA (mRNA).

The presence of introns and exons in hnRNA is critical because it reflects the early stage of gene expression where the full RNA product is being prepared for translation. Exons will ultimately be retained in the final mRNA, while introns are excised. Therefore, understanding this composition is essential in molecular biology to comprehend the mechanisms of gene regulation and expression.

In contrast, the other options either suggest a composition of only one type of sequence (either exons or introns) or imply that hnRNA consists solely of mature mRNA, which disregards the introns present in the primary transcript prior to its maturation.