Which molecule serves as a primary transcript in eukaryotes?

The molecule that serves as a primary transcript in eukaryotes is heterogeneous nuclear RNA (hnRNA). During the process of transcription in eukaryotic cells, the DNA sequence is transcribed into a precursor RNA molecule, which is known as hnRNA. This primary transcript includes both exons (the coding sequences) and introns (the non-coding sequences) before being processed.

Heterogeneous nuclear RNA undergoes several modifications before it becomes mature mRNA, which includes capping, polyadenylation, and splicing to remove introns. This processing is necessary for hnRNA to be converted into functional messenger RNA (mRNA), which is then translated into proteins.

In contrast, the other molecules mentioned, such as ribosomal RNA (rRNA), messenger RNA, and microRNA, have different roles. Ribosomal RNA is a component of ribosomes, messenger RNA is the processed form that carries the genetic information for protein synthesis, and microRNA is involved in the regulation of gene expression. Thus, among these choices, hnRNA is uniquely identified as the primary transcript before processing occurs.