Heterogeneous nuclear RNA (hnRNA)

Eukaryote cells synthesize hhetergenous nuclear RNA (hnRNA) which has been considered to be the precursor of mRNA. Only some hnRNA, however, serves as precursor of mRNA. Most of the hnRNA synthesized in eukaryotec cells is rapidly degraded without becoming mRNA.

Heterogeneous nuclear RNA is defined as all nuclear RNA, excluding. rRNA, having a sedimentation coefficient greater than 8S. Ribosomal RNA from mammalian cells has a high G+C content, while the G+C content of hnRNA is considerably lower. About 20-40% of hnRNA molecules are polyadenylated in mammalian cells. Their affinity for poly (A) or poly (U) can be used to separate them from rRNA, which has little or no affinity.

Heterogeneous nuclear RNA undergoes processing to become mRNA. It was previously assumed that hnRNA is cut up into segments, some of which become mRNA. Usually a poly (A) chain is added at the 3’ end (polyadenylation). According to recent suggestions, however, a looping-out mechanism takes place during processing, so that splicing as well as elimination of part of hnRNA are involved. This mechanism would permit removal of unrequired segments (loops) and would bring together the required segments of the final mRNA. Different looping behavior could bring about different types of mRNA from the same type of hnRNA.