Precursor mRNA

Overview
Precursor mRNA, more correctly termed heterogeneous nuclear RNA (hnRNA), is an immature single strand of messenger ribonucleic acid (mRNA). HnRNA is synthesized from a DNA template in the cell nucleus by a process called transcription.

Once hnRNA has been completely processed, it is termed "mature messenger RNA", "mature mRNA", or simply "mRNA".

Processing
Eukaryotic hnRNA exists only briefly before it is fully processed into mRNA. HnRNAs include two different types of segments, exons and introns. Exons are segments that are retained in the final mRNA, while introns are removed in a process called splicing, which is performed by the spliceosome.

Additional processing steps attach modifications to the 5' and 3' ends of the hnRNA. These include a 5' cap of 7-methylguanosine and a poly-A tail.

When an hnRNA strand has been properly processed, becoming an mRNA sequence, it is exported out of the nucleus and eventually translated into a protein... a process accomplished in conjunction with ribosomes.