Pre-mRNA

Precursor mRNA (pre-mRNA), also termed heterogeneous nuclear RNA (hnRNA), is an immature single strand of messenger ribonucleic acid (mRNA). pre-mRNA is synthesized from a DNA template in the cell nucleus by transcription.

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

Processing
Eukaryotic pre-mRNA 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 (except for self-splicing introns).

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

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