Post-transcriptional Processing of hnRNA

In bacteria, because there is no nuclear membrane separating the DNA from the ribosomes, the mRNA transcript produced by RNA polymerase is generally translated at the ribosome directly and immediately, often even while transcription of that same mRNA molecule is still ongoing — bacterial mRNA is therefore typically used essentially as-is, with little or no further chemical modification after transcription. In eukaryotic cells, by contrast, the primary transcript produced by RNA polymerase II — called heterogeneous nuclear RNA, or hnRNA, because it includes both coding and non-coding sequences and varies considerably in length — must undergo several specific processing steps inside the nucleus before it is exported to the cytoplasm as mature, translatable mRNA.

The first processing step is splicing. A eukaryotic gene is typically not one single continuous coding sequence; instead, it consists of coding sequences called exons, interrupted by intervening, non-coding sequences called introns. Splicing is the process by which the intron sequences are precisely cut out of the primary hnRNA transcript, and the exon sequences on either side of each removed intron are then joined (spliced) directly together, producing a continuous, mature mRNA molecule containing only exon-derived coding sequence. This splicing reaction is carried out with great precision by a large ribonucleoprotein complex called the spliceosome, which recognises specific short sequence signals present at the boundaries between each exon and intron.

The second processing step is capping, which takes place at the 5' end of the hnRNA transcript: an unusual, modified guanine nucleotide, called a 5' cap (specifically 7-methylguanosine), is chemically added to the 5' end of the transcript. This 5' cap serves several protective and functional roles, including protecting the mRNA from premature enzymatic degradation and helping the ribosome correctly recognise and bind the mRNA's 5' end to begin translation. …