A large proportion of the mRNA may be translated before transcription is completed. In some cases degradation of mRNA may begin even before completion of transcription. Methylation and polyadenylation which occur to a considerable extent in eukaryotes are not prevalent in bacterial cells. Nevertheless a certain amount of processing does occur.
In the T7 phage five contiguous genes transcribe 'early RNA' which is cut during transcription to five individual mRNAs by RNase III. In eukaryotes the transcribed RNA molecules are larger than mature mRNA by 20% in the slime mould Dictyostelium and about X 4 to X 5 in mammalian cells. The large RNA molecules containing pre-mRNA transcripts are known as heterogeneous nuclear RNAs (hnRNA).
