Post-transcriptional modification is the process whereby changes occur in eukaryotic mRNA, tRNA or other RNAs after transcription occurs, according to class notes from Northwestern University. The changes to mRNA can include additions of a cap and tail and removal of introns. Changes to tRNA include modification of bases and removal of introns.
Eukaryotic DNA transcription happens in a cell's nucleus and produces a primary RNA transcript, according to SparkNotes. Before being moved into the cytoplasm, products of the transcription must be modified to become mature messenger RNA. Removal of introns is a process called splicing and is part of this modification. Once the splicing process is complete, the messenger RNA contains only exons and is ready to be translated into a protein.
While exon sequences are important, intron sequences are not, indicates Sparknotes. The only intron sequence portions that are preserved are those that help to identify the fact they are introns so they may be removed. According to SparkNotes, an additional type of post-transcriptional modification is RNA editing. In this process, the messenger RNA sequence is changed and the resulting protein is altered. This editing can happen by changing one nucleotide to another and by inserting or deleting a nucleotide.