Post-transcriptional modification turns pre-mRNA into mature mRNA in eukaryotic cellsIn eukaryotes, the initial transcript called pre-mRNA is modified in the nucleus before it becomes mature mRNA.Three modifications make the mRNA stable, accurate, and ready for export to the cytoplasm for translation.NotePost-transcriptional modification is unique to eukaryotes, because their transcription and translation are separated by the nuclear envelope.The three modifications each protect and prepare the transcriptRemoval of introns and splicing of exons builds a continuous coding sequence.Eukaryotic genes contain exons (coding regions) interrupted by introns (non-coding regions).The spliceosome removes the introns from the pre-mRNA.The remaining exons are spliced together so the mature mRNA carries only coding regions.Addition of a 5' cap attaches a modified guanine nucleotide to the 5' end of the transcript.The cap protects the mRNA from degradation by enzymes.It aids ribosome binding at the start of translation.It also helps the mRNA exit the nucleus.Addition of a 3' poly-A tail adds a string of 100 to 200 adenine nucleotides to the 3' end.The tail stabilises the mRNA by delaying degradation.It assists export of the mRNA from the nucleus.It also enhances translation efficiency.Post-transcriptional modifications AnalogyPicture the 5' cap as a helmet that shields the transcript and guides it to the ribosome.Picture the poly-A tail as a slowly fraying countdown that sets how long the mRNA survives.These modifications matter for stability, accuracy, and transportStability comes from the 5' cap and poly-A tail, which let the mRNA survive long enough to be translated.Accuracy comes from splicing, which leaves only exons so the protein is built from the correct sequence.Transport is aided by all three modifications, which help the mature mRNA leave the nucleus and reach a ribosome.A pre-mRNA that leaves the nucleus unmodified would be degraded or mistranslated before a functional protein could be made.ExampleAn average human gene contains roughly 8 introns that must be removed.The exons are then spliced to form the final coding sequence.Standard splicing sets up alternative splicing, covered nextThe splicing described here joins exons in a single fixed order.Producing multiple protein variants by alternative splicing is treated separately in the next subtopic (D1.2.16).Theory of KnowledgeHow does the extra complexity of eukaryotic mRNA processing reflect the evolutionary distance between eukaryotes and prokaryotes?Active recallWhich complex removes introns from pre-mRNA?What molecule forms the 5' cap, and what does the cap do?How many adenine nucleotides make up a typical poly-A tail?Name the three reasons post-transcriptional modification matters.In which cell compartment do these modifications take place?
