Post-Transcriptional Modification of RNA

Key points

• • A modified guanine 5' cap is added to the front of the transcript; it protects the RNA and helps the ribosome recognise it for translation.
• • A tail of many adenine nucleotides, the poly-A tail, is added to the 3' end and adds stability and helps the RNA leave the nucleus.
• • Introns (the stretches that are removed) are taken out and exons (the stretches that are kept, which may include both coding and untranslated regions) are joined together by a complex called the spliceosome, in a step called splicing.
• • Alternative splicing lets one gene join its exons in different combinations, so a single gene can code for several related proteins.
• • Only after capping, tailing and splicing does the mature messenger RNA leave the nucleus through nuclear pores to be translated in the cytoplasm.
• • Faulty splicing can cause disease; for example, some forms of the blood disorder beta-thalassaemia are caused by splice-site mutations.