Post-transcriptional Modifications
- At this point the newly transcribed RNA, known as pre mRNA, is vulnerable to the enzymes and conditions outside the nucleus
- Must undergo additional modifications before it can leave the nucleus
Capping and tailing
- Add a Poly(A) Tail: the addition of 50 to 250 adenine nucleotides, to the 3’ end - Performed by enzyme poly-A polymerase (Enables mRNA to be translated efficiently and protects it from attack by RNA-digesting enzymes in the cytoplasm)
- Add 5’ cap: adding seven G’s by a difference enzyme complex (Functions as the initial attachment site for mRNA to ribosomes, to allow for translation)
Exons, introns and snrnps
- DNA contains coding regions = exons & non-coding regions = introns
- These introns need to be removed as they do not code for a part of a protein and if left would alter the amino acid sequence
- Introns are removed from pre-mRNA in the nucleus in a region called a spliceosome (a complex formed between pre-mRNA and small ribonucleoproteins = snRNPs)
- This process of splicing and cutting is so exact that not a single base of intron is retained and not a single base of exon is removed.
alternative splicing
- Exons may be joined in different combinations to produce different mRNAs from a single DNA gene sequence
- This greatly increases the number and variety of proteins that are encoded by a single gene
- Approximately 3 quarters of all human pre-mRNA are subjected to alternative splicing
- Once the finalizations are done the final product mRNA is ready to leave the nucleus