Lecture 8: Post Transcriptional Modification PDF

1 Lectures: 8 Post transcriptional modification Hussein Ali Alobaidy Post transcriptional modification It is a process by which primary transcript RNA is converted into mature RNA. There is some difference between prokaryotic and eukaryotic in transcription e...

1 Lectures: 8 Post transcriptional modification Hussein Ali Alobaidy Post transcriptional modification It is a process by which primary transcript RNA is converted into mature RNA. There is some difference between prokaryotic and eukaryotic in transcription events and mRNA production Prokaryotic Eukaryotic mRNA transcribed directly from DNA Primary transcript (RNA) must be template and used immediately in processed to produce the mRNA (active protein synthesis form). mRNA is (polycistronic) one mRNA mRNA is (monocistronic). one mRNA codes for more than one polypeptide codes for only one polypeptide No processing in prokaryotes(mRNA “Processing” of mRNA is required in matures on transcription) eukaryotes for the maturation Post Transcriptional modifications include; 1. Splicing (removal of introns) 2. Addition of 5’ cap (addition of a 5’ 7-methyl guanosine cap) 3. Creation of Poly A tail(addition of a poly-A tail at the 3’)  2 Lectures: 8 Post transcriptional modification Hussein Ali Alobaidy Addition of 5’ cap  In eukaryotes, mRNA cannot leave the nucleus directly following transcription  5' end of the RNA transcript contains a free triphosphate group since it was the first incorporated nucleotide in the chain.  The cap is added by the enzyme guanyl transferase.  Function: protects the mRNA from digestion by enzymes such as nucleases and phosphatases as it exits the nucleus and enters cell cytoplasm The Poly A Tail  Post-transcriptional RNA processing at the opposite end of the transcript comes in the form of a string of adenine bases attached to the end of the synthesized RNA chain.  The addition of the adenines is catalyzed by the enzyme poly (A) polymerase.  poly(A) polymerase, adds a poly(A) tail which is subsequently extended to as many as 200 A residues.  function poly(A) tail appears to protect the 3' end of mRNA from 3' 5' nucleases enzymes 3 Lectures: 8 Post transcriptional modification Hussein Ali Alobaidy Pre-mRNA processing (splicing) DNA of a eukaryotic gene is made of various regions 1. Exons: Segments of DNA that code for part of a specific protein 2. Introns: Non-coding regions of a gene The introns are interspersed among the exons Splicing The process by which introns, the noncoding regions of genes, are excised out of the primary messenger RNA transcript, and the exons (i.e., coding regions) are joined together to generate mature messenger RNA. The steps of pre-mRNA splicing (intron removal) are as follows:  The intron loops out as snRNPs (small nuclear ribonucleoprotein particles, complexes of snRNAs and proteins) bind to form the spliceosome.  The intron is excised, and the exons are then spliced together.  The resulting mature mRNA may then exit the nucleus and be translated in the cytoplasm 4 Lectures: 8 Post transcriptional modification Hussein Ali Alobaidy rRNA Processing Ribosomal RNAs In bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded from other places) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing. tRNA processing Most cells have 40 to 50 distinct tRNAs, and eukaryotic cells have multiple copies of many of the tRNA genes. Transfer RNAs are derived from longer RNA precursors by enzymatic removal of nucleotides from the 5’ and 3’ ends.