Post-transcription Modification Lecture Notes PDF

BMS: 141 Lecture No: 9 Title: Post-transcription modification, Genetic codon Instructor Name: Dr Lames Dawood Medicine and Surgery Program Fall 2024 Intended Learning Outcomes A) Posttranscr...

BMS: 141 Lecture No: 9 Title: Post-transcription modification, Genetic codon Instructor Name: Dr Lames Dawood Medicine and Surgery Program Fall 2024 Intended Learning Outcomes A) Posttranscriptional modifications of mRNA B) Posttranscriptional modifications of rRNA C) Posttranscriptional modifications of tRNA D) Define genetic codon E) Enumerate different types of muttions F) Explain the effect of different mutation on phenotypes A- Post-transcriptional Processing of mRNA 1- In prokaryotes Little modification occurs in mRNA The primary transcript is immediately available for translation It may be mono-cistronic (coding for one polypeptide chain) or poly-cistronic(coding for many polypeptide chains) tRNA & rRNA primary transcript are broken into smaller units  2- In Eukaryotes mRNA The primary transcript undergoes several modifications: i- Capping of 5’end by a GTP followed by its methylation in nucleus (7-metyhl GTP) ii- Removal of introns & splicing of exons In nucleus iii- Addition of poly (A) tail at 3’ end. i- Capping of 5’end a-Enzyme: guanyltransferase b- Source of guanine: GTP c- followed by its methylation in nucleus (7-metyhl GTP). Site of methylation: Cytosol Enzyme responsible for methylation: guanine-7- methyltransferase. Source of methyl group: S-adenosylmethionine (SAM) (Active methionine) Function of cap 1-Protects mRNA against 5’ exonucleases 2- Helps transport of mRNA to the cytoplasm 3- Helps recognition of mRNA by the ribosomes and initiation of translation N.B.: Eukaryotic mRNA lacking the cap are not efficiently translated  ii- Removal of introns & splicing of exons The coding regions of DNA are called exons. They are separated by intervening non-coding regions called introns. Pre-mRNA contains both exons & introns. Introns are removed and exons are spliced (joined together) N.B. Histone mRNA does not contain introns Primary mRNA Exons Introns Exons Introns Exons Introns Exons Introns snRNP : snurps: spliceosomes (snRNA + protein). A specific base sequence at the exon-intron junction determines the site of splicing. Exons Exons Exons Exons Alternative Splicing. ii- Removal of introns & splicing of exons Clinical significance of splicing: ❑It produces proteins with different activities. ❑Alternative splicing is included in as much as 60% of human genetic coding. ❑It produces multiple variations of the mRNA and, therefore, of its protein product. This appears to be a mechanism for producing a diverse set of proteins from a limited set of genes.  For example, different types of muscle cells all produce the same primary transcript from the tropomyosin gene. However, different patterns of splicing in the different cell types produce tissue-specific tropomyosin protein molecules in each tissue. Alternative splicing also produces Igs subtypes  Example for Alternative Splicing. Pre-mRNA For calcitonin E I E I E Thyroid gland neurons mRNA E E E E E Calcitonin hormone Peptide involved With taste  Defect in splicing: - Systemic lupus erythematosus: A fatal inflammatory disease results from an autoimmune Response in which the patient produces antibody against host proteins including snRNAP which having a main role in splicing  iii- Addition of poly(A) tail at 3’ end Recognition Sequence 5’ 3’ AAUAA cap exon exon exon 15- 20 Poly (A) polymerase nucleotides Recognition ptn ptn Sequence AAAAAAAA cap exon exon exon AAUAA up to 200As ptn ptn  Addition of a poly-A tail (Tailing) Most eukaryotic mRNA have a chain of 40–200 adenine nucleotides attached to the 3′-end.This poly-A tail is not transcribed from the DNA. This poly-A tail is added after transcription by the nuclear enzyme, polyadenylate polymerase, using ATP as the substrate. The mRNA is cleaved downstream of a consensus sequence, called the polyadenylation signal sequence (AAUAAA),found near the 3′-end of the RNA, and the poly-A tail is added to the new 3′-end Addition of a poly-A tail (Tailing) Significance of poly A tail These tails help stabilize the mRNA: After the mRNA enters the cytosol, the poly-A tail is gradually shortened Facilitate their exit from the nucleus.(movement)  B- Eukaryotic rRNA Processing The primary rRNA transcript ( 45S RNA) contains 18S, 5.8S and 28S RNA polymerase I transcribes these rRNA genes to produce a single 45 S precursor (pre-rRNA) then ---- RNA-processing in the nucleus ------ cleaves the precursor to release the mature 18S, 5.8S and 28S rRNA. 5S RNA gene is transcribed by RNA polymerase III.  C- Synthesis of tRNAs Eukaryotic tRNA genes are all transcribed by RNA polymerase III pre-tRNA molecules processing Removal of the extra 5`sequence (leader sequence). Replacement of UU nucleotides with the CCA at the 3`end. Modification of some bases e.g. methylation. Removal of short intron, which is present in the anticodon loop. Genetic Code and Mutations D- The Genetic Code Information based on a triplet code Def.: Codons are the basic unit, groups of 3 successive bases 64 possible codons (43 ) 61 specify one of the 20 amino acids three are stop codons (UAA, UAG, UGA) Initiator codon (AUG) The ordering and grouping of the nucleotides is called the reading frame Characteristic of the Genetic Code Specificity: certain codon codes for only one specific amin acid Universality: - All organisms use the same genetic code (except mitochondria) Degeneracy: - A given amino acid may have more than one code * Leu has 6 codons * Methionine, tryptophan has one codon each Reading frame: -The codons are read from the initiating codon on mRNA as a continous uninterrupted sequence of bases taken 3 at a time. tRNA The wobble hypothesis Interaction between codon and anticodon need to be exact at first two positions. Therefore, traditional base pairing rule is obeyed. The third base is not in close apposition to that of the anticodon due to ability of this base in anticodon to move (Wobble), allowing for nontraditional base pairing Anticodon of certain tRNA may recognize more than one codon in mRNA, specifying the same amino acid. E- DNA Mutations  MUTATIONs Effect of changing the nucleotide sequence in mRNA codons I- Single base mutation (Point mutation) ❖ Transition mutation (changing purine with another purine) ❖ Transversion mutation (changing purine with pyrimidine) According to the effect of Single base mutation we may have: * Silent mutation The resulting codon still codes for the same amino acid * Missense mutation The resulting codon codes for a different amino acid * Nonsense mutation The resulting codon codes for termination of the peptide chain * Sense mutation The terminating codon is changed to one codes for amino acid II- Addition or deletion of nucleotides -Addition or deletions of one or two bases results in a “frame-shift” mutation. - Addition or deletion of 3 nucleotides results in addition or deletion of one amino acid to the peptide chain In Class Assessment 2- the function of poly A tail in mRNA include all of the following except: a) Stabilize mRNA b) Protect against restriction c) Increse destruction rate of mRNA d) Facilitate exit from the nucleus 3-The functions of 5’ capping of mRNA include one of the following: a) Initiation of translation b) Facilitate addition of poly A tail c) Increase the rate of mRNA transcription d) Help binding with ribosome 4- Systemic lupus erythromatosus is a fatal inflammatory disease, results from: a) Defect in splicing b) Defect in 5’capping c) Defect in primary mRNA transcript d) Defect in poly A tail 5- the processing of tRNA include all of the following except: a)Trimming of some sequences at 3’ and 5’ terminal b)Addition of CCA in 3’ terminal c)Addition of poly A tail d)Splicing 🞂 Lippincott Illustrated Review Integrated system 🞂 Lippincott Illustrated Review 6th edition 🞂 Oxford Hand book of Medical Science 2nd edition 🞂 Clinical Key Student