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Two Stage Processes of Gene Expression

Two Stage Processes of Gene Expression. Transcription: - mRNA - tRNA - rRNA. Translation: - mRNA translated to Protein - tRNA & rRNA provide other components of the apparatus for protein synthesis. Prokaryotic Translation. Life Cycle of mRNA. Fig5.19 Two stage process of

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Two Stage Processes of Gene Expression

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  1. Two Stage Processes of Gene Expression Transcription: - mRNA - tRNA - rRNA Translation: - mRNA translated to Protein - tRNA & rRNA provide other components of the apparatus for protein synthesis

  2. Prokaryotic Translation

  3. Life Cycle of mRNA

  4. Fig5.19 Two stage process of bacterial mRNA Degradation Endonucleolytic cleavage from 5’ to 3’ behide ribosome Exonuclease degrade from 3’ to 5’

  5. tRNA - ‘adapter’ - small mol., only 75-95 base long Aminoacyl-tRNA; charged tRNA with the aa. corresponding to its anticodon

  6. Four major arms of tRNA + an extra arm Invariant or Conserved, >90-95%of t RNA specific bases. Semi-invariant or Semiconserved, restrict to one type of base. Extra arm, the most variable feater of tRNA

  7. Tertiary Structure of tRNA

  8. Ribosome -Ribonucleoprotein particles -Contain more RNA than Protein -Ribosomal protein, ‘r-protein’ -Ribosome in a cell are identical -Consist of two subunits, each contain; *a major rRNA *a number of small proteins

  9. Fig.6.4 The ribosome has two sides for binding charged tRNA. P site holds peptidyl-tRNA A site, entered by aminoacyl-tRNA

  10. Elongation Factor T loads aminoacyl-tRNA into A-site EF-Ts for replacement of GDP by GTP

  11. Fig.6.6Three stages of Protein Synthesis Initiation Elongation Termination

  12. Initiation requires; -free ribosome subunit -initiation factors,IF, only on 30S sub. Fig 6.7 Two steps of reaction; 1.Recognition of mRNA 2.Large sub. joins

  13. Bacteria use IF-1 IF-2 IF-3

  14. A Special Initiator tRNA -Protein synthesis start with the same aa, METHIONINE -Initiation codon, AUG, in Bacteria, GUG or UUG also used Two types of tRNA carry METHIONINE - one for initiation - other for recognizing AUG codons during elongation

  15. A Special Initiator tRNA *N-formyl-methionyl-tRNA; The initiator tRNA carries a methionine residue with formulating on amino gr., Known as tRNAfMet The name of the aminoacyl-tRNA, fMet-tRNAf -This tRNA is used only for Initiation. -Recognize AUG or GUG (occasionally UUG) tRNAmMet for recognizing AUG codons in internal locations, no methionine formylation

  16. Two sites on Ribosome P-site for fMet-tRNAf A-site for amioacyl-tRNA, aa-tRNA

  17. The Initiator tRNA fMet-tRNAf

  18. Initiation Factors: IF-2 ,control the initiator tRNA entry to the ribosome IF-1, bind 30S to complete initiation complex IF-3, need for 30S to bind initiation site

  19. Fig6.15 Initiation occurs independently at each cistron in a polycistronic mRNA

  20. Termination Reaction: - release the completed polypeptides -expel tRNAfrom ribosome -dissociate the ribosome tRNA Termination codons; UAA,most common UGA > UAG UGA, more error reading Release Factors, - catalyze termination, A-site -RF-1 for UAA, UAG -RF-2 for UGA, UAA -RF-3 for release RF-1&RF-2

  21. Ribosome Recycling Factors, RRF - act together with EF-G causing dissociation of 50S & 30S - IF-3 also reequire, remove deacylated tRNA from 30S , and prevent reassociation - RRF act on 50S

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