Translation in eukaryotes

Translation in eukaryotes

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  Translation in Eukaryotes
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  Translation in Eukaryotes  “Eukaryotictranscription is the elaborate process that eukaryotic cells use to copy genetic information stored in DNA into units of RNA replica.” WHAT ARE EUKARYOTES? Eukaryotes are organisms with a complex cell or cells, in which the genetic material is organized into a membrane-bound nucleus or nuclei and it also contains cell organelles.
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  Mechanism of Translation: Threesteps of translation: - 1) Initiation: Sets the stage for polypeptide synthesis. 2) Elongation: Causes the sequential addition of amino acids to the polypeptide chain as determined by mRNA. 3)Termination: Brings the polypeptide synthesis to the halt.
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  1: Initiation Translation initiationin eukaryotes is a highly regulated and complex stage of gene expression. The initiation of translation in eukaryotes is complex, involving at least 10 eukaryotic initiation factors (eIFs) & divided into 4 steps :  Ribosomal dissociation.  Formation of 43S preinitiation complex.  Formation of 48S initiation complex.  Formation of 80S initiation complex.
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  In eukaryotes  first,the tRNA carrying methionine  attaches to the small ribosomal subunit.  they bind to the  5' end of the mRNA  by recognizing the 5' GTP cap (added during processing in the nucleus).  Then, they "walk" along the mRNA in the 3' direction,  stopping when they reach the start codon (often, but not always, the first AUG).^66
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  a. Ribosomal Dissociation The80S ribosome  dissociates to form 40S &  60S subunits. Two initiating factors  namely elF-3 & elF-1A  bind to the newly  formed 40S subunit &  thereby block its  reassociation with 60S subunit.
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  Initiation Factors  eIF-1(and1A): promotes scanning.  eIF-2: binds tRNAiMet to 40S subunit, requires GTP (which gets hydrolyzed to GDP)  eIF-2B: catalyzes exchange of GTP to GDP on eIF-2  eIF-3: binds to 40S subunit, prevents 60S subunit from binding to it  eIF-5: stimulates 60S subunit binding to the 40S pre-initiation complex  eIF-6: binds to 60S subunit, helps prevent 40S subunit from binding to it.
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  b. Formation Of43S Preinitiation Complex  A ternary complex containing met-tRNA′ & elF-2 bound to GTP attaches to 40S ribosomal subunit to form 43S preinitiation complex. • The presence of elF-3 & elF-1A stabilizes this complex. c. Formation Of 48S Initiation Complex The binding of mRNA to 43S preinitiation complex results in the formation of 48S initiation complex through the intermediate 43S initiation complex. • elF-4F complex is formed by the association of elF-4G, elF-4A with elF-4E. • The elF-4F (referred to as cap binding protein) binds to the cap of mRNA.
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   • ThenelF-4A & elF-4B bind to mRNA & reduce its complex structure.  • This mRNA is then transferred to 43S complex  . • For the appropriate association of 43S preinitiation  complex with mRNA, energy has to be supplied by ATP.  • The ribosomal initiation complex scans the mRNA for the identification of appropriate initiation codon. • 5'-AUG is the initiation codon.
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  d. Formation Of80S Initiation Complex  48S initiation complex binds to 60S ribosomal subunit to form 80S initiation complex. • The binding involves the hydrolysis of GTP (bound to elF2). • This step is facilitated by the involvement of elF-5. • As the 80S complex is formed, the initiation factors bound to 48S initiation complex are released & recycled.
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  2.ELONGATION  Ribosomes elongatethe polypeptide chain by a sequential addition of amino acids.  • The amino acid sequence is determined by the order of the codons in the specific mRNA.  • Elongation, a cyclic process involving certain elongation factors (EFs). • Elongation may be divided into three steps. a. Binding of Aminoacyl t-RNA to A-site.  b. Peptide bond formation.  c. Translocation.
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  a. Binding ofAminoacyl t-RNA to A site  The 80S initiation complex contains met tRNA′ in the P site & A-site is free.  • Another Aminoacyl-tRNA is placed in the A-site. • This requires proper codon recognition on the mRNA & involvement of elongation factor 1a (EF- 1a) & supply of energy by GTP.  • The Aminoacyl-tRNA is placed in the A-site, EF- 1a & GDP are recycled to bring another Aminoacyl-tRNA.
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  b. Peptide bondformation  The enzyme Peptidyl transferase catalyzes the formation of peptide bond.  • The activity of this enzyme lies on 28S RNA of 60S ribosomal subunit.  • It is therefore the rRNA (and not protein) referred to as ribozyme that catalyzes peptide bond formation.  • Net result of peptide bond formation is the attachment of the growing peptide chain to the tRNA in the A-site.
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  c. Translocation  Theribosome moves to the next codon of the mRNA (towards 3'-end). • This process called translocation, involves the movement of growing peptide chain from A-site to P-site.  • Translocation requires EF-2 & GTP.  • GTP gets hydrolyzed and supplies energy to move mRNA.  • EF-2 & GTP complex recycles for translocation.  • About six amino acids per second are incorporated during the course of elongation of translation in eukaryotes. .
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  3. TERMINATION  ofthe stop or termination signals (UAA, UAG and UGA) terminates the growing polypeptide.  • When the ribosome encounters a stop codon, - there is no tRNA available to bind to the A site of the ribosome, - instead a release factor binds to it.  • In eukaryotes, a single release factor- eukaryotic release factor 1 (eRF1)-recognizes all three stop codons, and eRF3 stimulates the termination events.  • once the release factor binds, the ribosome unit falls apart, - releasing the large and small subunits, - the tRNA carrying the polypeptide is also released, freeing up the polypeptide product.  • Ribosome recycling occurs in eukaryotes. One
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  RIBOSOMAL RECYCLING  Afterthe release of polypeptide and the release factors the ribosome is still bound to the mRNA and is left with two deacylated tRNA (in the P and E sites).  • To participate in a new round of polypeptide synthesis, these mRNA and the tRNA must be released and the ribosome must dissociate into small subunit and large subunit.  • Collectively these events are termed as ribosome recycling