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York University
BIOL 2020
K Wheaton

3 binding sites in tRNA, subunits. When base A (aminoacy), P (peptidyl), E (exit) , both 30S paired with mRNA and 50S subunit, in A and P Sequence of mRNA translation mRNA bound on the 30S subunit, two of the 3 mRNA molec. Are bound to the mRNA througrd anticodon-codon base paris (A and P) 3 is bound to the E site What happens at the 50S end? A and P site converge at a site where peptide bond is formed, a tunnel connects this to the back of the ribosome, the polypeptide chain passes thru here Initiation of translation , prok The first translated codon is 25 bp away from 5’ end. In prok: mRNA are polygenic/polycistronic they encode 2 or more polypeptide chains. Start codon: AUG (methionine) or GUG-bacteria (valine) initiator contains a purine sequence (shine-dalgarno) 10 nucleotides on the 5’ end, 3’ 16S rRNA component interacts with sine-dalgarno sequence N-formylmethionine Found at amino terminal. Protein syn starts here, tRNA brings formylmethionine to ribosome to initiate syn (tRNAf). Different from tRNA that inserts methionine in internal positions (tRNAm)-ecoli Transformylase ½ of proteins the N-formylmethionine is removed when nascent chain is 10 AA long. Linked by aminoacyl-tRNA synthetase (special enzyme to add it) activated formyl donor is N- 10 formyltetrahydrofolate (free methionine and methionyl-tRNAm are not substrates for this) Initiation on the ribosome 30S forms IF1 and IF3, this prevents premature hoining to 50S (forms dead and 70S). IF2-GTPase that binds GTP allows association with formylmethionyl-tRNAf, then binds with mRNA to form 30S initation complex. 50S binds, GTP is hydrolyzed, reaplases initiation factors, leaves 70S (good) fMet-tRNAf occupies P site on ribosome EF-Tu Inserted into empty A site by EF-Tu, requires GTP, elongation factor binds aminoacyl-tRNA only in GTP form, it protects ester linkage from hydrolysis, only hydrolyzed when appropriate complex between EF-Tu-aminoacyl-tRNA complex and ribosome. Free E from GTP contributes to accuracy Power from? Reactivity of an amine group with an ester. Ribosomes bring substrates together (proximity/orientation). Peptide chain now attached in A site on 30S subunit. Change in interaction with 50S places CCA end of same tRNA in the P site of large subunit EF-G tRNA on P site is uncharged, translocase moves it over, EF-G (elongation factor G) uses GTp hydrolysis to force itself into A site on 30S forcing it to move the distance of one codon, then EF-G dissociates, peptidyl t-RNA is now on the P site, what was in the E site has been disengaged Elongation Peptide chain is on a different tRNA , remains in the P site, 50S growing into exit tunnel, repeated, polypeptide syn in the amino terminal to carboxyl terminal direction...added to carboxyl terminus of growing protein Termination Stop codons recognized by release factors (RFs), 3 factors, 3 one recognizes the interactions between first 2 and ribosome. (UAA, UGA, UAG). May assist peptidyl transferase water molecule attacks ester linkage freeing polypeptide chain. Prok and euk differ in initation of protein syn 1. Euk have larger ribo. (60S and 40S that form 80S) 2. Initiator tRNA for euk, is methionine rather than N-formylmethionine 3. Initiation: prok have no start codon, euk have more initiation factors (eIF....eIF-4E binds to 7- methylguanosine cap, eIF-2 association with GTP delivers met-tRNAi to ribosome) 5cap easy starting point 4. Euk mRNA is circular, eIF-4E binds to t
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