Synthetase matches codons of particular amino acid, catalyzes covalent attachment of amino acid to its rRNA (driven by hydrolysis of ATP) = aminoacyl-tRNA (AKA charged tRNA) released from enzyme and can deliver its amino acid to growing polypeptide chain (b) Pairing of tRNA anticodon with appropriate mRNA codon o Some tRNAs are able to mind to more than one codon; versatility is possible bc rules for base pairing between third nucleotide base of codon and corresponding base of tRNA anticodon are relaxed compared to those at other codon positions (ie. U at 5’ end of tRNA anticodon can pair with A or G in the third position [3’ end] of mRNA codon o Flexible base pairing (^^) = wobble ; explains why synonymous codons for amino acids most often differ in 3 rd nucleotide base Ribosomes - Facilitate specific coupling of tRNA anticodons with mRNA codons during protein synthesis - Consists of large subunit + small subunit; each made of proteins and 1+ ribosomal RNAs (rRNA) - Eukaryotes: subunits are made in nucleolus; rRNA genes are transcribed, RNA is processed/assembled with proteins from cytoplasm; resulting ribosomal subunits are exported via nuclear pores to cytoplasm - Large + small subunits join to form functional ribosome only when attached to mRNA molecule - Each ribosome has binding site for mRNA + 3 binding sites for tRNA, from right to left: (a) E site (e xit site)- discharged tRNAs leave ribosome (b) P site (p eptidyl-tRNA binding site)- holds tRNA carrying growing polypeptide chain (c) A site (a minoacyl-tRNA binding site)- holds tRNA carrying next amino acid to be added - As polypeptide becomes longer, it passes through an exit tunnel at ribosome’s large subunit; polypeptide is released once completed - RRNA (not protein) is primarily responsible for structure + function of ribosome; proteins (exterior) support shape changes of rRNA molecules as they carry out catalysis during translation (a) Main constituent of A and P sites + interphase between subunits (b) Acts as catalyst of peptide bond formation = ribozyme Building a Polypeptide - Synthesis of chain has 3 stages: initiation, elongation, and termination - Hydrolysis of GTP provides energy for chain initiation and elongation Ribosome Association and Initiation of Translation - MRNA, tRNA bearing first amino acid, and two subunits of a ribosome come together… (1) Small subunit binds mRNA at specific RNA sequence (just upstream of start codon) or visa versa o Eukaryotes: Small subunit (with initiator tRNA) binds to 5’ cap of mRNA and scans downstream until it reaches start codon; tRNA hydrogen-bonds to AUG start codon
(2) Start codon signals start of translation - Large subunit attaches, completes translation initiation complex ; proteins ( initiation factors ) bring components together - Cell expends energy obtained by hydrolysis of GTP to form initiation complex; initiator tRNA is P site, A site is available for next tRNA Elongation of the Polypeptide Chain - Amino acids are added one by one to previous amino acid at C-terminus of growing chain - Involves participation of elongation factors
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- Spring '13
- DNA, RNA molecules