Lecture 25Outline of Last Lecture I. Amino Acids and CodonsII. TranslationA. tRNA ChargingB. Wobble PairingIII. Decoding mRNAOutline of Current Lecture I. TranslationA. Eukaryotic TranslationB. Prokaryotic TranslationII. Chain InitiationIII. Chain ElongationA. Aminoacyl SiteB. Peptidyl SiteIV. Chain TerminationCurrent LectureThis lecture goes back to looking at the process of translation. In eukaryotes the initiator codon is always the first AUG in the mRNA sequence. Eukaryotes have monosystronic sequences. Eukaryotes have methionine residues in their nuclear DNA. The mitochondria have formyl-methionine residues. In prokaryotes the initiator codon is found by the help of the Shine Dalgamo sequence. This sequence is GGAGGU. The AUG initiation codon should be 8-13 bases down from the Shine Dalgamo sequence. Any AUG less than 8 bases away is too close. In prokaryotes, the sequence is polysystronic. Formyl-methionine residues are present in all prokaryotic mRNA. Both prokaryotes and eukaryotes need an initiation complex to begin the translation process. This complex contains mRNA, ribosomal subunits, initiation factors and charged methionine tRNAs. The translation process starts with chain initiation. This consists of the formation of the initiation complex. The mRNA strand, initiation factors, and 30S subunit come together first. Then the charged tRNA binds to the group. Finally, the 50S subunit binds to the complex and releases some initiation factors. The next step in translation is called chain elongation. Chain elongation occurs in three steps. The ribosomal complex has two binding sites for tRNA. The site closer to the 3’ end is called the peptidyl (P) site. The P site carries the tRNA-linked peptide chain. The other site is called the aminoacyl (A) site. This is the site for incoming aminoacyl-tRNA. In the first step an aminoacyl-tRNA binds to the A site. The appropriate anticodon loop binds to the next codon. The second step transfer the peptide chain from the P site tRNA to theA site tRNA. This occurs in the 50S subunit and is catalyzed by the enzyme peptidyltransferase. A new peptide bond is formed in this step by linking the carboxyl of the peptide to the amino group of the aminoacyl-tRNA. The final step of the elongation process involves releasing the now empty tRNA in the P site. The peptidyl-tRNABCHM 307 1st Editionin the A site translocates to the P site. These three steps are repeated until all amino acids are added to the peptide chain.The final step in translation is called chain termination. This occurs when a stop codon enters the ribosome’s A site. Release factors then bind to the A site as well. A hydrolysis reaction releases the peptide for the last tRNA. The mRNA strand is released. The ribosomal complex dissociates as
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