biol139-lecture30-2011 - BIOL 139 BIOL Translation...

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Unformatted text preview: BIOL 139 BIOL Translation Translation Chapter 8 con’t pp 275 - 287 pp pp 350, 363 - 371 igenetics Translation Translation The process by which the genetic information carried by The mRNA directs the synthesis of proteins from amino acids acids Process brought about by 2 important molecules: • tRNA • ribosomes Transfer RNAs (tRNAs) Short, single-stranded RNA molecules (74 to 95 NTs long) Function: • adapter molecule - brings a specific amino acid to ribosome:mRNA complex tRNAs carry some modified bases produced by chemical alterations of A, C, G and U tRNA structure tRNA Ala Cloverleaf Model amino acid attachment attachment site site Each tRNA carries one Each one particular amino acid, so at least 1 tRNA exists for least each of the 20 AAs each Each type of tRNA has a different sequence and ∴ different shape Each different The 3’ end amino acid is specified by the anticodon (ultimately the codon) Aminoacyl-tRNA synthetases: Translator Aminoacyl-tRNA • catalyzes the attachment of a tRNA to its correct or correct corresponding amino acid corresponding Based on: • shape of tRNA • anticodon (complementary to mRNA codon for that specific AA) • recognition of correct, specific amino acid At least 1 aminoacyl-tRNA synthetase for each of the 20 AAs Each functions with only ONE specific amino acid specific Aminoacyl-tRNA synthetases: Charging of tRNA Charging Base pairing between a mRNA codon and a tRNA anticodon determines where an amino acid becomes incorporated in a growing polypeptide –function of tRNA NOT amino acid growing cys tRNA cys tRNA with alanine Cell free translation system incorporated alanine into growing polypeptide alanine Wobble : Some tRNAs recognize more than one codon for the amino acid they carry the pyrimidines For some tRNAs the anticodons can interact with more than one codon for the same AA purines If amino acid is specified by more than one codon, first two NTs always same, while third base must be either one of purines (A or G) or one of pyrimidines (U or C) i.e Leucine has 2 tRNA, one with anticodon GAG, other with GAU Ribosomes: Site of protein synthesis Site Subunits remain dissociated till translation Functions: Functions: • recognizes start start site for translation site • link AAs together link in growing protein chain in • exposes mRNA codons sequentially • terminates translation terminates P and A site only functional when both subunits come together Translation (E. coli): Initiation phase - Ribosome-binding site Ribosome-binding Mature mRNA in both prokaryotes and eukaryotes AUG +1 Start codon Stop codon 30s ribosome E. coli Shine-Dalgarno sequence • the Shine-Dalgarno sequence is specifically recognized by complimentary sequences in the 16S rRNA of the 30S subunit • aligns ribosome subunit (30S) on mRNA in proper reading frame for correct protein synthesis Translation (E. coli): Initiation phase E. coli Shine-Dalgarno sequence • this sequence is specifically recognized by complimentary sequences in the 16S rRNA of the 30S subunit • aligns ribosome subunit (30S) on mRNA in proper reading frame for correct protein synthesis Presence of a nearby downstream 5’ AUG codon signals the initiation of translation Special tRNA carrying formylmethionine Start codon 5’ AUG – formylmethionine (fMet) first codon in the gene’s reading frame (fMet) • tRNAfMet brings modified methionine to site • fmet is first amino acid at N-terminus • first AUG preceding Shine-Dalgarno Box This tRNA carries the same anti-codon as tRNA carrying unmodified methionine, This however the rest of the tRNA is unique and is only used for initiation. however Translation (E. coli) : Initiation phase Translation The large 50S ribosomal subunit then binds such that the tRNAFMET is placed in the P site of the ribosome. This completes initiation of translation. Translation : Elongation phase Ribosome moves 5’ to 3’ direction • Various elongation factors escort the next tRNA into the A site • Peptidyl transferase catalyses the formation of a peptide bond between the carboxy terminus (C) of fMet and amino terminus (N) of Phe • This causes release of the tRNA from the P site • Ribosome moves down the mRNA and 2nd amino acid moves to P site • The 3rd amino acid now comes in and the process repeats. Translation : Termination phase Translation • When a nonsense (stop) codon is encountered (for which there is no tRNA), a release factor, which recognizes the stop codon, moves into the A site. •The polypeptide is released from the C-terminal tRNA and the whole complex dissociates. Translation (Eukaryotes): Initiation Phase • 40S ribosome subunit recognizes and binds to the 5’ methylated cap Scans along mRNA till finds initiation of translation sequence • lower eukaryotes is only AUG • mammals also includes surrounding sequences Initiator tRNA carries Met not fMet. Rest of translation is similar to prokaryotes Translation : Initiation phase comparison Polyribosome : a complex of several ribosomes translating from the same mRNA translating Once ribosome has moved away from the mRNA’s ribosomal binding site, another ribosome can bind and begin translation Prokaryote No nuclear membrane concurrent No Translation initiated by Shine-Dalgarno box box Nuclear membrane not linked Translation initiated by 5’ cap 5’ First codon is fMet fMet First codon is Met Met Polycistronic (several genes) (several Gene 1 AUG Gene 2 AUG Eukaryote Transcription/Translation Transcription/Translation Monocistronic (one gene) Monocistronic (one Gene 3 Gene 1 Gene AUG AUG Long mRNA transcript mRNA transcript mRNA’s have multiple ribosome binding sites –several proteins mRNA’s have one ribosome binding sites –one protein Polycistronic mRNA Prokaryote Prokaryote As soon as enough of the transcript is made translation can begin transcription t ra ns la tio n DNA mRNA protein mRNA is very short-lived (seconds) Eukaryote mRNA has to travel out of nucleus to cytoplasm mRNA before being translated -long-lived minutes to hours before Posttranslational processing Posttranslational Posttranslational Posttranslational processing processing -modifications that occur to the protein occur after translation after How mutations affect the products of gene expression How BIOL 139 Gene regulation in prokaryotes Gene Chapter 17 Chapter pp 610 - 624 pp pp 515 - 526 igenetics pp ...
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