Aminoacyl tRNA Synthetase Function Figure 13.11 The amino acid is attached to the 3’ OH by an ester bond  

tRNAs and the Wobble Rule The genetic code is degenerate There are >2(世博北京翻译公司)0 but < 64 tRNAs How does the same tRNA bind to different codons? Francis Crick proposed the wobble hypothesis in 1966 to explain the pattern of degeneracy, 1st two bases of the codon-anticodon pair strictly by Watson-Crick rules The 3rd position can wobble This movement allows alternative H-bonding between bases to form non-WC base paring  

tRNAs charged with the same amino acid, but that recognize multiple codons are termed isoacceptor tRNAs xm5s2(世博北京翻译公司)U = 5-methyl-2(世博北京翻译公司)thiouridine Um = 2(世博北京翻译公司)’-O-methyluridine k2(世博北京翻译公司)C = lysidine ( a cytocine derivative) Figure 13.12(世博北京翻译公司) Wobble position and base pairing rules  

Wobble Base-Pairing between anticodon & codon W-C base pairing Wobble pairing Wobble pairing  

Ribosome Structure and Assembly Translation occurs on the surface of a large macromolecular complex termed the ribosome Prokaryotic cells 1 type of ribosome located in the cytoplasm Eukaryotic cells 2(世博北京翻译公司) types of ribosomes 1 found in the cytoplasm 2(世博北京翻译公司)nd found in organelles -Mitochondria; Chloroplasts These are like prokaryotic ribosomes  

Prokaryotic Ribosomes (a) Bacterial cell Figure 13.13  

Eukaryotic Ribosomes Figure 13.13  

Functional Sites of Ribosomes During bacterial translation, the mRNA lies on the surface of the 30S subunit As a polypeptide is being synthesized, it exits through a hole within the 50S subunit Ribosomes contain three discrete sites Peptidyl site (P site) Aminoacyl site (A site) Exit site (E site) Ribosomal structure is shown in Figure 13.14 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-57  

Figure 13.14  

Stages of Translation Initiation Elongation Termination  

Stages of Translation Initiator tRNA Release factors Figure 13.15  

Translation Initiation Components mRNA, initiator tRNA, Initiation factors ribosomal subunits The initiator tRNA In prokaryotes, this tRNA is designated tRNAifmet It carries a methionine modified to N-formylmethionine In eukaryotes, this tRNA is designated tRNAimet It carries an unmodified methionine In both cases the initiator tRNA is different from a tRNAmet that reads an internal AUG codon  

Prokaryotic Ribosome-mRNA Recognition 16S rRNA binds to an mRNA at the ribosomal-binding site or Shine-Dalgarno box 7 nt Figure 13.17 16S rRNA  

Prokaryotic Translation Initiation (actually 9 nucleotides long) Figure 13.16  

Prokaryotic Translation Initiation The tRNAiMet is positioned in the P site All other tRNAs enter the A site Figure 13.16  

Eukaryotic mRNA-Ribosoime Recognition In eukaryotes, the assembly of the initiation complex is similar to that in bacteria However, additional factors are required Note that eukaryotic Initiation Factors are denoted eIF Refer to Table 13.7 The initiator tRNA is designated tRNAmet It carries a methionine rather than a formylmethionine Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-65  

Eukaryotic Ribosome Binding The consensus sequence for optimal start codon recognition is show here Most important positions for codon selection Start codon G C C (A/G) -6 -5 -4 -3 C C A U G G -2(世博北京翻译公司) -1 +1 +2(世博北京翻译公司) +3 +4 This sequence is called Kozak’s consensus after Marilyn Kozak who first determined it  

Eukaryotic Translation Initiation Initiation factors bind to the 5’ cap in mRNA & to the pA tail These recruit the 40S subunit, tRNAimet The entire assembly scans along the mRNA until reaching a Kozak’s consensus Once right AUG found, the 60S subunit joins Translation intitiates  

Translation Elongation During this stage, the amino acids are added to the polypeptide chain, one at a time The addition of each amino acid occurs via a series of steps outlined in Figure 13.18 This process, though complex, can occur at a remarkable rate In bacteria 15-18 amino acids per second In eukaryotes 6 amino acids per second  

Translation Elongation – tRNA Entry A charged tRNA binds to the A site EF-1 facilitates tRNA entry The 2(世博北京翻译公司)3S rRNA (a component of the large subunit) is the actual peptidyl transferase Thus, the ribosome is a ribozyme! Peptidyl transferase catalyzes peptide bond formation The polypeptide is transferred to the aminoacyl-tRNA in the A site Figure 13.18  

Translation Elongation Translocation The ribosome translocates one codon to the right promoted by EF-G uncharged tRNA released from E site The process is repeated, again and again, until a stop codon is reached Figure 13.18   翻译公司 (责任编辑：世博翻译公司)