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(CHAPTER 13- Brooker Text)

Translation. (CHAPTER 13- Brooker Text). Sept 25, 2008 BIO 184 Dr. Tom Peavy. CODON = 3 nucleotides encode for 1 amino acid DEGENERACY= more than one codon encodes for an amino acid (wobble base) Code is UNIVERSAL (nearly) Special codons=

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(CHAPTER 13- Brooker Text)

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  1. Translation (CHAPTER 13- Brooker Text) Sept 25, 2008 BIO 184 Dr. Tom Peavy

  2. CODON = 3 nucleotides • encode for • 1 amino acid • DEGENERACY= • more than one • codon encodes • for an amino acid • (wobble base) • Code is UNIVERSAL (nearly) • Special codons= start codon (AUG) stop codons (UAA, UGA, UAG)

  3. Figure 13.2 provides an overview of gene expression Figure 13.2

  4. tRNAs are named according to the amino acid they bear Proline anticodon Recognition Between tRNA and mRNA • During mRNA-tRNA recognition, the anticodon in tRNA binds to a complementary codon in mRNA Figure 13.8

  5. Figure 13.10 Structure of tRNA • cloverleaf folding pattern (stem-loop structures) • contain modified bases (>60 possible) • amino acyl tRNA synthetase= enzyme that conjugates appropriate amino acid to the 3’ end of tRNA (20 of them, one for each a.a.) • Wobble hypothesis= codon-anticodon recognition tolerates mismatches in the third position (but not first and second)

  6. RIBOSOME STRUCTURE AND ASSEMBLY • Macromolecular complex of rRNA and proteins • Prokaryotes = one type of ribosome (70S) • Found in their cytoplasm • Eukaryotes = two types of ribosomes • One type is found in the cytoplasm (80S) • The other is found in organelles (70S, like prok.) • Mitochondria ; Chloroplasts

  7. Bacterial Ribosomes (and mitochondrial/chloroplast) • A ribosome is composed of structures called the large and small subunits • Each subunit is formed from the assembly of Proteins + rRNA Note: S or Svedberg units are not additive Figure 13.13

  8. Eukaryotic Ribosomes Formed in the cytoplasm during translation The 40S and 60S subunits are assembled in the nucleolus Then exported to the cytoplasm Figure 13.13

  9. Ribosomes contain three • discrete sites: • Peptidyl site (P site) • Aminoacyl site (A site) • Exit site (E site)

  10. Release factors Three Stages: Initiation Elongation Termination Initiator tRNA

  11. 16S rRNA Initiation Stage • Initiation complex= • mRNA + initiation factors + ribosomal subunits • The binding of mRNA to the 30S subunit is facilitated by a ribosomal-binding site or Shine-Dalgarno sequence • This is complementary to a sequence in the 16S rRNA Hydrogen bonding Component of the 30S subunit Figure 13.17

  12. Initiation completed after assembly of first tRNA (=formyl methionine) The only charged tRNA that enters through the P site All others enter through the A site 70S initiation complex This marks the end of the first stage Figure 13.16

  13. Elongation • Charged tRNA binds to the A site (use of GTP and Elongation factor) • Bond between polypeptide chain (P site) and incoming amino acid (A site) catalyzed by peptidyl transferase • Polypeptide transferred to the A site Figure 13.18

  14. Elongation Cont’d • Ribosome translocates one codon to the right (GTP and elongation factor) • Uncharged tRNA released from the E site • Repeat Process until stop codon

  15. Termination Stage • In most species there are three stop or nonsensecodons • UAG • UAA • UGA • These codons are not recognized by tRNAs, but by proteins called release factors • Indeed, the 3-D structure of release factors mimics that of tRNAs

  16. Carboxyl group Amino group • Polypeptide synthesis has a directionality that parallels the • 5’ to 3’ orientation of mRNA Carboxy terminus Amino terminus Condensation reaction releasing a water molecule

  17. Levels of Structures in Proteins • There are four levels of structures in proteins • 1. Primary (its amino acid sequence) • 2. Secondary (protein folds to form regular, repeating known) • 3. Tertiary (short regions of secondary structure in a protein fold into a three-dimensional) • 4. Quaternary (Proteins made up of two or more polypeptides)

  18. A protein subunit Figure 13.6

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