1. Chapter 15-II Translation Genes and How They Work

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3. tRNA and Ribosomes tRNA molecules carry amino acids to the ribosome for incorporation into a polypeptide –Aminoacyl-tRNA synthetases add amino acids to the acceptor stem of tRNA –Anticodon loop contains 3 nucleotides complementary to mRNA codons 3

4. 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. c: Created by John Beaver using ProteinWorkshop, a product of the RCSB PDB, and built using the Molecular Biology Toolkit developed by John Moreland and Apostol Gramada (mbt.sdsc.edu). The MBT is fi nanced by grant GM63208 2D “Cloverleaf” Model Acceptor end Anticodon loop 3׳3׳ 5׳5׳ 3D Ribbon-like Model Acceptor end Anticodon loop 3D Space-filled Model Anticodon loop Acceptor end Icon Anticodon end Acceptor end

5. tRNA charging reaction Each aminoacyl-tRNA synthetase recognizes only 1 amino acid but several tRNAs Charged tRNA – has an amino acid added using the energy from ATP –Can undergo peptide bond formation without additional energy Ribosomes do not verify amino acid attached to tRNA 5

6. 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. tRNA PiPi PiPi NH 3 + O–O– C O C O C O OH O AMP O OH AMP Amino group Carboxyl group Trp NH 3 + Trp ATP Amino acid site Accepting site Anticodon specific to tryptophan Aminoacyl-tRNA synthetase tRNA site

7. 7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. tRNA PiPi PiPi NH 3 + O–O– C O C O C O C O C O OH O AMP O OH AMP O O Charged tRNA travels to ribosome Amino group Carboxyl group Trp NH 3 + Trp ATP Amino acid site Accepting site Anticodon specific to tryptophan Aminoacyl-tRNA synthetase tRNA site Charged tRNA dissociates Trp

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9. 9 The ribosome has –multiple tRNA binding sites P site – binds the tRNA attached to the growing peptide chain A site – binds the tRNA carrying the next amino acid E site – binds the tRNA that carried the last amino acid –two primary functions 1. Decode the mRNA 2. Form peptide bonds Peptidyl transferase mRNA 3´ 5´5´ Large subunit Small subunit Enzymatic component of the ribosome --Forms peptide bonds between amino acids

10. 10 Translation In prokaryotes, i nitiation complex includes –Initiator tRNA charged with N-formylmethionine –Small ribosomal subunit –mRNA strand Ribosome binding sequence (RBS) of mRNA positions small subunit correctly Large subunit now added Initiator tRNA bound to P site with A site empty

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12. Initiations in eukaryotes similar except –Initiating amino acid is methionine –More complicated initiation complex –Lack of an RBS – small subunit binds to 5′ cap of mRNA Elongation adds amino acids –2 nd charged tRNA can bind to empty A site –Requires elongation factor called EF-Tu to bind to tRNA and GTP –Peptide bond can then form –Addition of successive amino acids occurs as a cycle 12

13. 13 There are fewer tRNAs than codons Wobble pairing allows less stringent pairing between the 3′ base of the codon and the 5′ base of the anticodon This allows fewer tRNAs to accommodate all codons Termination –Elongation continues until the ribosome encounters a stop codon –Stop codons are recognized by release factors which release the polypeptide from the ribosome Wobble pairing

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16. 16 Protein targeting In eukaryotes, translation may occur in the cytoplasm or the rough endoplasmic reticulum (RER) Signal sequences at the beginning of the polypeptide sequence bind to the signal recognition particle (SRP) The signal sequence and SRP are recognized by RER receptor proteins Docking holds ribosome to RER Beginning of the protein-trafficking pathway

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20. 20 Mutation: Altered Genes Missense mutation Silent mutation Nonsense mutations

21. 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Polar Normal HBB Sequence Abormal HBB Sequence Nonpolar (hydrophobic) Amino acids Nucleotides Amino acids Nucleotides Leu CCCCGTTTAGGAGAA ThrProGlu CTTGAA LysSer Leu CCCCGTTTAGGGAA ThrPro val Glu CTTTGAA LysSer 11 11 Normal Deoxygenated Tetramer Abnormal Deoxygenated Tetramer Tetramers form long chains when deoxygenated. This distorts the normal red blood cell shape into a sickle shape. Hemoglobin tetramer "Sticky" non- polar sites 22 22 11 11 22 22 Frameshift mutations Addition or deletion of a single base Much more profound consequences Alter reading frame downstream Triplet repeat expansion mutation Huntington disease Repeat unit is expanded in the disease allele relative to the normal

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23. 23 Chromosomal mutations Change the structure of a chromosome –Deletions – part of chromosome is lost –Duplication – part of chromosome is copied –Inversion – part of chromosome in reverse order –Translocation – part of chromosome is moved to a new location

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25. 25 --Too much change, however, is harmful to the individual with a greatly altered genome --Balance must exist between amount of new variation and health of species Mutations are the starting point for evolution