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The Genetic Code links nucleic acid and protein information Features of the Genetic Code

The Genetic Code links nucleic acid and protein information Features of the Genetic Code 1. Three nucleotides code an amino acid 2. The code is nonoverlapping 3. The code has no punctuation 4. The code is degenerate.

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The Genetic Code links nucleic acid and protein information Features of the Genetic Code

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  1. The Genetic Code links nucleic acid and protein information • Features of the Genetic Code • 1. Three nucleotides code an amino acid • 2. The code is nonoverlapping • 3. The code has no punctuation • 4. The code is degenerate

  2. Transfer RNAs are adapter molecules that connect specific amino acids to specific codons in RNA • Transfer RNAs have a common design • 1. There is at least one tRNA for each aa • 2. Each tRNA is a single chain of 73-93 nts • 3. All tRNA molecules are L shaped

  3. 4. tRNAs contain many unusual bases derived • by modification. Methylation prevents • formation of certain base pairs, favoring • alternative interactions. Methylation increases hydrophobicity and interactions with proteins

  4. 5. All tRNAs have a clover leaf design with about half the nts in base paired double helices and the other half represented as • 5 specific functional motifs: • a. 3’ CCA terminal region of the acceptor stem • b. The TyC loop contains a ribothymine- psedouracil-cytosine sequence • c. The “extra arm” containing a variable • number of residues • d. The DHU loop contains several dihydrouracil residues • e. The anticodon loop - complementary bonds to mRNA codons

  5. 6. The 5’ end of tRNAs are phosphorylated and is usually pG • 7. Activated amino acids are attached to the adenosine residue at the end of the 3’ CCA component of the acceptor stem • 8. The anticodon is present in a loop near the center of the sequence

  6. “Wobble” allows some transfer RNAs to recognize more than one codon • Yeast alanyl-tRNA has the anticodon IGC which • binds to three codons: GCU, GCC and GCA • The first two nucleotides are the same whereas the third varies – it “wobbles” • The first two bases of the codon bind precisely so that codons that differ in either of the first two bases require a different tRNA • Part of the degeneracy of the code arises from wobble in pairing of the third base of the codon with the first base of the anticodon

  7. Amino acids are activated by adenylation • Amino acids used for protein synthesis must be • attached to specific tRNA molecules; this is the “translation” step of protein synthesis • The amino acid attached to a specific tRNA will then be incorporated into the polypeptide according to the anticodon on that tRNA • Energy must be provided to make peptide bond formation thermodynamically favorable • This is accomplished by the formation of aminoacyl-tRNA

  8. Aminoacyl-tRNA synthetases attach amino acids to the 2’ or 3’ hydroxyl group of the ribose unit at the 3’ end of tRNA

  9. The first step of the aminoacyl-tRNA synthetase reaction is the formation of aminoacyl-adenylate from an amino acid and ATP • amino acid + ATP  aminoacyl-AMP + PPi

  10. The second step is to transfer the aminoacyl group of aminoacyl-AMP to a specific tRNA to form aminoacyl-tRNA • aminoacyl-AMP + tRNA  aminoacyl tRNA + AMP • DG0’ of this reaction is close to 0, therefore pyrophosphate must be hydrolyzed to drive the reaction • Thus the overall reaction is: • amino acid + ATP + tRNA  • aminoacyl-tRNA + AMP + 2Pi

  11. Aminoacyl-tRNA synthetases have highly discriminating amino acid activation sites • Because aminoacyl-tRNA synthetases perform • the actual translation process they must be • be highly specific for their appropriate amino acid and cognate tRNA molecule • Thus there must be a different aminoacyl-tRNA synthetase for each different tRNA molecule • Different aminoacyl-tRNA synthetases recognize • specific tRNAs via interactions with their anticodon loops and acceptor stems, especially modified bases and ribose units

  12. Ribosomes are ribonucleoprotein particles composed of two subunits • Ribosomes constitute about 25% of the mass of an E. coli cell • Ribosomes dissociate into a large subunit (50S) and a small subunit (30S) • The large subunit contains 34 different proteins plus two RNA molecules (23S and a 5S)

  13. Ribosomal RNAs are central to ribosome function • rRNAs fold into complex structures with many short duplex regions • Key catalytic sites in ribosomes are composed largely of rRNA; contributions from proteins are minor

  14. Messenger RNA is translated in the 5’ to 3’ direction • The direction of translation is 5’ 3’ • Thus mRNA can be translated as it is transcribed • In prokaryotes transcription and translation are • closely coupled in space and time

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