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Protein Synthesis

Protein Synthesis. Making proteins from genes. One gene-one enzyme hypothesis (Beadle and Tatum) Later revised to the one gene-one polypeptide hypothesis Transcription synthesis of RNA under the direction of DNA (mRNA) Translation synthesis of a polypeptide under the direction of mRNA.

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Protein Synthesis

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  1. Protein Synthesis

  2. Making proteins from genes • One gene-one enzyme hypothesis (Beadle and Tatum) • Later revised to the one gene-one polypeptide hypothesis • Transcription • synthesis of RNA under the direction of DNA (mRNA) • Translation • synthesis of a polypeptide under the direction of mRNA

  3. DNARNAProtein • ‘U’ (uracil) replaces ‘T’ in RNA • The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ • Triplet code/codons • More than one triplet for each Amino Acid

  4. The Genetic Code

  5. Initiation of Transcription • Transcription factors attach to TATA box • Contains sequences of only adenine and thymine • RNA polymerase attaches to begin transcription

  6. Elongation and Termination • Elongation • RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end • Termination • RNA polymerase reaches terminator sequence • Polyadenylation signal--AAUAAA

  7. Transcription Helpers • RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA code • Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins • Terminator region: sequence that signals the end of transcription • Transcription unit: stretch of DNA transcribed into an RNA molecule

  8. mRNA modification • 5’ cap: modified guanine molecule • Protection • recognition site for ribosomes • 3’ tail: poly (A) tail (up to 250 adenine nucleotides) • Protection • Recognition • transport

  9. RNA Splicing • Exons (expressed sequences) kept • Introns (intervening sequences) spliced out • Splicing done by snRNPs which bind together to form a large compound known as a spliceosome • Made of snRNA • RNA that acts as an enzyme= ribozyme

  10. Basics of Translation • mRNA moved through ribosome • “code” read in triplets • tRNA transfers correct amino acid to the polypeptide chain

  11. tRNA • Has an anticodon (nucleotide triplet) • Codes for a specific amino acid

  12. Picking the right Amino Acid • Aminoacyl-tRNA synthetase • Joins a specific amino acid to its tRNA molecule • Wobble • Relaxation in the base-pairing rules • Third nucleotide in a codon can be different in many cases and still code for the same amino acid

  13. Ribosomes • rRNA (ribosomal RNA) • site of mRNA codon & tRNA anticodon coupling • P site • holds the tRNA carrying the growing polypeptide chain • A site • holds the tRNA carrying the next amino acid to be added to the chain • E site • discharged tRNA’s leave the ribosome

  14. Initiation of Translation • Union of mRNA, tRNA, and small ribosomal subunit • Start codon = AUG (codes for methionine) • Large subunit then attaches

  15. Elongation • Codon recognition • Anticodon of tRNA binds with the complementary mRNA codon in the A site • Peptide bond formation • Large subunit forms a peptide bond between the amino acid in the A site and the growing polypeptide • Translocation • tRNA in A site moves to P site… the one in the P site moves to the E site

  16. Termination • When a stop codon (UAG, UAA, or UGA) is reached, the A site accepts a “release factor” • Release factor cuts the polypeptide free from the tRNA molecule • Ribosome dissociates

  17. Polyribosomes • Translation of mRNA by many ribosomes • many copies of a polypeptide very quickly

  18. Where does it happen? • Protein always begins in the cytosol • Will continue there unless it receives a signal peptide telling it to bind to the ER • SRP’s (signal-recognition particles) • Binds to a ribosome and brings it to the membrane of the ER • Attaches to a receptor molecule

  19. Roles of RNA

  20. Mutations Point mutation- The change of just one nucleotide • Base-pair substitution • One nucleotide substituted for another • Sometimes a “silent mutation” if it still codes for the same amino acid • Missense mutation- still codes for an amino acid, but not the right one • Nonsense mutation- changes to a stop codon and terminates translation

  21. Frame-shift Mutations • Insertion • Nucleotide is added • Deletion • Nucleotide is lost • Causes the codons from that point on to be incorrectly grouped Mutagen- anything that causes a mutation (chemicals, X-rays, carcinogens, etc.)

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