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5.4 Translation

5.4 Translation. Protein Synthesis Part 2 – Reading the Instructions and Making a Protein. Copy out note in purple . Also see G7duotang. And/or print out Protein Synthesis Review notes under Genetic Resources. 3 Types RNA 0:37.

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5.4 Translation

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  1. 5.4 Translation Protein Synthesis Part 2 – Reading the Instructions and Making a Protein Copy out note in purple. Also see G7duotang. And/or print out Protein Synthesis Review notes under Genetic Resources

  2. 3 Types RNA 0:37

  3. Recall Transcription: mRNA is complementary to DNA and read in triplets (codons) Why 3 nucleotides for a codon? Why not 2? 42 = 16 possible combinations. We have 20 aas so we need more. 43 = 64 combos. With that many we can have 3 stop codons, and multiple codons for an aa.

  4. The Genetic Code (pg. 240)

  5. Translation - Overview • Aminoacyl-tRNA • Ribosome structure • Translation in 3 steps • Initiation • Elongation • Termination

  6. Aminoacyl-tRNA Copy title (purple) • tRNA structure: • Folds into a 3-lobe shape • brings specific amino acids from the cytoplasm to the ribosome • Contains anticodons that match with codons on mRNA • aa attaching end carries correct aa to codon in A-site of ribosome

  7. tRNA Aminoacyl-tRNA: -a tRNA molecule with its corresponding amino acid attached to its acceptor site at the 3’ end Aminoacyl-synthetase: -The enzyme that adds the appropriate amino acid to each tRNA -There are at least 20 of them -Each is specific for a particular amino acid and tRNA Anticodon: -recognizes the codon of the mRNA

  8. Aminoacyl-tRNA synthetase • ATP is needed to bind the proper amino acid to the tRNA • Responsible for attaching the correct aa to tRNA with the correct anticodon • Has 2 binding sites: • Recognizes the anticodon on tRNA molecule • Recognizes aa corresponding to that anticodon

  9. Ribosome Structure • Ribosomes are made up of proteins and ribosomal RNA (rRNA) • Made of a large subunit and a small subunit • Large subunit contains: • E-site (exit) = releases tRNA back into cytoplasm • P-site (peptidyl) = where peptide bonds are formed and holds the growing aa chain • A-site (aminoacyl) = holds aa-tRNA that awaits for its aa to be added to the growing chain • Small subunit holds and moves across mRNA

  10. Ribosome Structure

  11. Translation mRNA  Protein (facilitated by tRNA) Happens in a ribosome, in the cytoplasm Has three stages: Initiation Elongation Termination *Steps are on G7 duotang

  12. Translation – Initiation – See G7  2 parts of the ribosome assemble together around the mRNA • mRNA strand’s finder sequence AGGA binds to rRNAantifinder sequence UCCU on the small subunit • AUG (the initiation/start codon (AUG) which codes for Methionine MET) will be always be the first available codon at the P site • a tRNA with the anticodon UAC carrying MET attaches to the initiation codon at the P site. Every protein starts with methionine.

  13. Translation - Elongation The next codon will be available to be read on the mRNA at the A site AtRNA carrying an aa coded for at the A site attaches with its anticodon at the A site The aa’s beside each other form a peptide bond (remember from unit 1 – dehydration synthesis!) and become a chain such that the chain is always newly attached to the tRNA at the A site.

  14. Translocation The ribosome moves down the mRNA by one codon • Codon-tRNA in P-site now in E-site, tRNA exits • Codon-tRNA in A-site, now in P-site, has the chain • Codon in A-site now empty • aa-tRNA with corresponding anticodon binds in the empty A-site • Steps repeat and chain elongates

  15. Polypeptide Chain Elongation the next tRNA with the appropriate anticodon (carrying the required amino acid) enters the A site tRNA at the P site is released from its amino acid and the mRNA codon by enzyme action the amino acid at the P site attaches to the aa chain at the A site (by peptide bond) the ribosome shifts over and the next aa enters the A site the tRNAs that have been released are recycled by aminoacyl-tRNAsynthetase adding a corresponding aa to them this process repeats itself until the ribosome reaches a STOP codon (UGA, UAG, and UAA)

  16. Translation – Elongation

  17. Translation – Termination • Finally a UAA, UAG, or UGA codon comes up on the mRNA • There are no tRNA’s with these anticodons (they are “stop” codons) • No aa can bond so the protein synthesis terminates • The ribosome disengages from the mRNA and releases the newly assembled polypeptide. • The protein then moves to the ER and golgi for further modification

  18. Posttranslational Modifications • Occurs in ER and Golgi • Polypeptide begins to fold into its 3D shape during formation • Glycosylation– sugar added • Phosphorylation – phosphate added • Quaternary structure - polypeptides may be joined together • Enzymes may • Remove amino acids • Cleave polypeptide chain into 2 or more pieces

  19. Translation – Termination weblink: protein synthesis

  20. Protein packaging 0:10

  21. Proteins made in cytoplasm and on ER 3:00

  22. Polyribosome 0:58

  23. Polyribosomes 0:33

  24. Protein synthesis – A summary • DNA unwinds • mRNA copy is made of one of the DNA strands. • mRNA copy moves out of nucleus into cytoplasm. • tRNA molecules are activated as their complementary amino acids are attached to them. • mRNA copy attaches to the small subunit of the ribosomes in cytoplasm. 6 of the bases in the mRNA are exposed in the ribosome. • A tRNA bonds complementarily with the mRNA via its anticodon. • A second tRNA bonds with the next three bases of the mRNA, the amino acid joins onto the amino acid of the first tRNA via a peptide bond. • The ribosome moves along. The first tRNA leaves the ribosome. • A third tRNA brings a third amino acid • Eventually a stop codon is reached on the mRNA. The newly synthesized polypeptide leaves the ribosome.

  25. Uses of Proteins: 1. Enzymes (nutrition, digestion, all chemical reactions) 2. Structural support (connective tissue, bones, hair, nails) 3. Movement (skeletal muscle, heart muscle)

  26. Genetic Code - Why is it so good? Mutation = change in intended genetic code Mutagen = something causing a mutation eg. Xrays, UV light, chemicals *more in lesson in 2 days 1. Universality - all organisms use the same code (evolved first?) 2. Redundancy - there are many codons for the same amino acid 3. Not Ambiguous - one codon never codes for 2 aa’s 4. Wobble Factor - last position (3rd nucleotide letter) is the leastimportant. It can change (mutate) and still get the same aa. eg. CUU CUA CUC CUG all code for leucine 5. Coherance - chemically similar aa’s have similar codons (less drastic mutations)

  27. Classwork: G9 pkg (with partner) Practice with ziplock (with partner) G11 if time Homework: 5.4 #1-7 pg 254 G8 duo – protein synth Q’s G10 pkg – protein synth worksheet Tomorrow – more protein synth practice. Quiz next day.

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