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Protein Sythesis

Protein Sythesis. The Genome of a Eukaryotic Cell is within the nucleus. Proteins are synthesized in the cytoplasm . DNA does not leave the nucleus. Why? 1) To dangerous! If distorted, cleaved, or damaged  useless Possible death of organism 2) Proteins required in large amounts

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Protein Sythesis

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  1. Protein Sythesis

  2. The Genome of a Eukaryotic Cell is within the nucleus.Proteins are synthesized in the cytoplasm • DNA does not leave the nucleus. Why? 1) To dangerous! • If distorted, cleaved, or damaged  useless • Possible death of organism 2) Proteins required in large amounts • Only 2 strands of DNA • Can be many strands of different RNA chains (coding for different proteins) • Increases flexibility in carrying out various activities. 3) Leaving/re-entering the nucleus would be time-consuming.

  3. Central Dogma • Transcription (copy from one medium to another) • DNA  RNA • Translation (convert into a different language) • RNA  polypeptide Dogma: “proclaimed as true without perfect proof” Science is a study which examines ‘truth’ down to its core. As a scientist, you can never prove anything as perfectly true. You can observe and study millions of different species or processes within species... up to the finest parts that make up matter. But a real scientist will never stop trying to find out more. The more you learn, the more there is to know.

  4. RNA • Ribonucleic Acid (RNA) • How does RNA differ from DNA again? • Three Kinds • mRNA: messenger • Longer the gene, the longer the mRNA • tRNA: transfer • Transfers appropriate AA to ribosome to build a PP • Short: 70-90 ribonucleotides • rRNA: ribosomal • Structural component of Ribosome.

  5. The Genetic Code • There are _____ amino acids. • How many nucleotides must code for one amino acid? • If only one nucleotide coded for one AA... • 41 = 4 possible combinations • Ex// thymine  cysteine • If two nucleotides coded for one AA... • ________ possible combinations • Ex// AC  cysteine • If three nucleotides code for one AA... • _________ possible combinations • Ex// CAU  histidine • Codon: sequence of three bases in DNA or complementary mRNA that serves as a code for a particular amino acid. • More than one codon can code for a single amino acid (redundancy).

  6. Start Codon: signals initiation of translation. • Only one in the genetic code • Is usually AUG  methionine • Stop Codon: does not code for an amino acid: signals termination of translation. • Three of these in the genetic code • UAA, UAG, UGA

  7. Seatwork/Homework • Pg. 241, #1--13

  8. TRANSCRIPTION (5.3) Initiation • RNA polymerase binds to specific part of DNA that is to be transcribed  helix opens. • Specific. How? • Transcription factors help polymerase to bind  these factors help control which genes are transcribed. • Bound upstream of the gene: promoter region. • Usually high in T and A • Less energy to unwind helix • RNA polymerase only recognizes the promoter region (therefore, not random!) • This region is not transcribed

  9. Elongation • RNA polymerase builds mRNA in 5’ – 3’ direction • Does not require a primer • RNA poly. Uses one strand of DNA for mRNA synthesis • Template strand: strand of DNA used by RNA polymerase to build complimentary mRNA. • mRNA complimentary to this DNA strand • Coding strand: not being used for transcription. • mRNA identical to the DNA strand (except for uracil)

  10. Termination • mRNA synthesized until the end of gene is reached • Terminator sequence: sequence of bases at end of a gene that signals the RNA polymerase to stop transcribing • Differs between prokaryotes and eukaryotes • mRNA dissociates with DNA • RNA polymerase free to bind to another promoter region.

  11. Posttranscriptional Modifications • The mRNA made by transcription is called the primary transcript. • Modifications need to be made to RNA before it can leave the nucleus • CAPPING AND TAILING • SPLICING

  12. Capping and Tailing • 5’ cap: 7-methyl guanosine • Added to start of primary transcript • Protects it from digestion by nucleases and phosphatases • Helps bind mRNA to ribosome • Poly-A tail: approx. 200 adenine ribonucleotides is added to 3’ end by poly-A polymerase. • Slowly degraded over time • When ‘used up,’ mRNA is degraded

  13. Splicing • DNA of eukaryotic gene comprises of two intermingling regions • Exons: coding regions • Introns: noncoding regions • If these regions are translated, protein will not function properly. • Removed from primary transcript by spliceosomes • Introns stay within nucleus  degraded  nucleotides recycled.

  14. Primary transcript  capped/tailed and intronsexiced  mRNA transcript. • No quality control enzyme! • Errors not as detrimental to the cell (multiple copies of mRNA) • If error made during transcription, protein usually useless and will be degraded.

  15. Summary of Transcription

  16. Seatwork/Homework • Page 249 • #1-6, 8-9.

  17. TRANSLATION • Initiation • Ribosome recognizes the 5’ cap in eukaryotes and binds to mRNA. • Elongation • Ribosome moves along mRNA in 5’-3’ direction. • Amino acids are added to the growing PP • Codon AA.

  18. Reading Frame • Reading Frame: how the mRNA is read. • Three possibilities

  19. The Role of tRNA • tRNA: single stranded nucleic acid • Transfers AAs to ribosome • Anticodon: sequence of three bases on one arm of the tRNA. • Recognizes the codon of mRNA (complimentary) • If mRNA codon is AUA, anticodon is _____. • tRNA + amino acid = aminoacyl-tRNA. • Aminnoacyl-tRNAsynthetase: enzyme responsible for adding AA to tRNA. • At least 20 of these enzymes.

  20. What must be the codon coding for tyrosine?_____

  21. Wobble Hypothesis • Every tRNA transfers only ONE specific AA. Therefore, there must be AT LEAST ____ different tRNA molecules. • Between 20-64 tRNAs: depends on organism. • Third base may differ between two codons, but may code for the same AA. • Ex// UAU and UAC  tyrosine. • If tRNA’santicodon is UAU, can still bind to UAC codon. • tRNA can recognize more than one codon by unusual pairing • Advantage: correct amino acid may be added despite errors made from DNA  mRNA. • WOBBLE HYPOTHESIS: the proposal that tRNA can recognize more than one codon by unusual base pairing b/n 1st base of anticodon and 3rd base of codon.

  22. Elongation of the Polypeptide • Start codon: _______. Every protein initially begins with the AA _______________. • Ribosome has two sites for tRNA • A (acceptor) site: site that tRNA brings amino acid to. • P (peptide) site: peptide bonds are formed between adjoining AAs. • tRNA carrying Methionine enters P site first. Next appropriate AA will enter the A site. Met + AA peptide bind.

  23. Methionine is the start codon. b) Second tRNA enters A site. • Ribosome TRANSLOCATES one codon over, and next tRNA enters A site. • Bond, translocation, and addition of new tRNA. • Repeat d. Stop codon does not code for an AA. • Release-factor protein helps dismantle the ribosome-mRNA complex. New PP is released!

  24. Termination of Protein Synthesis (as seen in step f) of previous slide) • Ribosome eventually reaches a stop codon. • Ribosome stalls • Release Factor Protein: aids in the release of the PP chain from the ribosome. • Subunits fall off of mRNNA. • Glycosylation: sugars may be added to PP. • Phosphorylation: addition of phosphates. • PP may also be cleaved (cut) afterward • Ex// methionine may not be the first AA anyore.

  25. Summary of Translation

  26. Seatwork/Homework • Page 254 #1-4, 6-7, 9

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