Chapter 14From DNA to Protein: Gene      Expression
Gene ExpressionPath from a gene to a   Gene expression occurs                          in two steps:  phenotype           ...
RNARNA (ribonucleic acid) differs from DNA:• Usually one  polynucleotide  strand• The sugar is ribose• Contains uracil (U)...
RNABases in RNA can pair with a single strand of DNA, except that adenine pairs with uracil instead of thymine.Single-stra...
Three Types of RNAMessenger RNA (mRNA)  Carries copy of a DNA sequence to site  of protein synthesis at the ribosomeTransf...
DNA RNA Protein
Transcription (DNA RNA)Transcription components:A DNA template for base pairings (one of  the two strands of DNA)Nucleoti...
RNA Polymerase• Catalyze the  Synthesis of RNA• Do NOT need a  primer to start  synthesizing RNA• Do NOT have  proofreadin...
Phases of TranscriptionInitiationElongationTermination
InitiationRNA Polymerase binds to the target DNA  at the promoterUnwinds a ~10bp bubble of the DNA to  initiate transcript...
PromoterSequence of DNA where RNA polymerase bindsCritical Regulation Point for the Flow of Genetic Information in the Cel...
ElongationRNA polymerase unwinds DNA about ten base pairs at a time; reads template in 3′ to 5′ direction.The RNA transcri...
Elongation
TerminationIs specified by a specific DNA base  sequence (termination site).Mechanisms of termination are complex  and var...
Following TranscriptionThe completed mRNA leaves the nucleus through the nuclear poreMust interact with a ribosome to init...
Genetic CodeSpecifies which amino acids will be used to build a proteinCodon:A sequence of three bases—each codon  specifi...
Genetic Code20 amino acids64 different  possible  codonsCodons are  redundantGenetic Code is  Universal
TranslationNeeds:mRNA template“charged tRNAs”Ribosome
tRNAsTransfer RNAs act as the adapter  between the mRNA and the growing  polypeptideMust be “charged” or bound to a specif...
tRNAsClover leaf secondary  structureTwo important sites:Anticodon  Binds to mRNAAmino Acid Attachment Site  Binds to the ...
RibosomeThe workbench that holds mRNA and charged tRNAs in the correct positions to allow assembly of polypeptide chain.Ri...
Large SubunitThree tRNA binding sites:A (amino acid) site binds   with anticodon of   charged tRNAP (polypeptide) site is ...
Small SubunitHas a fidelity function: When proper binding occurs, hydrogen bonds form between the base pairs.
Phases of Translation•   Initiation•   Elongation•   Termination
InitiationInitiation Complex  Forms1) mRNA2) Charged tRNA  (bound to  methionine)3) Small subunit  bound to the mRNA  at t...
InitiationThe large subunit then  joins the complex;The charged tRNA is  now in the P site of  the large subunit.
ElongationThe second charged tRNA  enters the A site.Large subunit catalyzes  two reactions:• It breaks bond between  tRNA...
Elongation• The large subunit  has peptidyl  transferase activity• Critical to growing  the polypeptide  chain
ElongationWhen the first tRNA has released its methionine, it moves to the E site and dissociates from the ribosome—can th...
TerminationTranslation ends when a  stop codon enters the A  site.Stop codon binds a  protein release factor—  allows hydr...
Post-Translational          ModificationsProteolysis: Cutting of a long polypeptide chain into final products, by protease...
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Chapter 14

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Chapter 14

  1. 1. Chapter 14From DNA to Protein: Gene Expression
  2. 2. Gene ExpressionPath from a gene to a Gene expression occurs in two steps: phenotype Transcription“Central Dogma of Copies information from a Molecular Biology” DNA sequence (a gene) to a complementary RNA sequenceDNA RNA Protein Translation Converts RNA sequence to amino acid sequence of a polypeptide
  3. 3. RNARNA (ribonucleic acid) differs from DNA:• Usually one polynucleotide strand• The sugar is ribose• Contains uracil (U) instead of thymine (T)
  4. 4. RNABases in RNA can pair with a single strand of DNA, except that adenine pairs with uracil instead of thymine.Single-strand RNA can fold into complex shapes by internal base pairing.
  5. 5. Three Types of RNAMessenger RNA (mRNA) Carries copy of a DNA sequence to site of protein synthesis at the ribosomeTransfer RNA (tRNA) Carries amino acids for polypeptide assemblyRibosomal RNA (rRNA) Catalyzes peptide bonds and provides structure
  6. 6. DNA RNA Protein
  7. 7. Transcription (DNA RNA)Transcription components:A DNA template for base pairings (one of the two strands of DNA)Nucleotides (ATP,GTP,CTP,UTP) as substratesRNA polymerase enzyme
  8. 8. RNA Polymerase• Catalyze the Synthesis of RNA• Do NOT need a primer to start synthesizing RNA• Do NOT have proofreading ability
  9. 9. Phases of TranscriptionInitiationElongationTermination
  10. 10. InitiationRNA Polymerase binds to the target DNA at the promoterUnwinds a ~10bp bubble of the DNA to initiate transcriptionTranscription begins at the initiation site
  11. 11. PromoterSequence of DNA where RNA polymerase bindsCritical Regulation Point for the Flow of Genetic Information in the CellTells RNA polymerase “When, Where, How Much”
  12. 12. ElongationRNA polymerase unwinds DNA about ten base pairs at a time; reads template in 3′ to 5′ direction.The RNA transcript is antiparallel to the DNA template strand, and adds nucleotides to its 3′ end.RNA polymerases do not proofread and correct mistakes.
  13. 13. Elongation
  14. 14. TerminationIs specified by a specific DNA base sequence (termination site).Mechanisms of termination are complex and varied.For some genes the transcript falls away from the DNA template and RNA polymerase (others require another protein to assist)
  15. 15. Following TranscriptionThe completed mRNA leaves the nucleus through the nuclear poreMust interact with a ribosome to initiate translationInformation encoded on the mRNA is the utilized to guide protein synthesis
  16. 16. Genetic CodeSpecifies which amino acids will be used to build a proteinCodon:A sequence of three bases—each codon specifies a particular amino acid.Start codon: AUGInitiation signal for translation.Stop codons: UAA, UAG, UGAStop translation and polypeptide is released.
  17. 17. Genetic Code20 amino acids64 different possible codonsCodons are redundantGenetic Code is Universal
  18. 18. TranslationNeeds:mRNA template“charged tRNAs”Ribosome
  19. 19. tRNAsTransfer RNAs act as the adapter between the mRNA and the growing polypeptideMust be “charged” or bound to a specific amino acid before they can be used in translation
  20. 20. tRNAsClover leaf secondary structureTwo important sites:Anticodon Binds to mRNAAmino Acid Attachment Site Binds to the respective amino acid
  21. 21. RibosomeThe workbench that holds mRNA and charged tRNAs in the correct positions to allow assembly of polypeptide chain.Ribosomes are not specific, they can make any type of protein.Composed of a Large and Small Subunit
  22. 22. Large SubunitThree tRNA binding sites:A (amino acid) site binds with anticodon of charged tRNAP (polypeptide) site is where tRNA adds its amino acid to the growing chainE (exit) site is where tRNA sits before being released from the ribosome.
  23. 23. Small SubunitHas a fidelity function: When proper binding occurs, hydrogen bonds form between the base pairs.
  24. 24. Phases of Translation• Initiation• Elongation• Termination
  25. 25. InitiationInitiation Complex Forms1) mRNA2) Charged tRNA (bound to methionine)3) Small subunit bound to the mRNA at the start codon
  26. 26. InitiationThe large subunit then joins the complex;The charged tRNA is now in the P site of the large subunit.
  27. 27. ElongationThe second charged tRNA enters the A site.Large subunit catalyzes two reactions:• It breaks bond between tRNA in P site and its amino acid• Peptide bond forms between that amino acid and the amino acid on tRNA in the A site
  28. 28. Elongation• The large subunit has peptidyl transferase activity• Critical to growing the polypeptide chain
  29. 29. ElongationWhen the first tRNA has released its methionine, it moves to the E site and dissociates from the ribosome—can then become charged again.Elongation occurs as the steps are repeated, assisted by proteins called elongation factors.
  30. 30. TerminationTranslation ends when a stop codon enters the A site.Stop codon binds a protein release factor— allows hydrolysis of bond between polypeptide chain and tRNA on the P site.Polypeptide chain separates from the
  31. 31. Post-Translational ModificationsProteolysis: Cutting of a long polypeptide chain into final products, by proteasesGlycosylation: Addition of sugars to form glycoproteinsPhosphorylation: Addition of phosphate groups catalyzed by protein kinases— charged phosphate groups change the conformationLeads to a mature and functional protein

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