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From DNA to Protein: Genotype to Phenotype

From DNA to Protein: Genotype to Phenotype. 12 From DNA to Protein: Genotype to Phenotype. Review 12.1 What Is the Evidence that Genes Code for Proteins? 12.2 How Does Information Flow from Genes to Proteins? 12.3 How Is the Information Content in DNA Transcribed to Produce RNA?

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From DNA to Protein: Genotype to Phenotype

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  1. From DNA to Protein: Genotype to Phenotype

  2. 12 From DNA to Protein: Genotype to Phenotype • Review • 12.1 What Is the Evidence that Genes Code for Proteins? • 12.2 How Does Information Flow from Genes to Proteins? • 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? • 12.4 How Is RNA Translated into Proteins? • 12.5 What Happens to Polypeptides after Translation? • 12.6 What Are Mutations?

  3. 12-1 Recap: • Beadle & Tatum’s studies of mutations in bread molds led to the one gene-one polypeptide hypothesis: • The function of a gene is to code for a specific polypeptide

  4. 12-1 Recap ?s: • What’s a “model organism”? • Why was Neurospora a good model for studying biochemical genetics? • How were Beadle and Tatum’s expts set up to determine, on the basis of phenotypes of mutant strains, the order of a biochemical pathway? • What’s the distinction between “protein” and “polypeptide”?

  5. 12.1 What Is the Evidence that Genes Code for Proteins? The molecular basis of phenotypes was known before it was known that DNA is the genetic material. Studies of many different organisms showed that major phenotypic differences were due to specific proteins.

  6. 12.1 What Is the Evidence that Genes Code for Proteins? Model organisms: easy to grow or observe; show the phenomenon to be studied results from one organism applied to others Examples: pea plants, Drosophila, E. coli, common bread mold Neurospora crassa

  7. 12.1 What Is the Evidence that Genes Code for Proteins? Bread mold experiments: Suggested the one-gene, one-enzyme hypothesis

  8. Figure 12.1 One Gene, One Enzyme (Part 1)

  9. Figure 12.1 One Gene, One Enzyme (Part 2) each mutant was missing a single enzyme in the pathway.

  10. 12.1 What Is the Evidence that Genes Code for Proteins? The gene-enzyme relationship has been revised to the one-gene, one-polypeptide relationship. Example: In hemoglobin, each polypeptide chain is specified by a separate gene. Other genes code for RNA that is not translated to polypeptides; some genes are involved in controlling other genes.

  11. 12-2 Recap: • Central Dogma of molec bio: • DNA codes for the production of RNA, RNA codes for the production of polypeptides (proteins) • Proteins do NOT code for the production of protein, DNA, or RNA • Transcription = process that copies a DNA sequence into mRNA • Translation = process by which this info is converted into protein • tRNA recognizes the genetic info in mRNA and brings the appropriate amino acid into position in a growing polypeptide chain

  12. 12-2 Recap ?s: • What’s that central dogma all about? • What are the roles of mRNA and tRNA in gene expression?

  13. 12.2 How Does Information Flow from Genes to Proteins? Expression of a gene to form a polypeptide: Transcription—copies information from gene to a sequence of RNA. Translation—converts RNA sequence to amino acid sequence.

  14. 12.2 How Does Information Flow from Genes to Proteins? RNA, ribonucleic acid differs from DNA: Usually one strand The sugar is ribose Contains uracil (U) instead of thymine (T)

  15. 12.2 How Does Information Flow from Genes to Proteins? RNA can pair with a single strand of DNA, except that adenine pairs with uracil instead of thymine. RNA (CODONS) Practice! DNA  RNA

  16. Figure 12.2 The Central Dogma The central dogma of molecular biology: information flows in one direction when genes are expressed (Francis Crick).

  17. 12.2 How Does Information Flow from Genes to Proteins? The central dogma raised two questions: How does genetic information get from the nucleus to the cytoplasm? What is the relationship between a DNA sequence and an amino acid sequence?

  18. 12.2 How Does Information Flow from Genes to Proteins? One hypothesis— messenger RNA (mRNA) forms as a complementary copy of DNA and carries information to the cytoplasm. This process is transcription.

  19. Figure 12.3 From Gene to Protein

  20. 12.2 How Does Information Flow from Genes to Proteins? Other hypothesis—an adapter molecule that can bind amino acids, and recognize a nucleotide sequence—transfer RNA (tRNA). tRNA molecules carrying amino acids line up on mRNA in proper sequence for the polypeptide chain—translation.

  21. 12.2 How Does Information Flow from Genes to Proteins? Exception to the central dogma: Viruses: acellular particles that reproduce inside cells; many have RNA instead of DNA.

  22. 12.2 How Does Information Flow from Genes to Proteins? Synthesis of DNA from RNA is reverse transcription. Viruses that do this are retroviruses.

  23. 12-3 Recap: • Transcription (catalyzed by an RNA polymerase) proceeds in 3 steps: • Initiation, elongation, termination • Genetic code relates the information in mRNA (linear sequence of codons) to protein (linear sequence of amino acids)

  24. 12-3 Recap ?s: • What are the steps of gene transcription (producing mRNA)? • How do RNA polymerases work? • How was the genetic code deciphered?

  25. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? Within each gene, only one strand of DNA is transcribed—the template strand. Transcription produces mRNA; the same process is used to produce tRNA and rRNA.

  26. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? RNA polymerases catalyze synthesis of RNA. single enzyme-template binding results in polymerization of hundreds of RNA bases.

  27. Figure 12.4 RNA Polymerase

  28. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? Transcription occurs in three phases: Initiation Elongation Termination

  29. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? Initiation requires a promoter—a special sequence of DNA. RNA polymerase binds to it. tells RNA polymerase where to start, which direction to go in, and which strand of DNA to transcribe. Part of it is the initiation site.

  30. Figure 12.5 DNA Is Transcribed to Form RNA RNA polymerase binds to the promoter and starts to unwind the DNA strands. RNA polymerase Initiation site Termination site Complementary strand Rewinding of DNA Template strand Unwinding of DNA Promoter 3′ 3′ 5′ 5′

  31. Figure 12.5 DNA Is Transcribed to Form RNA RNA polymerase reads the DNA template strand from 3′ to 5′ and produces the RNA transcript by adding nucleotides to the 3′ end. 5′ 3′ 5′ 3′ 3′ 3′ 5′ 5′ 5′ RNA transcript 3′ 3′ 5′ 5′ Direction of transcription Nucleoside triphosphates (A, U, C, G)

  32. Figure 12.5 DNA Is Transcribed to Form RNA When RNA polymerase reaches the termination site, the RNA transcript is set free from the template. 3′ 3′ 5′ 5′ 5′ 3′ RNA

  33. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? Elongation: RNA polymerase unwinds DNA about 10 base pairs at a time; reads template in 3′ to 5′ direction. The RNA transcript is antiparallel to the DNA template strand. RNA polymerases do not proofread and correct mistakes.

  34. Figure 12.5 DNA Is Transcribed to Form RNA (B)

  35. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? Termination: specified by a specific DNA sequence. Mechanisms of termination are complex and varied. Eukaryotes—first product is a pre-mRNA that is longer than the final mRNA and must undergo processing.

  36. Figure 12.5 DNA Is Transcribed to Form RNA (C)

  37. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? The genetic code: specifies which amino acids will be used to build a protein Codon: a sequence of three mRNA bases. Each specifies a particular amino acid. Start codon: AUG—initiation signal for translation Stop codons: stops translation and polypeptide is released

  38. Figure 12.6 The Genetic Code AUG UAC CAU UUA GCC AUC AAC UUU UAC UAU AAU UGA

  39. ANIMATIONS! • Transcription • Deciphering the Genetic Code

  40. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? For most amino acids, there is more than one codon; the genetic code is redundant. But not ambiguous— each codon specifies only one amino acid.

  41. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? The genetic code is nearly universal: codons that specify amino acids are the same in all organisms!! Exceptions: within mitochondria and chloroplasts, and in one group of protists.

  42. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? This common genetic code is a common language for evolution. Ancient; has remained intact also facilitates genetic engineering

  43. 12.3 How Is the Information Content in DNA Transcribed to Produce RNA? How was the code deciphered? 20 “code words” (amino acids) are written with only four “letters.” Triplet code seemed likely: could account for 4 × 4 × 4 = 64 codons.

  44. 12-4 Recap: • Key step in protein synthesis: attachment of amino acid to proper tRNA (activating enzyme) • Translation of genetic info from mRNA into protein occurs @ ribosome • Multiple ribosomes may act on a single mRNA to make multiple copies of the protein for which it codes

  45. 12-4 Recap ?s: • How is an amino acid attached to a specific tRNA? • Why’s it called “second genetic code”? • Describe initiation, elongation, and termination of translation.

  46. 12.4 How Is RNA Translated into Proteins? tRNA: for each amino acid, there’s a specific type Functions: Carries an amino acid Associates with mRNA molecules Interacts with ribosomes

  47. Figure 12.8 Transfer RNA

  48. Video: tRNA

  49. 12.4How Is RNA Translated into Proteins? The conformation (3D shape) of tRNA results from base pairing (H bonds) within the molecule. 3′ end is the amino acid attachment site—binds covalently. Always CCA. Anticodon: site of base pairing with mRNA. Unique for each tRNA.

  50. Figure 12.9 Charging a tRNA Molecule tRNA The enzyme activates the amino acid, catalyzing reaction with ATP to form high energy AMP–amino acid and a pyrophosphate ion. Activating enzyme Amino acid site ATP site Activated alanine tRNA site Alanine Activating enzyme (aminoacyl-tRNA synthase) for a specific amino acid Charged tRNA tRNA bonded to alanine The enzyme then catalyzes a reaction of the activated amino acid with the correct tRNA. The charged tRNA will deliver the appropriate amino acid to join the elongating polypeptide product of translation. The specificity of the enzyme ensures that the correct amino acid and tRNA have been brought together. Pi Specific amino acid(e.g., alanine) Pyrophosphate (PPi) Alanine-specific tRNA

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