1. Gene Expression Chapter 13

2. Learning Objective 1 • What early evidence indicated that most genes specify the structure of proteins?

3. Garrod’s Work • Inborn errors of metabolism • evidence that genes specify proteins • Alkaptonuria • rare genetic disease • lacks enzyme to oxidize homogentisic acid • Gene mutation • associated with absence of specific enzyme

4. Alkaptonuria

5. Tyrosine Homogentisic acid Functional enzyme present Functional enzyme absent Disease condition Normal metabolism ALKAPTONURIA Maleylacetoacetate Homogentisic acid excreted in urine; turns black when exposed to air CO2 H2O Fig. 13-1, p. 280

6. Learning Objective 2 • Describe Beadle and Tatum’s experiments with Neurospora

7. Beadle and Tatum • Exposed Neurospora spores • to X-rays or ultraviolet radiation • induced mutations prevented metabolic production of essential molecules • Each mutant strain • had mutation in only one gene • each gene affected only one enzyme

8. Beadle-Tatum Experiments

9. Expose Neurospora spores to UV light or X-rays 1 Each irradiated spore is used to establish culture on complete growth medium (minimal medium plus amino acids, vitamins, etc.) Fungal growth (mycelium) 2 Transfer cells to minimal medium plus amino acids Transfer cells to minimal medium plus vitamins Transfer cells to minimal medium (control) 3 Minimal medium plus arginine Minimal medium plus tryptophan Minimal medium plus lysine Minimal medium plus leucine Minimal medium plus other amino acids Fig. 13-2, p. 281

10. KEY CONCEPTS • Beadle and Tatum demonstrated the relationship between genes and proteins in the 1940s

11. Learning Objective 3 • How does genetic information in cells flow from DNA to RNA to polypeptide?

12. DNA to Protein • Information encoded in DNA • codes sequences of amino acids in proteins • 2-step process: 1. Transcription 2. Translation

13. Transcription • Synthesizes messenger RNA (mRNA) • complementary to template DNA strand • specifies amino acid sequences of polypeptide chains

14. Translation • Synthesizes polypeptide chain • specified by mRNA • also requires tRNA and ribosomes • Codon • sequence of 3 mRNA nucleotide bases • specifies one amino acid • or a start or stop signal

15. DNA to Protein

16. Nontemplate strand ‘ ‘ ‘ Transcription DNA ‘ ‘ mRNA (complementary copy of template DNA strand) Template strand ‘ Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6 Polypeptide Met Thr Cys Glu Cys Phe Translation Fig. 13-4, p. 283

17. KEY CONCEPTS • Transmission of information in cells is typically from DNA to RNA to polypeptide

18. Learning Objective 4 • What is the difference between the structures of DNA and RNA?

19. RNA • RNA nucleotides • ribose (sugar) • bases (uracil, adenine, guanine, or cytosine) • 3 phosphates • RNA subunits • covalently joined by 5′ – 3′ linkages • form alternating sugar-phosphate backbone

20. RNA Structure

21. Uracil Adenine Cytosine Guanine Fig. 13-3, p. 282

22. Learning Objective 5 • Why is genetic code said to be redundant and virtually universal? • How may these features reflect its evolutionary history?

23. Genetic Code • mRNA codons • specify a sequence of amino acids • 64 codons • 61 code for amino acids • 3 codons are stop signals

24. Codons

25. Genetic Code • Is redundant • some amino acids have more than one codon • Is virtually universal • suggesting all organisms have a common ancestor • few minor exceptions to standard code found in all organisms

26. KEY CONCEPTS • A sequence of DNA base triplets is transcribed into RNA codons

27. Learning Objective 6 • What are the similarities and differences between the processes of transcription and DNA replication?

28. Enzymes • Similar enzymes • RNA polymerases (RNA synthesis) • DNA polymerases (DNA replication) • Carry out synthesis in 5′→ 3′ direction • Use nucleotides with 3 phosphate groups

29. Antiparallel Synthesis • Strands of DNA are antiparallel • Template DNA strand and complementary RNA strand are antiparallel • DNA template read in 3′→ 5′ direction • RNA synthesized in 5′→ 3′ direction

30. Antiparallel Synthesis

31. mRNA transcript mRNA transcript Promoter region Promoter region Promoter region 5’ 5’ Gene 2 RNA polymerase 5’ 5’ 3’ 3’ 3’ 3’ 3’ Gene 1 Gene 3 5’ mRNA transcript Fig. 13-9, p. 287

32. Base-Pairing Rules • In RNA synthesis and DNA replication • are the same • excepturacil is substituted for thymine

33. Transcription

34. Growing RNA strand Template DNA strand 5’ end 3’ direction Nucleotide added to growing chain by RNA polymerase 5’ direction 3’end Fig. 13-7, p. 286

35. Learning Objective 7 • What features of tRNA are important in decoding genetic information and converting it into “protein language”?

36. Transfer RNA (tRNA) • “Decoding” molecule in translation • Anticodon • complementary to mRNA codon • specific for 1 amino acid

37. tRNA

38. ’ Loop 3 ’ Hydrogen bonds Loop 1 Loop 2 Anticodon Fig. 13-6a, p. 285

39. OH 3’ end Amino acid accepting end P 5’ end Hydrogen bonds Loop 3 Loop 1 Modified nucleotides Loop 2 Anticodon Fig. 13-6b, p. 285

40. Amino acid (phenylalanine) ‘ ‘ Anticodon Fig. 13-6c, p. 285

41. Transfer RNA (tRNA) • tRNA • attaches to specific amino acid • covalently bound by aminoacyl-tRNA synthetase enzymes

42. Aminoacyl-tRNA

43. AMP+ Phenylalanine + Aminoacyl-tRNA synthetase Anticodon Amino acid tRNA Aminoacyl-tRNA Fig. 13-11, p. 289

44. AMP+ Phenylalanine + Aminoacyl-tRNA synthetase Anticodon Amino acid Aminoacyl-tRNA tRNA Stepped Art Fig. 13-11, p. 289

45. Learning Objective 8 • How do ribosomes function in polypeptide synthesis?

46. Ribosomes • Bring together all machinery for translation • Couple tRNAs to mRNA codons • Catalyze peptide bonds between amino acids • Translocate mRNA to read next codon

47. Ribosomal Subunits • Each ribosome is made of • 1 large ribosomal subunit • 1 small ribosomal subunit • Each subunit contains • ribosomal RNA (rRNA) • many proteins

48. Ribosome Structure

49. Front view Large subunit E P A Ribosome Small subunit Fig. 13-12a, p. 290

50. Large ribosomal subunit E site P site A site Small ribosomal subunit mRNA binding site Fig. 13-12b, p. 290