1. Chapter 8
Microbial Genetics

2. Q&A
E. coli is found naturally in the human large intestine, and there it is beneficial. However, the strain designated E. coli O157:H7 produces Shiga toxin. How did E. coli acquire this gene from Shigella?

3. Learning Objectives
Upon completing this chapter, you should be able to:
Define genetics, genome, chromosome, gene, genetic code, genotype, phenotype, and genomics
Describe how DNA serves as genetic information
Describe the process of DNA replication
Describe protein synthesis, including transcription and translation
Compare protein synthesis in prokaryotes and eukaryotes
Explain the regulation of gene expression in bacteria by induction, repression, and catabolite repression
Describe how the Lac operon works
Classify mutations by type
Define mutagen and provide examples
Describe 2 ways in which mutations can be repaired
Outline the methods of direct and indirect selection of mutants
Define horizontal and vertical gene transfer
Compare the mechanisms of genetic recombination in bacteria
Describe the functions and usefulness of plasmids and transposons
Define translation, transduction, and conjugation and describe how each can occur
Discuss how genetic mutation and recombination provide material for natural selection to act upon

4. Terminology
Genetics: The study of what genes are, how they carry information, how information is expressed, and how genes are replicated
Gene: A segment of DNA that encodes a functional product, usually a protein
Chromosome: Structure containing DNA that physically carries hereditary information; the chromosomes contain the genes
Genome: All the genetic information in a cell

5. Terminology Genomics: The molecular study of genomes
Genotype: The genes of an organism (genetic makeup)
Phenotype: Expression of the genes (physical appearance)

6. E. coli
Figure 8.1a

7. Genetic Map of the Chromosome of E. coli
Figure 8.1b

8. The Flow of Genetic Information
Figure 8.2

9. DNA Polymer of nucleotides: Adenine, thymine, cytosine, and guanine
Double helix associated with proteins
"Backbone" is deoxyribose-phosphate
Strands are held together by hydrogen bonds between AT and CG
Strands are antiparallel (complementary)
Figure 8.3b

10. Semiconservative Replication
Figure 8.3a

11. DNA Synthesis
Figure 8.4

12. DNA Synthesis DNA is copied by DNA polymerase In the 5'  3' direction
Initiated by a primer
Leading strand is synthesized continuously
Lagging strand is synthesized discontinuously
Forms Okazaki fragments
primers are removed and Okazaki fragments joined by a DNA ligase

13. Table 8.1

14. Table 8.1

15. DNA Synthesis
Figure 8.5

16. Replication of Bacterial DNA
Figure 8.6

17. Replication of Bacterial DNA
ANIMATION DNA Replication: Overview
ANIMATION DNA Replication: Forming the Replication Fork
ANIMATION DNA Replication: Replication Proteins

18. Transcription DNA is transcribed to make RNA (mRNA, tRNA, and rRNA)
Transcription begins when RNA polymerase binds to the promoter sequence
Transcription proceeds in the 5'  3' direction
Transcription stops when it reaches the terminator sequence

19. Transcription
Figure 8.7

20. The Process of Transcription
Figure 8.7

21. The Process of Transcription
ANIMATION Transcription: Overview
ANIMATION Transcription: Process
Figure 8.7

22. RNA Processing in Eukaryotes
Figure 8.11

23. Translation mRNA is translated in codons (three nucleotides)
Translation of mRNA begins at the start codon: AUG
Translation ends at nonsense codons: UAA, UAG, UGA
Figure 8.2

24. The Genetic Code
64 codons (61 are sense and 3 are nonsense) on mRNA encode the 20 amino acids
The genetic code is degenerate
tRNA carries the complementary anticodon
Figure 8.2

25. The Genetic Code ANIMATION Translation: Overview
ANIMATION Translation: Genetic Code
ANIMATION Translation: Process

26. The Genetic Code
Figure 8.8

27. Bacteria Undergo Simultaneous Transcription & Translation
Figure 8.10

28. The Process of Translation
Figure 8.9

29. The Process of Translation
Figure 8.9

30. The Process of Translation
Figure 8.9

31. The Process of Translation
Figure 8.9

32. The Process of Translation
Figure 8.9

33. The Process of Translation
Figure 8.9

34. The Process of Translation
Figure 8.9

35. The Process of Translation
Figure 8.9

36. Regulation Constitutive genes are expressed at a fixed rate
Other genes are expressed only as needed
Repressible genes
Inducible genes
Catabolite repression

37. Operon
ANIMATION Operons: Overview
Figure 8.12

38. Induction- the process of turning on a gene - Repressor sits on operator; need an inducer to bind to repressor so the repressor moves, thus allowing gene expression.
Figure 8.12

39. Induction
Figure 8.12

40. Repression- inhibits gene expression and production of enzymes - the operon is on, synthesizing a gene product (tryptophan)
Figure 8.13

41. Repression – once tryptophan is synthesized, it binds to the inactive repressor protein, making it active. It binds to the operator, so synthesis is turned off.
ANIMATION Operons: Induction
ANIMATION Operons: Repression
Figure 8.13

42. Catabolite Repression
(a) Growth on glucose or lactose alone
(b) Growth on glucose and lactose combined
Figure 8.14

43. Lactose present, no glucose = lac operon on
Lactose + glucose present = lac operon off
Figure 8.15

44. Mutation A change in the genetic material
Mutations may be neutral, beneficial, or harmful
Mutagen: Agent that causes mutations
Spontaneous mutations: Occur in the absence of a mutagen

45. Mutation Base substitution (point mutation) Change in one base
Missense mutation
Change in one base
Result in change in amino acid
Figure 8.17a, b

46. Mutation
Nonsense mutation
Results in a nonsense codon
Figure 8.17a, c

47. Mutation Frameshift mutation
Insertion or deletion of one or more nucleotide pairs
Figure 8.17a, d

48. The Frequency of Mutation
Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes
Mutagens increase to 10–5 or 10–3 per replicated gene
ANIMATION Mutations: Types

49. Chemical Mutagens - Analogs
Figure 8.19a

50. Chemical Mutagens - Analogs
ANIMATION Mutagens
Figure 8.19b

51. Radiation
Ionizing radiation (X rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose-phosphate backbone

52. Radiation
UV radiation causes thymine dimers
Figure 8.20

53. Repair Photolyases separate thymine dimers Nucleotide excision repair
ANIMATION Mutations: Repair
Figure 8.20

54. Selection
Positive (direct) selection detects mutant cells because they grow or appear different
Example: finding those microbes that are resistant to an antibiotic by growing cells on a plate containing the antibiotic biotic.
Negative (indirect) selection detects mutant cells because they do not grow (looking for auxotrophs)
Replica plating

55. Replica Plating
Figure 8.21

56. Checking for possible mutagens or carcinogens: use the Ames Test
Figure 8.22

57. Checking for possible mutagens or carcinogens: use the Ames Test
Figure 8.22

58. Genetic Transfer and Recombination
1. Differentiate horizontal and vertical gene transfer.
2. Compare the mechanisms of genetic recombination in bacteria.
3. Describe the functions of plasmids and transposons.

59. Genetic Recombination
Vertical gene transfer: Occurs during reproduction between generations of cells (parent to offspring).
Horizontal gene transfer: The transfer of genes between cells of the same generation (cell to cell).
ANIMATION Horizontal Gene Transfer: Overview

60. Genetic Recombination
Exchange of genes between two DNA molecules
Crossing over occurs when two chromosomes break and rejoin
Figure 8.23

61. Genetic Recombination
Figure 8.25

62. Genetic Transformation- naked DNA is picked up an incorporated into another cell
ANIMATION Transformation
Figure 8.24

63. Bacterial Conjugation
Figure 8.26

64. Conjugation in E. coli
Figure 8.27a

65. Conjugation in E. coli
Figure 8.27b

66. Conjugation in E. coli ANIMATION F Factor ANIMATION Chromosome Mapping
ANIMATION Conjugation: Overview
ANIMATION Hfr Conjugation
Figure 8.27c

67. Transduction by a Bacteriophage
ANIMATION Generalized Transduction
ANIMATION Specialized Transduction
Figure 8.28

68. Q&A
E. coli is found naturally in the human large intestine, and there it is beneficial. However, the strain designated E. coli O157:H7 produces Shiga toxin. How did E. coli acquire this gene from Shigella?

69. Plasmids
Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid
Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds
R factors: Encode antibiotic resistance

70. R Factor, a Type of Plasmid mer = mercury resistance sul = sulfonamide resistance str = streptomycin resistance cml = chloramphenicol resistance tet = tetracycline resistance
Figure 8.29

71. Transposons
Segments of DNA that can move from one region of DNA to another
Contain insertion sequences for cutting and resealing DNA (transposase)
Complex transposons carry other genes, such as antibiotic resistance
Figure 8.30a, b

72. Transposons
Figure 8.30c

73. Transposons ANIMATION Transposons: Overview
ANIMATION Transposons: Insertion Sequences
ANIMATION Transposons: Complex Transposons

74. Genes and Evolution
Genetic mutation and recombination provide material for natural selection to act upon!!!!

75. Genes and Evolution Mutations and recombination provide diversity
Fittest organisms for an environment are selected by natural selection

76. Evolution - The evolution of WNV
Clinical Focus, p. 223

77. Evolution Which strain is more closely related to the Uganda strain?
How did the virus change?
Strain
% Similar to Uganda
Kenya
71%
U.S.
51%