microbial genetics

1. MICROBIAL GENETICS
BY
DR JAWAD NAZIR
ASSISTANT PROFESSOR
DEPARTMENT OF MICROBIOLOGY
UNIVERSITY OF VETERINARY AND ANIMAL SCIENCES, LAHORE

2. Microbial genetics
Terminology
Genetics Study of what genes are, how they
carry information, how information is
expressed, and how genes are
replicated
Gene Segment of DNA that encodes a
functional product, usually a protein

3. Microbial genetics
Terminology
Genome
 All of the genetic material in a cell
Genomics
 Molecular study of genomes
Genotype
 Genetic make up of an organism (potential properties)
 Collection of genes
Phenotype
 Expression of the genes
 Collection of proteins

4. Microbial genetics
Chromosome (E. coli)
• 4.6 million base pairs
• 1 meter long
• 1000 times longer than
bacteria

5.  Polymer of nucleotides:
adenine, thymine,
cytosine, guanine
 Double helix associated
with proteins
 "Backbone" is
deoxyribose-phosphate
 Strands held together
by hydrogen bonds
between AT and CG
 Strands are antiparallel
DNA

6. Microbial genetics
 Consist of nucleotides.
 Nucleotides consist of a:
• Pentose
• Phosphate group
• Nitrogen-containing (purine or pyrimidine) base
Nucleic Acids

7. Microbial genetics
Flow of Genetic Information

8. DNA replication

9. Microbial genetics
DNA replication
 Two parent strands unwound
 DNA polymerase
 Replication fork
 Semiconservative replication
 Each parent strand make complementary strand

10. Microbial genetics
 DNA is copied by DNA polymerase
 In the 5  3 direction
 Initiated by an RNA primer
 Leading strand synthesized continuously
 Lagging strand synthesized discontinuously
 Okazaki fragments
 RNA primers are removed and Okazaki fragments joined
by a DNA polymerase and DNA ligase
DNA replication

11. DNA replication

12. Microbial genetics
Adding nucleotides to a DNA

13. Microbial genetics
DNA replication
Bidirectional
Closed loop DNA

14. Microbial genetics
 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
Transcription

15. Microbial genetics
Transcription

16. Transcription

17. Transcription

18. Transcription

19. Microbial genetics
 Process of protein synthesis from mRNA strand
 mRNA is translated in codons (3 nucleotides)
 Translation of mRNA begins at the start codon: AUG
 Translation ends at a STOP codon: UAA, UAG, UGA
 Sense codons (61)
 Nonsense codons (3) stop codons
 UAA, UAG, UGA
Translation

20. Translation

21. Translation

22. Translation

23. Translation

24. Translation

25. Translation

26. Translation

27. Translation

28. 28
Translation

29. Translation

30. Polyribosome

31. Microbial genetics
RNA processing in prokaryotes

32. Microbial genetics
RNA processing in eukaryotes

33. Microbial genetics 33
 Constitutive enzymes are expressed at a fixed rate
 Other enzymes are expressed only as needed
 Repressible enzymes
 Inducible enzymes
 Beta galactosidase in E.coli
 Insulin
Regulation of Bacterial Gene Expression

34. Microbial genetics
Regulation of Gene Expression

35. Microbial genetics
Operon

36. Repression

37. Regulation of Gene Expression

38. Regulation of Gene Expression

39. Regulation of Gene Expression

40. Regulation of Gene Expression

41. Microbial genetics 41
 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
Mutation

42. Point mutation

43.  Base substitution (point
mutation)
 Missense mutation
Mutation
•Change in one base
•Result in change in
amino acid

44.  Nonsense mutation
Mutation
• Results in a nonsense
codon

45.  Frameshift mutation
Mutation
•Insertion or deletion of one
or more nucleotide pairs

46. Microbial genetics 46
Mutagens
 Chemical mutagens
 A number of household and lab chemicals sodium azide
 Physical mutagens
 Radiation, UV light
Mutation

47. Microbial genetics
Chemical mutagens
Nitrous acid as a mutagen

48. Nucleoside analogs

49. Microbial genetics 49
 Ionizing radiation (X rays and gamma rays) causes the
formation of ions that can react with nucleotides and
the deoxyribose-phosphate backbone.
 Breakage of covalent bonds in phosphate sugar
backbone of DNA
 Covalent bonds between adjoining nitrogenous bases
caused by UV light (thymin dimer)
 Nucleotide excision repairs mutations
Radiations

50. 50
 UV radiation causes
thymine dimers
 Light-repair
separates thymine
dimers
 Skin cancer by
accumulation of
TDs in the skin cells
Mutation

51. Microbial genetics 51
 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
The Frequency of Mutation

52. Microbial genetics 52
Identifying mutants
 Bacteria have one copy of each gene so identifying
mutation is relatively easy
Selection:
 Positive (direct) selection detects mutant cells because
they grow or appear different.
 Pencillin resistant population
 Negative (indirect) selection detects mutant cells
because they do not grow.

53. Replica Plating

54. The Ames Test for Chemical Carcinogens

55. Genetic transfer and Recombination

56. Genetic transfer and Recombination

57. Microbial genetics 57
 Vertical gene transfer
 Horizontal gene
transfer
Genetic Transfer and Recombination
Occurs during reproduction,
between generations of
cells
Transfer of genes between
cells of the same
generation

58. 58
Transformation

59. In vitro transformation

60. Mechanim of transformation

61. Mechanim of transformation

62. Microbial genetics
 Competant cells
 Naturally occuring
 Induced
 Calcium chloride
Transformation

63. Microbial genetics
Conjugation

64. Microbial genetics
Conjugation

65. Microbial genetics 65
Conjugation

66. Microbial genetics 66
Conjugation

67. Microbial genetics
Conjugation

68. 68
Transduction
Recombinant
1
Phage protein coat
Bacterial
chromosome
2
3
Bacterial
DNA
Phage
DNA
4
Recipient cell
5
Donor
bacterial
DNA
Recipient
bacterial
DNA
Recombinant cell
A phage infects the
donor bacterial
cell.
Phage DNA and proteins
are made, and the
bacterial chromosome is
broken down into pieces.
Occasionally during phage assembly,
pieces of bacterial DNA are packaged
in a phage capsid. Then the donor cell
lyses and releases phage particles
containing bacterial DNA.
A phage carrying bacterial
DNA infects a new host cell,
the recipient cell.
Recombinant can occur,
producing a recombinant
cell with a genotype
different from both the
donor and recipient cells.

69. Microbial genetics 69
 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
Plasmids

70. Microbial genetics 70
Plasmids

71. Microbial genetics 71
 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
Transposons

72. Microbial genetics
Insertion sequence

73. Microbial genetics
Complex transposons

74. Insertion of transposons into plasmids

75. THANK YOU