Genetics b


Published on

Published in: Technology
1 Like
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Genetics b

  1. 1. DNA as the Genetic Material The early work of Fred Griffith in 1928 on the transfer of virulence in the pathogen S. pneumoniae set the stage for DNA research.
  2. 2. Transformation <ul><li>Change of nonvirulent bacteria into virulent pathogens. </li></ul>
  3. 3. The Genetic Code <ul><li>DNA base sequence corresponds to the amino acid sequence of the polypeptide specified by the gene. </li></ul><ul><li>Organization of the code – codons, wobble </li></ul><ul><li>Gene structure – linear sequence of nucleotides </li></ul>
  4. 19. Mutations <ul><li>Spontaneous mutations arise occasinally in all cells and develop in the absence of any added agent. </li></ul><ul><li>Induced mutations are the result of exposure of the organism to some physical or chemical agent called a mutagen. </li></ul>
  5. 20. Biochemical Mutations <ul><li>Prototrophs – can grow on minimal media </li></ul><ul><li>Resistant mutant – is a particular type of biochemical mutant that acquires resistance to some pathogen, chemical, or antibiotic. </li></ul><ul><li>Easy to select for and very useful in microbial genetics. </li></ul>
  6. 21. Biochemical Mutations <ul><li>Cause a change in the biochemistry of the cell. </li></ul><ul><li>Inactivate a biosynthetic pathway </li></ul><ul><li>Auxotrophs – cannot grow on minimal medium </li></ul>
  7. 22. Mutations <ul><li>Mutare – to change </li></ul><ul><li>Conditional mutations – expressed only under certain environmental conditions. </li></ul>
  8. 23. Point Mutations <ul><li>Silent </li></ul><ul><li>Missense </li></ul><ul><li>Nonsense </li></ul><ul><li>Frameshift </li></ul>
  9. 31. Suppressor Mutation <ul><li>Overcomes the effect of the first mutation. </li></ul><ul><li>The second mutation is within the same gene, the change may be called a second site reversion or intergenic suppression. </li></ul>
  10. 32. The Expression of Mutations <ul><li>The expression of a mutation will only be noticed if it produces a detectable, altered phenotype. </li></ul><ul><li>Wild type  forward mutation </li></ul><ul><li>Reversion mutation  back mutation </li></ul>
  11. 33. Spontaneous Mutations <ul><li>Arise from several potential sources. </li></ul><ul><li>Errors in DNA replication, damage to DNA from factors such as gamma radiation and heat, or transposons. </li></ul><ul><li>Transition and transversion mutations. </li></ul><ul><li>Frameshifts – deletion of DNA segments resulting in an altered codon reading frame </li></ul>
  12. 34. Induced Mutations <ul><li>Virtually any agent that directly damages DNA, alters its chemistry, or interferes with repair mechanisms will induce mutations. </li></ul><ul><li>Modes of mutagen action – incorporation of base analogs, specific mispairing, intercalation, and bypass of replication. </li></ul>
  13. 35. Mutant Selection <ul><li>Replica plating – direct and indirect method. </li></ul><ul><li>Ames Test – developed in the 1970s by Bruce Ames, used to detect carcinogens </li></ul>
  14. 36. Detection and Isolation of Mutants <ul><li>In order to study microbial mutants, one must be able to detect them readily, even when there are few, and then efficiently isolate them from the parent organism and other mutants. </li></ul>
  15. 38. Microbial Genetics: Recombination and Plasmids <ul><li>Recombination is the process in which a new recombinant chromosome, one with a genotype different from either parent, is formed by combining genetic material from two organisms. </li></ul>
  16. 39. Bacterial Plasmids <ul><li>Conjugation, the transfer of DNA between bacteria involving direct contact, depends on the presence of an extra piece of circular DNA – plasmid DNA. </li></ul>
  17. 40. Plasmids <ul><li>Are small circular DNA molecules that can exist independently of host chromosomes and are present in many bacteria (some yeast and fungi). </li></ul><ul><li>Own replication origins and are autonomously replicating and stably inherited. </li></ul>
  18. 41. Replicons <ul><li>Is a DNA molecule or sequence that has a replication origin and is capable of being replicated. </li></ul><ul><li>Plasmids and bacterial chromosomes are separate replicons. </li></ul><ul><li>Plasmids – less than 30 genes, single or multiple copies </li></ul>
  19. 42. Curing <ul><li>Plasmids can be eliminated from host cells in a process known as curing. </li></ul><ul><li>May occur spontaneously or be induced by treatments that inhibit plasmid replication. </li></ul><ul><li>Acridine mutations, UV and ionizing radiation, thymine starvation, and growth above optimal temperature. </li></ul>
  20. 43. Classified: mode of existence and spread <ul><li>Episome – is a plasmid that can exist either with or without being integrated into the host’s chromosome. </li></ul><ul><li>Conjugative – plasmids have genes for pili and can transfer copies of themselves to other bacteria during conjugation </li></ul>
  21. 44. Fertility Factors <ul><li>A plasmid called the fertility factor or F factor plays a major role in conjugation in E. coli . </li></ul><ul><li>About 94.5 kilobases long and bears genes responsible for cell attachment and plasmid transfer between specific bacterial strains during conjugation. </li></ul>
  22. 45. F factor <ul><li>Also contains insertion sequences that assist plasmid integration into host cell chromosome. </li></ul><ul><li>The F factor is an episome that can exist outside the bacterial chromosome or be integrated into it. </li></ul>
  23. 46. Resistance Factors <ul><li>R factors – often confer antibiotic resistance on the bacteria that contain them </li></ul><ul><li>Code for enzymes capable of destroying or modifying antibiotics. </li></ul><ul><li>Usually not integrated into host chromosome </li></ul><ul><li>Resistance genes are within a transposon </li></ul>
  24. 47. Col Plasmids <ul><li>Bacteria also harbor plasmids with genes that may give them a competitive advantage in the microbial world. </li></ul><ul><li>Bacteriocins – bacterial proteins that destroy other bacteria </li></ul><ul><li>Virulence - make the bacteria more pathogenic </li></ul><ul><li>Metabolic – degrade substances </li></ul>
  25. 48. Transposable Elements <ul><li>Move around the genome – transposition </li></ul><ul><li>Insertion sequences – IS elements (750 to 1,600 base pairs) contain only the genes for those enzymes required for its transposition </li></ul><ul><li>Composite transposons </li></ul>
  26. 49. Bacterial Recombination <ul><li>Transformation </li></ul><ul><li>Transduction </li></ul><ul><li>Conjugation </li></ul>
  27. 50. DNA Transformation <ul><li>Uptake of naked DNA </li></ul><ul><li>Transformation frequency of very competent cells is around 10 -3 for most genera when an excess of DNA is used </li></ul>
  28. 51. Griffith’s Experiment
  29. 56. Transformation
  30. 59. Bacterial Conjugation <ul><li>F + x F - </li></ul><ul><li>HFr </li></ul><ul><li>F’ </li></ul>
  31. 68. Hfr Conjugation
  32. 79. Transduction <ul><li>Bacterial viruses participate in this mode of bacterial gene transfer. </li></ul><ul><li>Generatlized transduction </li></ul><ul><li>Specialized transduction </li></ul>
  33. 91. Generalized Transduction
  34. 94. Specialized Transduction
  35. 101. Altered Porin
  36. 102. Altered Carrier Protein
  37. 103. Pumps
  38. 104. Restriction Endonucleases