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Bacterial Genetics

Bacterial Genetics. Review. Genome: genetic blueprint Gene: Most organisms-DNA Viruses DNA or RNA. Nucleic acids. Nucleotides Sugar Phosphate Nitrogenous base DNA Guanine-G; cytosine-C; adenine-A; thymine-T RNA Uracil-U replaces thymine. Review. DNA –ds helix

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Bacterial Genetics

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  1. Bacterial Genetics

  2. Review • Genome: genetic blueprint • Gene: • Most organisms-DNA • Viruses • DNA or RNA

  3. Nucleic acids • Nucleotides • Sugar • Phosphate • Nitrogenous base • DNA • Guanine-G; cytosine-C; adenine-A; thymine-T • RNA • Uracil-U replaces thymine

  4. Review • DNA –ds helix • Strands held together by H bonds • Complimentary base pairing • A pairs with T; in RNA, A pairs with U • C with G • Linear sequence of bases contains info • 3 nucleotides code for 1 amino acid

  5. Duplication of DNA • Replication of chromosome(s) • Precise duplication of DNA • DNA polymerase • Denaturation: separate strands

  6. Terms • Genotype: set of genes carry instructions • Phenotype: expression of those genes • Genome of prokaryotes includes chromosomal DNA and plasmids

  7. Flow of Information • DNA to RNA to polypeptides • Transcription-synthesis of complimentary strand of RNA from DNA-mRNA • Separates strands of DNA • Copies only DNA needed for protein synthesis-mRNA

  8. Flow of Information • Translation: protein synthesis • Decodes sequence of nucleotides into amino acids (20) • Genetic Code: codons, group of 3 nucleotides-AAU,CGC • AUG-start codon

  9. Degenerate Code • Each codon specifies a particular amino acid • Several codons code same aa

  10. Flow of Information • Translation • Ribosomes-site of protein synthesis • tRNA recognizes the specific codon • tRNA forms complimentary base pairing with the codon • Has anticodon & carries the required aa

  11. Question • AUG is first codon on mRNA. • What is the anticodon on the tRNA?

  12. Regulation of Gene Expression • Prokaryotes: Where is mRNA transcribed in cell? • Translation can begin before completion of mRNA molecule

  13. Regulation of Gene Expression • Eukaryotes: Where? • mRNA contains exons and introns • Exons are expressed, encode for aa • Introns do not • Processing by ribozymes to remove introns & splice exons

  14. Regulation of Gene Expression • Conserve energy • Turn on & off transcription of genes • Constitutive genes not regulated • Operon

  15. Operons • Regulation of metabolic genes • Uses repressors (regulatory genes) • Block RNA polymerase from attaching

  16. Induction • Turn on the transcription of gene • Inducer- induces transcription • Inducible enzymes • Synthesized only when substrate is present • Glycolysisgenes constitutive

  17. Lac Operon • Inducible operon: enzymes to metabolize lactose • Default is “off” • Regulatory sites • Promoter- • Operator-

  18. Lac Operon • i genes code for repressor-regulatory protein • Always turned on • Binds to operator • Structural genes • Lac operon-3 genes

  19. Lactose in Medium • Binds repressor changing shape • Repressor can’t bind • RNA polymerase can bind • Enzymes for lactose metabolism produced • To turn on operon:

  20. Repressible Operon • Tryptophan operon • Usually occurs in anabolism • Repressor is inactive so tyrptophan is synthesized • Default in “on”

  21. Tryptophan Operon • Amino acid in media • Binds to repressor activating it • Genes to synthesize amino acid produced

  22. Mutation • Change (heritable) in base sequence of DNA • Called mutant • Genotype differs from parent • Phenotype may be altered

  23. Types of Mutations • Spontaneous mutations • DNA replication errors • Occur at low frequencies • Induced mutations-mutagens • Alter structure of bases • Errors in base paring

  24. Point Mutations • Change in 1 base-pair • Single base is replaced with another • Change in genotype • May be change in phenotype

  25. Types of Point Mutations • Silent mutation- no phenotypic change • Degenerate code

  26. Missense Mutations • Change in amino acid • Can result in significant changes in polypeptide

  27. Examples of Missense • Sickle cell anemia- hemoglobin • Change from glutamic acid (hydrophilic) to valine (hydrophobic) • Change in shape of protein

  28. Nonsense Mutation • Base-pair substitution • Create stop codon in middle of mRNA • Premature termination of translation

  29. Frame Shift Mutation • Bases deleted or inserted • Shifts translational reading frame • Largeinsertions are transposons

  30. Frame Shift • THE FAT CAT ATE THE BAD RAT • Remove a C • THE FAT ATA TET HEB ADR AT

  31. Induced Mutations • Mutagens • Increase mutation rate • Chemical mutagens • Nitrous acid • Occurs at random sites

  32. Radiation • UV light -nonionizing • Covalent bonds between certain bases • Adjacent thymines(Ts) can cross link • Pyrimidine dimers • Some enzymes that repair UV damage

  33. Nucleoside Analog • Structurally similar to normal bases • Have altered base pairing • 2 aminopurine replaces A & may pair with C • 5-bromouracil replaces T but may pair with G

  34. Genetic Recombination • Physical exchange of genes between 2 homologous DNA molecules • Contributes to population’s genetic diversity

  35. Horizontal Transfer • Microbes of same generation • Involves a donor cell - gives DNA to recipient cell • Part of DNA incorporated into recipient’s DNA

  36. Transformation • Free (naked) DNA in solution • Cells after death, release DNA • Cells may take up DNA • Only in certain stage of cell cycle

  37. Competence • Cells able to take up DNA & be transformed • Release competence factor that helps in uptake

  38. Transformation • Enzymes cut DNA into small pieces • Recombination between donor & recipient • Few competent bacteria

  39. First Evidence of Transformation • Griffith in 1920s • Streptococcus pneumoniae in mice

  40. Conjugation • Mediated by one kind of plasmid • F plasmid or F factor • Genes to control conjugation • Donor cells must have F plasmid

  41. Differs from Transformation • Cells must be of opposite mating types • Donor is F plus • Recipient is F minus • Requires direct cell contact • Transfers larger quantities of DNA

  42. Conjugation • Gram negative cells • Gram positive cells produce sticky surface molecules • Keeps cells together

  43. Conjugation • Plasmid is replicated • A copy of plasmid transferred to recipient • F minus cell becomes F plus • Receptors on new F plus change

  44. F Plasmid • Plasmid integrates into the chromosome converts cell to Hfr cell( high frequency of recombination) • F factor DNA can separate and become plasmid

  45. Conjugation • Hfr and F- cell • Replication of Hfr begins in middle of integrated F factor • Small piece leads the chromosome into F- cell • Donor DNA can recombine (DNA not integrated is degraded)

  46. Transduction • Bacterial DNA is transferred via a virus -Bacteriophage • Virulent phages –lytic cycle • Generalized transduction • Any gene on donor chromosome transferred

  47. Generalized Transduction • All genes are equally likely to be packaged inside phage • Virus cannot replicate in new bacteria • Defective virus • Specialized transduction-only certain bacterial genes transferred

  48. Plasmids • Extra chromosomal material • F factor or plasmid is a conjugative plasmid • Carries genes for sex pili and for transfer

  49. Dissimilation Plasmids • Enzymes break down unusual sugars and hydrocarbons • Pseudomonas use toluene and petroleum as carbon and energy sources • Used to clean up oil spills • Allows bug to grow in adverse environments

  50. Virulence Plasmids • E. coli carries plasmids that code for toxins –diarrhea • Bacteriocins- toxic proteins kills other bacteria • E. coli produces colicins

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