Chapter 8 The Genetics of Bacteria and Their Viruses

1. Chapter 8The Genetics of Bacteriaand Their Viruses José A. Cardé Serrano, PhD Universidad Aventista de las Antillas Biol 223 Agosto 2010

2. Chapter Outline • Viruses and Bacteria in Genetics • The Genetics of Viruses • The Genetics of Bacteria • Mechanisms of Genetic Exchange in Bacteria

3. Bacteria and viruses have made important contributions to the science of genetics. Bacteria and Viruses in Genetics

4. Bacteria and Viruses • Small size • Rapid reproduction • Selective media (e.g., antibiotics) • Simple structures and physiology • Genetic variability • Complete genome sequences

5. Key Points • Their small size, short generation time, and simple structures have made bacteria and viruses valuable model systems for genetic studies. • Many basic concepts of genetics were first deduced from studies of bacteria and viruses.

6. Viruses can only reproduce by infecting living host cells. Bacteriophages are viruses that infect bacteria. Several important genetic concepts have been discovered through studies of bacteriophages. The Genetics of Viruses

7. Virus: Definición • Partícula • Ácido nucleico • Capa de proteínas • Cápsula • capsómeros • Parásito celular obligado • Pequeño • Infeccioso

8. Virus: Historia • Dimitri Ivanowsky • Científico ruso • Virus del Mosaico del Tabaco • 1892 • agente causante de la enfermedad no era retenido por un filtro de porcelana

9. Virus: Historia • Martinus Beijerinck • 1898 • Científico holandés • Resultados similares a Ivanowsky • Aportación Conceptualización • Agente causante ES más pequeño que una bacteria. • Acuño el término Virus

10. Virus: Propiedades • Parásito intracelular obligado • Infeccioso • Su genoma puede ser DNA o RNA • Capaz de replicarse y dirigir la síntesis de proteínas utilizando la maquinaria del hospedero cuando se encuentra en el hospedero correcto.

11. Virus: Propiedades • La progenie de los virus se ensambla “de novo” a partir de componentes sintetizados en la célula hospedera. • Medio de transmisión • El virus recién producido en la célula hospedera. • Composición estructural simple • Cristalografía • Virus del Mosaico del Tabaco -1935

12. Bacteriophage T4

13. Bacteriophage T4 • Double-stranded DNA genome • Protein head • Genome contains 168,800 base pairs and 150 characterized genes • Lytic phage

15. Bacteriophage • Double-stranded DNA genome • Genome contains, 48,502 base pairs and about 50 genes • May be lytic or lysogenic

16.  Phage

17.  Phage -DNA integration

18. Mapping Genes in Bacteriophage • Genes may be mapped based on recombination frequencies. • Host bacteria are infected with two types of phage; progeny phage are screened for recombination. • Map distances are calculated as the average number of crossovers between genetic markers.

19. Phage phenotypes • Temperature-sensitive (ts) mutations • Plaque morphology (rapid lysis; lysis inhibition) • Host range

20. Bacteriophage T4 Plaque Morphology (r+ and r)

21. Key Points • Viruses are obligate parasites that can reproduce only by infecting living host cells. • Bacteriophages are viruses that infect bacteria. • Bacteriophage T4 is a lytic phage that infects E. coli, reproduces, and lyses the host cell.

22. Key Points • Bacteriophage lambda () can enter a lytic pathway, like T4, or it can enter a lysogenic pathway, during which its chromosome is inserted into the chromosome of the bacterium. • In its integrated state, the  chromosome is called a prophage, and it’s lytic genes are kept turned off.

23. Bacteria contain genes that mutate to produce altered phenotypes. Gene transfer in bacteria is unidirectional—from donor cells to recipient cells. The Genetics of Bacteria

24. Bacteria • One main chromosome with a few thousand genes. • Variable number of plasmids and episomes. • Asexual reproduction by simple fission. • Parasexual processes occur.

25. Phenotypes in Bacteria • Colony color and morphology • Nutritional mutants for energy sources • Prototrophs and auxotrophs • Drug and antibiotic resistance

26. Bacterial colonies:Serratia marcencens

27. Recombination in Bacteria

28. Key Points • Bacteria usually contain one main chromosome. • Wild-type bacteria are prototrophs; they can synthesize everything they need to grow and reproduce given an energy source and some inorganic molecules. • Auxotrophic mutant bacteria require additional metabolites for growth.

29. Key Points • Gene transfer in bacteria is unidirectional; genes from a donor cell are transferred to a recipient cell, with no transfer from recipient to donor.

30. Bacteria exchange genetic material through three different parasexual processes. Mechanisms of Genetic Exchange in Bacteria

33. The U-tube Experiment Pregunta: Ocurrirá intercambio de material genético entre bacterias? Hipótesis: Si ocurre intercambio. Diseño Experimental: El Tubo U Racional: Si incubo bacterias con fenotipos distintos y el intercambio ocurre, debo obtener bacterias recombinantes Resultados: ?

34. Streptococcus pneumoniae Phenotypes

35. Transformation

36. Transformation in Bacillus subtilis

37. Conjugation in E. coli

38. Transduction • In transduction, a bacteriophage transfers DNA from a donor cell to a recipient cell. • In generalized transduction, a random fragment of bacterial DNA is packaged in the page head in place of the phage DNA. • In specialized transduction, recombination between the phage chromosome and the host chromosome produces a phage chromosome containing a piece of bacterial DNA.

39. Plasmids • A plasmid is a genetic element that can replicate independently of the main chromosome in an extrachromosomal state. • Most plasmids are not required for the survival of the host cell. • Plasmids in E. coli • F Factor (Fertility Factor) • R Plasmids (Resistance Plasmids) • Col Plasmids (synthesize compounds that kill sensitive cells)

40. Episomes • An episome is a genetic element that is not essential to the host and that can either replicate autonomously or be integrated into the bacterial chromosome. • Integration depends on the presence of IS elements.

42. Key Points • Three parasexual processes—transformation, conjugation, and transduction—occur in bacteria. These processes can be distinguished by two criteria: whether the gene transfer is inhibited by deoxyribonuclease and whether it requires cell contact.

43. Key Points • Transformation involves the uptake of free DNA by bacteria. • Conjugation occurs when a donor cell makes contact with a recipient cell and then transfers DNA to the recipient cell. • Transduction occurs when a virus carries bacterial genes from a donor cell to a recipient cell.

44. Key Points • Plasmids are self-replicating extrachromosomal genetic elements. • Episomes can replicate autonomously or as integrated components of bacterial chromosomes.