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Chapter4 Heredity and variation of bacteria Xin Gang Department of microbiology and immunology Shantou University Medical College.

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Presentation on theme: "Chapter4 Heredity and variation of bacteria Xin Gang Department of microbiology and immunology Shantou University Medical College."— Presentation transcript:

1 Chapter4 Heredity and variation of bacteria Xin Gang Department of microbiology and immunology Shantou University Medical College

2 outline Concepts Genetic materials of bacteria Mechanism of variation

3 concepts Heredity Variation –Genotypic variation –Phenotypic variation

4 Variation of bacteria i.Variation in morphology and structure –lose capsule (Pneumococcus) –H O –Lose Spore –L-form

5 Variation ii.Variation in virulence –Bovine TB BCG 13years, 230 generation –C. diphtheriae  -corynephage, diphtheriae toxin

6 Variation iii.Variation in drug resistance –Penicillin resistant strains of staphylococcus aureus iv.Variation in colony –S-R v.Variation in enzyme activity –Phenotypic –Genotypic

7 I Genetic materials of bacteria 1.Chromosomes 2.Plasmids 3.Phage 4.Transposable elements

8 DNA forms: ds-DNA, circle 1 Chromosomes

9 Size –E.coli 1300  m, 4288gene Rolling-circle pattern of replication

10 2 Plasmids Extrachromosomal genetic elements that are capable of autonomous replication. Small double-stranded DNA molecules, usually circular exist independently of host chromosome autonomously replicating (replicon) may disappear spontaneously or by induction (UV) incompatibility and compatibility

11 Classification of Plasmids Transfer properties –Conjugative kbp eg. F, R plasmid –Nonconjugative <15kbp, transfer by mobilization eg. ColE1 plasmid

12 Phenotypic effects Fertility plasmid,F plasmid coding sex pilus Tn 9 Tn 21 Tn 10 Tn 8 RTF R determinant Resistance plasmid, R plasmid resistance transfer factor resistance determinant Virulence plasmid Coliciogenic plismid

13 3 Bacteriophage, phage T4 bacteriophages infecting E.coli.

14 Bacteriophage (Phage) Definition –Bacterial virus. –Virus of bacteria, fungi, actinomyces, and spirochete.

15 Composition –Nucleic acid DNA or RNA –Protein Protection Infection Composition and Structure

16 Structure (T 4 ) –Head or capsid –Tail Tail Tail Fibers Base Plate Head/Capsid Contractile Sheath

17 Infection of Host Cells Attachment Sheath contraction Nucleic acid injection

18 Attachment and injection

19 Types of Bacteriophage Lytic or virulent phage –(e.g., T 4 ) Lysogenic or temperate phage –(e.g., )

20 Virulent Phage Lytic or virulent phage –Phage that can only multiply within bacteria and kill the cell by lysis. (e.g., T 4 )

21 Lytic Phage Multiplication Cycle

22 Lytic cycle Attachment Injection and uncoating Biosynthesis –Eclipse Early proteins Phage DNA synthesis Late proteins –Intracellular accumulation Maturation and releasing

23 Lysogenic Phage Lysogenic or temperate phage: –Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. (e.g., ) Prophage Lysogen,Lysogenic bacterium Lysogenic conversion

24 Lysogenic bacterium /lysogen prophage Lysogenic phage /temperate phage

25 Lysogenic or phage conversion –Definition: A change in the phenotype of a bacterial cell as a consequence of lysogeny Modification of Salmonella O antigen Toxin production by Corynebacterium diphtheriae

26 Lytic versus lysogenic infection by phage Lambda

27 4 Thransposable element Concept –Segments of DNA that are able to move around the genome. Properties –“ Random ” movement –Jumping genes or movable genes –First discovered in the 1940s by Barbara McClintock during her study on maize genetics.(won the Nobel prize in 1983)

28 Types of Transposon Insertion sequences (IS) –A short sequence of DNA containing only the genes for those enzymes required for its transposition. IR: inverted repeat Importance –Mutation –Plasmid insertion

29 Types of Transposon Transposons (Tn) or complex Tn –contain genes other than those required for transposition (eg. Antibiotic resistance or toxin genes) Importance –Antibiotic resistance

30 II mechanism of variation i.Mutation ii.Gene transfer and recombination

31 i Mutations in bacteria Changes in DNA sequences Base substitutions, deletions, insertions, rearrangements Spontaneous mutation: Backword mutation or reverse mutation

32 ii Gene transfer and recombination 1.Transformation 2.Transduction 3.Conjugation 4.Lysogenic conversion 5.protoplast fusion

33 General Features of Gene Transfer in Bacteria Unidirectional –Donor to recipient Donor does not give an entire chromosome Gene variation can occur between species

34 1 Transformation Definition: Gene transfer resulting from the uptake of DNA from a donor. Competence of the recipient (Bacillus, Haemophilus, Neisseria, Streptococcus)

35 Griffith ’ s transformation experiments

36 Bacterial Transformation With DNA Fragments

37 2 Conjugation Definition: Donor DNA transferred to recipient cell through sex pilus. Donor Recipient

38 F plasmid F+ Mechanism of F + x F - Crosses Pair formation Conjugation bridge DNA transfer Origin of transfer Rolling circle replication F+F+ F-F- F+F+ F-F- F+F+ F+F+ F+F+ F+F+

39 HfrF’ F’ plasmid Hfr, high frequency of recombination F+F+ Hfr Physiological States F plasmid F plasmid

40 Mechanism of Hfr x F - Crosses HfrF-F- F-F- F-F- F-F-

41 Mechanism of F ’ x F - Crosses F’ F-F- F-F-

42 Resistance plasmid, R plasmid RTF (resistance transfer factor) –transfer genes –code pili R determinant (resistance determination ) –resistance genes –transponsons Tn 9 Tn 21 Tn 10 Tn 8 RTF R determinant

43 3 Transduction Definition: Gene transfer from a donor to a recipient by way of a bacteriophage

44 Types of transduction Generalized - Transduction in which potentially any donor bacterial gene can be transferred Specialized (restricted) - Transduction in which only certain donor genes can be transferred

45 Generalized transduction by bacteriophages

46 Generalized Transduction Infection of Donor Phage replication and degradation of host DNA Assembly of phages particles Release of phage Infection of recipient Legitimate recombination

47 specialized transduction by a temprerate bacteriophage

48 gal bio gal bio gal bio gal bio gal specialized transduction

49 iv.lysogenic conversion v.DNA recombination protoplast fusion

50 Practical implications 1.Application in diagnosis, treatment and prevention of infectious diseases Eg. L-form PCR 2.Detection of mutagenicity 3.Application in genetic engineering

51 The Ames Test for mutagenicity


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