医学资料5细菌的遗传和变异e

1、Bacterial GeneticsXiao-Kui GUO PhD2021/7/18 星期日Bacterial Genomics2021/7/18 星期日Microbial Genomics2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日68%Deinococcus radioduransMicrobial Genome FeaturesG+C content29%Borrelia burgdorferiGenome organizationsingle circular chromosomelarge linear chromosome plus21 extr

2、achromosomal elementscircular chromosome plus one or moreextrachromosomal elementstwo circular chromosomes2021/7/18 星期日PLASMIDS Plasmids are extrachromosomal genetic elements capable of autonomous replication. An episome is a plasmid that can integrate into the bacterial chromosomeClassification of

3、Plasmids Transfer properties Conjugative plasmids Nonconjugative plasmids Phenotypic effects Fertility plasmid (F factor) Bacteriocinogenic plasmids. Resistance plasmids 7 factors) .2021/7/18 星期日Insertion sequences (IS)- Insertion sequences are transposable genetic elements that carry no known genes

4、 except those that are required for transposition.a. Nomenclature - Insertion sequences are given the designation IS followed by a number. e.g. IS1b. Structure Insertion sequences are small stretches of DNA that have at their ends repeated sequences, which are involved in transposition. In between t

5、he terminal repeated sequences there are genes involved in transposition and sequences that can control the expression of the genes but no other nonessential genes are present.c. Importancei) Mutation - The introduction of an insertion sequence into a bacterial gene will result in the inactivation o

6、f the gene.ii) Plasmid insertion into chromosomes - The sites at which plasmids insert into the bacterial chromosome are at or near insertion sequence in the chromosome.iii) Phase Variation - The flagellar antigens are one of the main antigens to which the immune response is directed in our attempt

7、to fight off a bacterial infection. In Salmonella there are two genes which code for two antigenically different flagellar antigens. The expression of these genes is regulated by an insertion sequences. In one orientation one of the genes is active while in the other orientation the other flagellar

8、gene is active. Thus, Salmonella can change their flagella in response to the immune systems attack. Phase variation is not unique to Salmonella flagellar antigens. It is also seen with other bacterial surface antigens. Also the mechanism of phase variation may differ in different species of bacteri

9、a (e.g. Neisseria; transformation).2021/7/18 星期日Transposons (Tn) - Transposons are transposable genetic elements that carry one or more other genes in addition to those which are essential for transposition.Nomenclature - Transposons are given the designation Tn followed by a number.Structure - The

10、structure of a transposon is similar to that of an insertion sequence. The extra genes are located between the terminal repeated sequences. In some instances (composite transposons) the terminal repeated sequences are actually insertion sequences. Importance - Many antibiotic resistance genes are lo

11、cated on transposons. Since transposons can jump from one DNA molecule to another, these antibiotic resistance transposons are a major factor in the development of plasmids which can confer multiple drug resistance on a bacterium harboring such a plasmid. These multiple drug resistance plasmids have

12、 become a major medical problem because the indiscriminate use of antibiotics have provided a selective advantage for bacteria harboring these plasmids.2021/7/18 星期日2021/7/18 星期日Mechanism of bacterial variationGene mutationGene transfer and recombination TransformationConjugation TransductionLysogen

13、ic conversion Protoplast fusion 2021/7/18 星期日Types of mutationBase substitutionFrame shefitInsertion sequences2021/7/18 星期日What can cause mutation?Chemicals: nitrous acid; alkylating agents5-bromouracilbenzpyreneRadiation: X-rays and Ultraviolet lightViruses2021/7/18 星期日Bacterial mutationMutation ra

14、teMutation and selectivityBackward mutation2021/7/18 星期日TransformationTransformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain bacteria (e.g. Bacillus, Haemophilus, Neisseria, Pneumococcus) can take up DNA from the environment and the DNA

15、that is taken up can be incorporated into the recipients chromosome.2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日 Conjugation Transfer of DNA from a donor to a recipient by direct physical contact between the cells. In bacteria there are two mating types a donor (mal

16、e) and a recipient (female) and the direction of transfer of genetic material is one way; DNA is transferred from a donor to a recipient.2021/7/18 星期日Physiological States of F FactorAutonomous (F+)Characteristics of F+ x F- crossesF- becomes F+ while F+ remains F+Low transfer of donor chromosomal ge

17、nesF+2021/7/18 星期日Physiological States of F FactorIntegrated (Hfr)Characteristics of Hfr x F- crossesF- rarely becomes Hfr while Hfr remains HfrHigh transfer of certain donor chromosomal genesF+Hfr2021/7/18 星期日Physiological States of F FactorAutonomous with donor genes (F)Characteristics of F x F- c

18、rossesF- becomes F while F remains FHigh transfer of donor genes on F and low transfer of other donor chromosomal genesHfrF2021/7/18 星期日Mechanism of F+ x F- CrossesDNA transferOrigin of transferRolling circle replication Pair formation Conjugation bridgeF+F-F+F-F+F+F+F+2021/7/18 星期日Mechanism of Hfr

19、x F- CrossesDNA transferOrigin of transferRolling circle replicationHomologous recombination Pair formation Conjugation bridgeHfrF-HfrF-HfrF-HfrF-2021/7/18 星期日Mechanism of F x F- CrossesDNA transferOrigin of transferRolling circle replication Pair formation Conjugation bridgeFFFFFF-FF-2021/7/18 星期日R

20、 Plasmid2021/7/18 星期日 Transduction: Transduction is defined as the transfer of genetic information between cells through the mediation of a virus (phage) particle. It therefore does not require cell to cell contact and is DNase resistant.2021/7/18 星期日Generalized Transduction Generalized transduction

21、 is transduction in which potentially any bacterial gene from the donor can be transferred to the recipient.2021/7/18 星期日The mechanism of generalizedtransduction2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日2021/7/18 星期日Generalized transductionIt is relatively easy. It is rather efficient (10-3 per recipie

22、nt with P22HT, 10-6 with P22 or P1), using the correct phage. It moves only a small part of the chromosome which allows you to change part of a strains genotype without affecting the rest of the chromosome. The high frequency of transfer and the small region transferred allows fine-structure mapping

23、2021/7/18 星期日Specialized transductionSpecialized transduction is transduction in which only certain donor genes can be transferred to the recipient. Different phages may transfer different genes but an individual phage can only transfer certain genesSpecialized transduction is mediated by lysogenic or temperate phage and the genes that get transferred will depend on where the prophage has inserted in the chromosome.2021/7/18 星期日The mechanism of specializ