5细菌的遗传和变异e

1、Bacterial Genetics,Xiao-Kui GUO PhD,Bacterial Genomics,Microbial Genomics,68% Deinococcus radiodurans,Microbial Genome Features,G+C content,29% Borrelia burgdorferi,Genome organization,single circular chromosome,large linear chromosome plus 21 extrachromosomal elements,circular chromosome plus one o

2、r more extrachromosomal elements,two circular chromosomes,PLASMIDS Plasmids are extrachromosomal genetic elements capable of autonomous replication. An episome is a plasmid that can integrate into the bacterial chromosome,Classification of Plasmids Transfer properties Conjugative plasmids Nonconjuga

3、tive plasmids Phenotypic effects Fertility plasmid (F factor) Bacteriocinogenic plasmids. Resistance plasmids 7 factors) .,Insertion sequences (IS)- Insertion sequences are transposable genetic elements that carry no known genes except those that are required for transposition.,a. Nomenclature - Ins

4、ertion sequences are given the designation IS followed by a number. e.g. IS1 b. Structure Insertion sequences are small stretches of DNA that have at their ends repeated sequences, which are involved in transposition. In between the terminal repeated sequences there are genes involved in transpositi

5、on and sequences that can control the expression of the genes but no other nonessential genes are present. c. Importance i) Mutation - The introduction of an insertion sequence into a bacterial gene will result in the inactivation of the gene. ii) Plasmid insertion into chromosomes - The sites at wh

6、ich 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 to fight off a bacterial infection. In Salmonella there are two ge

7、nes 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 gene is active. Thus, Salmonella can change their flagella in resp

8、onse 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 bacteria (e.g. Neisseria; transformation).,Transposons (Tn) - Transposons

9、 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 structure of a transposon is similar to that of an insertion sequence. The e

10、xtra 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 located on transposons. Since transposons can jump from one DNA molecule to an

11、other, 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 become a major medical problem because the indiscriminate use of antibiotic

12、s have provided a selective advantage for bacteria harboring these plasmids.,Mechanism of bacterial variation,Gene mutation Gene transfer and recombination Transformation Conjugation Transduction Lysogenic conversion Protoplast fusion,Types of mutation,Base substitution Frame shefit Insertion sequen

13、ces,What can cause mutation?,Chemicals: nitrous acid; alkylating agents 5-bromouracil benzpyrene Radiation: X-rays and Ultraviolet light Viruses,Bacterial mutation,Mutation rate Mutation and selectivity Backward mutation,Transformation,Transformation is gene transfer resulting from the uptake by a r

14、ecipient 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 that is taken up can be incorporated into the recipients chromosome.,Conjugation,Transfer of DNA from a donor to a recipient by direct

15、 physical contact between the cells. In bacteria there are two mating types a donor (male) and a recipient (female) and the direction of transfer of genetic material is one way; DNA is transferred from a donor to a recipient.,Physiological States of F Factor,Autonomous (F+) Characteristics of F+ x F

16、- crosses F- becomes F+ while F+ remains F+ Low transfer of donor chromosomal genes,Physiological States of F Factor,Integrated (Hfr) Characteristics of Hfr x F- crosses F- rarely becomes Hfr while Hfr remains Hfr High transfer of certain donor chromosomal genes,Physiological States of F Factor,Auto

17、nomous with donor genes (F) Characteristics of F x F- crosses F- becomes F while F remains F High transfer of donor genes on F and low transfer of other donor chromosomal genes,Mechanism of F+ x F- Crosses,DNA transfer Origin of transfer Rolling circle replication,Pair formation Conjugation bridge,M

18、echanism of Hfr x F- Crosses,DNA transfer Origin of transfer Rolling circle replication Homologous recombination,Pair formation Conjugation bridge,Mechanism of F x F- Crosses,DNA transfer Origin of transfer Rolling circle replication,Pair formation Conjugation bridge,R Plasmid,Transduction:,Transduc

19、tion 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.,Generalized Transduction,Generalized transduction is transduction in which potentially any bacterial gene

20、 from the donor can be transferred to the recipient.,The mechanism of generalized transduction,Generalized transduction,It is relatively easy. It is rather efficient (10-3 per recipient with P22HT, 10-6 with P22 or P1), using the correct phage. It moves only a small part of the chromosome which allo

21、ws 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,Specialized transduction,Specialized transduction is transduction in which only certain donor genes can be transferr

22、ed to the recipient. Different phages may transfer different genes but an individual phage can only transfer certain genes Specialized 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.,The mechanism