原核生物的遗传学课件.pptx

1、2022-8-11The Genetics of Microorganisms1Chapter 8The Genetics of Microorganisms2022-8-11The Genetics of Microorganisms2Chapter OutlineMicroorganisms in GeneticsThe Genetics of VirusesThe Genetics of BacteriaMechanisms of Genetic Exchange in BacteriaThe Evolutionary Significance of Genetic Exchange i

2、n BacteriaThe Genetics of Fungi2022-8-11The Genetics of Microorganisms3Multi-Drug-Resistant Bacteria:A Ticking Timebomb?2022-8-11The Genetics of Microorganisms4Section 1Microorganisms in GeneticsMicroorganisms have made important contributions to the science of genetics.2022-8-11The Genetics of Micr

3、oorganisms5Key PointsTheir small size,short generation time,and simple structure have made microorganisms valuable model systems for genetic studies.Many basic concepts of genetics were first deduced from studies of microorganisms.2022-8-11The Genetics of Microorganisms6Section 2The Genetics of Viru

4、ses 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 microorganisms.2022-8-11The Genetics of Microorganisms71 Bacteriophages T4 and LambdaViruses that infect bacteria a

5、re called bacteriophages(噬菌体噬菌体).Among the many bacteriophages that have been identified,two have played especially important roles in the elucidation of genetic concepts.Both of these viruses infect the colon bacillus Escherichia coli.2022-8-11The Genetics of Microorganisms8Bacteriophages can be ca

6、tegorized into two typesvirulent(烈性烈性)and temperate(温和温和)based on their lifestyles in infected cells.Bacteriophage T4 is a virulent phageBacteriophage lambda()is a temperate phage.The results of studies performed on phage T4 and have established genetic paradigms that are relevant to understanding o

7、ther types of virus,such as the human immunodeficiency virus(HIV).2022-8-11The Genetics of Microorganisms91.1 Bacteriophage T4Bacteriophage T4 is a large virus that stores its genetic information in a double-stranded DNA molecule packaged inside a proteinaceous head.2022-8-11The Genetics of Microorg

8、anisms10The virus is composed almost entirely of proteins and DNAapproximately half of each.The T4 chromosome is approximately 168,800 bp long and contains about 150 characterized genes and an equal number of uncharacterized sequences thought to be genes.A phage T4(center)from which the DNA has been

9、 released by osmotic(低渗透压低渗透压)shock.Both ends of the linear DNA molecule are visible.2022-8-11The Genetics of Microorganisms11The tail of the virus contains several important components.Its central hollow core provides the channel through which the phage DNA is injected into bacterium.The tail sheat

10、h functions as a small muscle that contracts and pushes the tail core through the bacterial cell wall.The six tail fibers are used to locate receptors on the host cell,and the tail pins on the baseplate the attach firmly to these receptors.2022-8-11The Genetics of Microorganisms12T4 is a lytic phage

11、!2022-8-11The Genetics of Microorganisms13T4 encodes nucleases that degrade the host DNA,the degradation products are then used in the synthesis of phage DNA.But how do these enzymes degrade host DNA without destroying the DNA of the virus?cytosine2022-8-11The Genetics of Microorganisms141.2 Bacteri

12、ophage lambda is smaller than T4;however,its life cycle is more complex.The genome contains about 50 genes in a double-stranded DNA molecule 48502 bp long.This linear DNA molecule is packaged in the head.Soon after it is injected into an E.coli cell,the DNA molecule is converted to a circular form,w

13、hich participates in all subsequent intracellular events.2022-8-11The Genetics of Microorganisms15Inside the cell,the circular chromosome can proceed down either of two pathways.Lytic cycleLysogenic pathwayProphage2022-8-11The Genetics of Microorganisms16Integration of the chromosome occurs by a sit

14、e-specific recombination event between the circular DNA and the circular E.coli chormosome.This recombination occurs at specific attachment sitesattP on the chromosome and attB on the bacterial chromosomeand is mediated by the product of the int gene,the integrase.2022-8-11The Genetics of Microorgan

15、isms17It covalently inserts the DNA into the chromosome of the host cell.The site-specific recombination occurs in the central region of the attachment sites where both attP and attB have the same sequence of 15 nucleotide pairs:GCTTTTTTATACTAACGAAAAAATATGATT With the exception of this core sequence

16、,attP and attB have quite different sequences.2022-8-11The Genetics of Microorganisms18Because recombination occurs within this core sequence during integration,the resulting attB/P and attP/B sites that flank the integrated prophage also both contain the 15-nucleotide-pair sequence.These structure

17、are important because they facilitate excision of the prophage by a very similar site-specific recombination event.2022-8-11The Genetics of Microorganisms19About once in every 105 cell divisions,the prophage excises form the host chromosome and inters the lytic pathway.This phenomenon is the reason

18、the prophage is said to be in a lysogenic state,that is,one capable of causing lysis,albeit at low frequency.Excision of the prophage can also be induced,for example,by irradiation with ultraviolet light.2022-8-11The Genetics of Microorganisms20The excision process is usually precise,with site-speci

19、fic recombination between the core sequences in attB/P and attP/B.It produces an autonomous chromosome that has the original pre-integration form.Excision requires the integrase and the product of the xis gene,excisase(切除酶切除酶)These two enzymes mediate a site-specific recombination event that is esse

20、ntially the reverse of the integration event.2022-8-11The Genetics of Microorganisms21Occasionally,excision occurs anomalously,and bacterial DNA is excised along with phage DNA.When this occurs,the resulting virus can transfer bacterial genes from one host bacterium to another.2022-8-11The Genetics

21、of Microorganisms222.Mapping Genes in BacteriophageGenes on bacteriophage chromosomes can be mapped using recombination frequencies,just as in eukaryotes.However,because viruses have a single chromosome that does not go through meiosis,the mapping procedure is some what different from that used for

22、an organism like Drosophila.2022-8-11The Genetics of Microorganisms23Crosses are performed by simultaneously infecting host bacteria with two different types of phage and then screening the progeny phage for recombinant genotypes.Map distance,in centiMorgans,are then calculated as the average number

23、 of crossovers that have occurred between genetic markers.For short distances,map distances are approximately equal to the percentage of recombinant chromosomes among the progeny.2022-8-11The Genetics of Microorganisms24There are many different kinds of alleles in phage.A.Temperature-sensitive(ts)mu

24、tations are among the most useful.Wild-type coliphage can grow at temperatures ranging from about 25 to over 42 C,whereas heat-sensitive mutants can grow at 25 C,but not at 42 C.Thus,ts mutants can be distinguished from wild-type phage by culturing the phage at low and high temperature.2022-8-11The

25、Genetics of Microorganisms25a)Plaques formed by T4 wild-type(r+)and rapid lysis(r)mutants on a confluent lawn of E.coli strain B cells.b)Individual r+and r plaques shown at higher magnification.B.Altered plaque morphology噬菌斑变异噬菌斑变异2022-8-11The Genetics of Microorganisms26C.Host range mutants(宿主范围突变宿

26、主范围突变)E.coli strain B cells can be infected by all wild-type T-even phages.E.coli B/2 cells harbor a mutation that alters the phage T2 receptor on the bacterial surface so that T2 phage can not attach them.Plaques formed by T2 wild-type(h+)and host range(h)mutants when grown on a mixed lawn(菌苔菌苔)of

27、E.coli B and B/2 cells.2022-8-11The Genetics of Microorganisms27Alfred HersheyMax DelbrckHershey shared the 1969 Nobel Prize for Physiology and Medicine with Max Delbrck and Salvador Luria.Salvador Luria.2022-8-11The Genetics of Microorganisms28Parental phage:Turbid plaques with sharp edges(h+r)or c

28、lear plaques with fuzzy edges(h r+)Recombinant progenyTurbid with fuzzy edges(h+r+)or clear with sharp edges(h r)The types of plaques formed by T2 h+r+,hr,h+r,hr+phage when grown on a lawn containing both E.coli B and B/2 cells.On this mixed lawn,each of the four possible genotypes produces a plaque

29、 with a distinct phenotype.2022-8-11The Genetics of Microorganisms29When large numbers of progeny were analyzed,about 2%had recombinant genotypes.On the basis of these results,the distance between the r and h genes was estimated to be about 2 centiMorgans.The results also indicated that phage recomb

30、ination is a reciprocal process,because the two recombinant genotypes(h+r and h r)were present among the progeny with approximately the same frequency.2022-8-11The Genetics of Microorganisms303.Bacteriophage T4:A linear Chromosome and a Circular Genetic MapT4 is a single linear molecular DNA.Thus,th

31、e T4 linkage map was expected to be linear.However,the results of early genetic crosses exhibited some unexpected,but repeatable,inconsistencies.2022-8-11The Genetics of Microorganisms31 An early circular map of the bacteriophage T4 chromosome.The mutant alleles are e(endolysin=lysozyme),tu(turbid p

32、laque),r(rapid lysis),h(host range)and ac(acridine resistance).The results of two-factor crosses indicated that the mutations h42,ac41,and r67 were linked with the order h42-ac41-r67.The results of three-factor crosses indicated that the order was ac41-h42-r67.The contradiction could be resolved by

33、making the T4 genetic map circular.With a circular map,h42 maps between ac41 and r67 starting at ac41 and moving counterclockwise.2022-8-11The Genetics of Microorganisms32However,given the circular map and moving clockwise from h42 yields the sequence h42-ac41-r67.So,a circular genetic map resolved

34、the paradox.But how can a circular genetic map be generated from a linear chromosome?2022-8-11The Genetics of Microorganisms33He proposed that T4 chromosome was both terminally redundant(双侧末双侧末端重复端重复)and circularly permuted(循环序列变化循环序列变化).George Streisinger1960s2022-8-11The Genetics of Microorganisms

35、34The formation of concatameric(系列系列)replicative DNA molecules by“head-to-tail”recombination within the terminally redundant regions of T4 chromosomes.头部充满机制头部充满机制Headfull mechanism2022-8-11The Genetics of Microorganisms352022-8-11The Genetics of Microorganisms36Key Points Viruses are obligate paras

36、ites 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.In its integrated state,the chromosome is called a prophage,and its lytic genes are kept turned off.202

37、2-8-11The Genetics of Microorganisms37Bacteriophage 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.Phage T4 has a circular genetic map but a linear chromosome.The circular map is produced beca

38、use T4 chromosomes are terminally redundant and circularly permuted.2022-8-11The Genetics of Microorganisms38Section 3The Genetics of BacteriaBacteria contain genes that mutant to produce altered phenotypes.Gene transfer in bacteria is unidirectionalfrom donor cells to recipient cells.2022-8-11The G

39、enetics of Microorganisms39The genetic information of most bacteria is stored in a single main chromosome carrying a few thousand genes and a variable number of“mini-chromosomes”called plasmids and episomes(附加体附加体).2022-8-11The Genetics of Microorganisms40Plasmids are autonomously replicating,circul

40、ar DNA molecules that carry anywhere from three genes to several hundred genes.Some bacteria contain as many as 11 different plasmids in addition to the main chromosome.Episomes are similar to plasmids,but episomes can replicate either autonomously or as part of the main chromosomein an integrated s

41、tate like the prophage.2022-8-11The Genetics of Microorganisms41Bacteria reproduce asexually by simple fission(分裂生殖分裂生殖),with each daughter cell receiving one copy of the chromosome.They are monoploid but“multinucleate”;that is,the cell usually contains two or more identical copies of the chromosome

42、.The chromosome of bacteria do not go through the meiotic and meiotic condensation cycle that occur during cell division and gametogenesis in eukaryotes.在细菌中不发生重组如在真核生物有性生殖过程中的自由组合和交换在细菌中不发生重组如在真核生物有性生殖过程中的自由组合和交换2022-8-11The Genetics of Microorganisms42Recombination has been just as important in th

43、e evolution of bacteria as it has been in the evolution of eukaryotes.Indeed,processes that are akin to sexual reproductionparasexual processes(拟有拟有性性)occur in bacteria.Conjugation 接合接合Transformation 转化转化Transduction 转导转导2022-8-11The Genetics of Microorganisms431.Mutant Genes in BacteriaWhat is colo

44、ny?Each bacterial species produces colonies with a specific color and morphology.Mutations in bacteria genes can change colony color morphology small or petite coloniesColony of Serratia macescens2022-8-11The Genetics of Microorganisms441.1 Mutants Blocked in Their Ability to Utilize Specific Energy

45、 SourcesWild-type E.coli can use almost any sugar as energy source.However,some mutants are unable to grow on the milk sugar lactose.They grow well on other sugars but cannot grow on medium containing lactose as the sole energy source.Other mutants are unable to grow on galactose,and still others ar

46、e unable to grow on arabinose.2022-8-11The Genetics of Microorganisms45Standard nomenclature for describing these and other types of mutants in bacteria is to use three letter abbreviations with appropriate superscripts.For phenotypes,the first letter is capitalized;For genotypes are lower case and

47、italicized.Wild-type E.coli is phenotypically Lac+and genotypically lac+.Mutants that are unable to utilize lactose as an energy source are phenotypically Lac-and genotypically lac-(or just lac).2022-8-11The Genetics of Microorganisms461.2 Mutants Unable to Synthesize an Essential MetaboliteWild-typ

48、e E.coli can grow on medium(minimal medium)containing an energy source and some inorganic salts.These cells can synthesize all of the metabolitesamino acids,vitamins,purines,pyrimidines,and so onthey need from these substances.These bacteria are called prototrophs(原养原养型微生物型微生物).2022-8-11The Genetics

49、 of Microorganisms47When a mutation occurs in a gene encoding an enzyme required for the synthesis of an essential metabolite,the bacterium carrying that mutation will have a new growth requirement.It will grow if the metabolite is added to the medium,but it will not grow in the absence of the metab

50、olite.Such mutants are called auxotrophs(营养缺陷型营养缺陷型);they require auxiliary nutrients for growth.Wild-type E.coli can synthesize tryptophan de nove;these cells are phenotypically Trp+and genotypically trp+.Tryptophan auxotrophis are Trp-and trp-.2022-8-11The Genetics of Microorganisms481.3 Mutants R