遗传学英文课件：8 原核生物的遗传学英文课件

1、2020/12/8,The Genetics of Microorganisms,1,Chapter 8The Genetics of Microorganisms,2020/12/8,The Genetics of Microorganisms,2,Chapter Outline,Microorganisms in Genetics The Genetics of Viruses The Genetics of Bacteria Mechanisms of Genetic Exchange in Bacteria The Evolutionary Significance of Geneti

2、c Exchange in Bacteria The Genetics of Fungi,2020/12/8,The Genetics of Microorganisms,3,Multi-Drug-Resistant Bacteria: A Ticking Timebomb,2020/12/8,The Genetics of Microorganisms,4,Section 1Microorganisms in Genetics,Microorganisms have made important contributions to the science of genetics,2020/12

3、/8,The Genetics of Microorganisms,5,Key Points,Their 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,2020/12/8,The Genetics of Microorganisms

4、,6,Section 2The Genetics of Viruses,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,2020/12/8,The Genetics of Microorganisms,7,1 Bacteriophages T4 and

5、 Lambda,Viruses that infect bacteria are called bacteriophages (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,2020/12/8,The Genetics of

6、 Microorganisms,8,Bacteriophages can be categorized into two typesvirulent (烈性) and temperate (温和)based on their lifestyles in infected cells. Bacteriophage T4 is a virulent phage Bacteriophage lambda () is a temperate phage. The results of studies performed on phage T4 and have established genetic

7、paradigms that are relevant to understanding other types of virus, such as the human immunodeficiency virus (HIV,2020/12/8,The Genetics of Microorganisms,9,1.1 Bacteriophage T4,Bacteriophage T4 is a large virus that stores its genetic information in a double-stranded DNA molecule packaged inside a p

8、roteinaceous head,2020/12/8,The Genetics of Microorganisms,10,The 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

9、 be genes,A phage T4 (center) from which the DNA has been released by osmotic (低渗透压) shock. Both ends of the linear DNA molecule are visible,2020/12/8,The Genetics of Microorganisms,11,The tail of the virus contains several important components. Its central hollow core provides the channel through w

10、hich the phage DNA is injected into bacterium. The tail sheath 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 recepto

11、rs,2020/12/8,The Genetics of Microorganisms,12,T4 is a lytic phage,2020/12/8,The Genetics of Microorganisms,13,T4 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

12、 the virus,cytosine,2020/12/8,The Genetics of Microorganisms,14,1.2 Bacteriophage 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

13、 injected into an E. coli cell, the DNA molecule is converted to a circular form, which participates in all subsequent intracellular events,2020/12/8,The Genetics of Microorganisms,15,Inside the cell, the circular chromosome can proceed down either of two pathways,Lytic cycle Lysogenic pathway Proph

14、age,2020/12/8,The Genetics of Microorganisms,16,Integration of the chromosome occurs by a site-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 chromosomean

15、d is mediated by the product of the int gene, the integrase,2020/12/8,The Genetics of Microorganisms,17,It 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 sequ

16、ence of 15 nucleotide pairs: GCTTTTTTATACTAA CGAAAAAATATGATT With the exception of this core sequence, attP and attB have quite different sequences,2020/12/8,The Genetics of Microorganisms,18,Because recombination occurs within this core sequence during integration, the resulting attB/P and attP/B s

17、ites that flank the integrated prophage also both contain the 15-nucleotide-pair sequence. These structure are important because they facilitate excision of the prophage by a very similar site-specific recombination event,2020/12/8,The Genetics of Microorganisms,19,About once in every 105 cell divis

18、ions, the prophage excises form the host chromosome and inters the lytic pathway. This phenomenon is the reason 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 w

19、ith ultraviolet light,2020/12/8,The Genetics of Microorganisms,20,The excision process is usually precise, with site-specific 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 integras

20、e and the product of the xis gene, excisase (切除酶) These two enzymes mediate a site-specific recombination event that is essentially the reverse of the integration event,2020/12/8,The Genetics of Microorganisms,21,Occasionally, excision occurs anomalously, and bacterial DNA is excised along with phag

21、e DNA. When this occurs, the resulting virus can transfer bacterial genes from one host bacterium to another,2020/12/8,The Genetics of Microorganisms,22,2. Mapping Genes in Bacteriophage,Genes on bacteriophage chromosomes can be mapped using recombination frequencies, just as in eukaryotes. However,

22、 because viruses have a single chromosome that does not go through meiosis, the mapping procedure is some what different from that used for an organism like Drosophila,2020/12/8,The Genetics of Microorganisms,23,Crosses are performed by simultaneously infecting host bacteria with two different types

23、 of phage and then screening the progeny phage for recombinant genotypes. Map distance, in centiMorgans, are then calculated as the average number of crossovers that have occurred between genetic markers. For short distances, map distances are approximately equal to the percentage of recombinant chr

24、omosomes among the progeny,2020/12/8,The Genetics of Microorganisms,24,There are many different kinds of alleles in phage,Temperature-sensitive (ts) mutations are among the most useful. Wild-type coliphage can grow at temperatures ranging from about 25 to over 42C, whereas heat-sensitive mutants can

25、 grow at 25C, but not at 42C. Thus, ts mutants can be distinguished from wild-type phage by culturing the phage at low and high temperature,2020/12/8,The Genetics of Microorganisms,25,Plaques formed by T4 wild-type (r+) and rapid lysis (r) mutants on a confluent lawn of E. coli strain B cells.b) Ind

26、ividual r+ and r plaques shown at higher magnification,Altered plaque morphology 噬菌斑变异,2020/12/8,The Genetics of Microorganisms,26,Host range mutants (宿主范围突变,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

27、 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) of E. coli B and B/2 cells,2020/12/8,The Genetics of Microorganisms,27,Alfred Hershey,Max Delbrck,Hershey shared the 1969 Nobel Prize for Physiol

28、ogy and Medicine with Max Delbrck and Salvador Luria,Salvador Luria,2020/12/8,The Genetics of Microorganisms,28,Parental phage: Turbid plaques with sharp edges (h+ r) or clear plaques with fuzzy edges (h r+) Recombinant progeny Turbid with fuzzy edges (h+ r+) or clear with sharp edges (h r,The types

29、 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 with a distinct phenotype,2020/12/8,The Genetics of Microorganisms,29,When large numbers of progeny were analyzed,

30、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 recombination is a reciprocal process, because the two recombinant genotypes (h+ r and h r) were present among the

31、 progeny with approximately the same frequency,2020/12/8,The Genetics of Microorganisms,30,3. Bacteriophage T4: A linear Chromosome and a Circular Genetic Map,T4 is a single linear molecular DNA. Thus, the T4 linkage map was expected to be linear. However, the results of early genetic crosses exhibi

32、ted some unexpected, but repeatable, inconsistencies,2020/12/8,The Genetics of Microorganisms,31,An early circular map of the bacteriophage T4 chromosome. The mutant alleles are e (endolysin=lysozyme), tu (turbid plaque), r (rapid lysis), h (host range) and ac (acridine resistance,The results of two

33、-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 making the T4 genetic map circular. With a circular map, h42 maps between ac4

34、1 and r67 starting at ac41 and moving counterclockwise,2020/12/8,The Genetics of Microorganisms,32,However, given the circular map and moving clockwise from h42 yields the sequence h42-ac41-r67. So, a circular genetic map resolved the paradox. But how can a circular genetic map be generated from a l

35、inear chromosome,2020/12/8,The Genetics of Microorganisms,33,He proposed that T4 chromosome was both terminally redundant (双侧末端重复) and circularly permuted (循环序列变化,George Streisinger 1960s,2020/12/8,The Genetics of Microorganisms,34,The formation of concatameric (系列) replicative DNA molecules by “hea

36、d-to-tail” recombination within the terminally redundant regions of T4 chromosomes,头部充满机制 Headfull mechanism,2020/12/8,The Genetics of Microorganisms,35,2020/12/8,The Genetics of Microorganisms,36,Key Points,Viruses are obligate parasites that can reproduce only by infecting living host cells. Bacte

37、riophages 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,2020/12/8,The Genetics of Microorganisms,37,Bacteriophage lam

38、bda () 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 because T4 chromosomes are terminally redundant and

39、circularly permuted,2020/12/8,The Genetics of Microorganisms,38,Section 3The Genetics of Bacteria,Bacteria contain genes that mutant to produce altered phenotypes. Gene transfer in bacteria is unidirectionalfrom donor cells to recipient cells,2020/12/8,The Genetics of Microorganisms,39,The genetic i

40、nformation 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,2020/12/8,The Genetics of Microorganisms,40,Plasmids are autonomously replicating, circular DNA molecules that carry anywhere

41、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 state like the prophage,2020/12/8,T

42、he Genetics of Microorganisms,41,Bacteria reproduce asexually by simple fission (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. The chromosome of bacteria

43、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,2020/12/8,The Genetics of Microorganisms,42,Recombination has been just as important in the evolution of bacteria as it has been in the evolutio

44、n of eukaryotes,Indeed, processes that are akin to sexual reproductionparasexual processes (拟有性)occur in bacteria. Conjugation 接合 Transformation 转化 Transduction 转导,2020/12/8,The Genetics of Microorganisms,43,1. Mutant Genes in Bacteria,What is colony? Each bacterial species produces colonies with a

45、specific color and morphology. Mutations in bacteria genes can change colony color morphology small or petite colonies,Colony of Serratia macescens,2020/12/8,The Genetics of Microorganisms,44,1.1 Mutants Blocked in Their Ability to Utilize Specific Energy Sources,Wild-type E. coli can use almost any

46、 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 are unable to grow on arabinose,2020/12/

47、8,The Genetics of Microorganisms,45,Standard 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 italicized. Wild-type E. coli is

48、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,2020/12/8,The Genetics of Microorganisms,46,1.2 Mutants Unable to Synthesize an Essential Metabolite,Wild-type E. coli can grow on me

49、dium (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 (原养型微生物,2020/12/8,The Genetics of Microorganisms,4

50、7,When 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 metabolite. Such mut

51、ants are called auxotrophs (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,2020/12/8,The Genetics of Microorganisms,48,1.3 Mutants Resistant

52、to Drugs and Antibiotics,Wild-type E. coli are killed by antibiotics such as ampicillin and tetracycline. Phenotypically , they are Amps and Tets. The mutant alleles that make E. coli resistant to these antibiotics are designated ampr and tetr, respectively,2020/12/8,The Genetics of Microorganisms,4

53、9,Bacteria that contain these mutant alleles can grow on medium containing the antibiotics, whereas wild-type bacteria cannot. Thus, antibiotics can be used to select bacteria that carry genes for resistance. The resistance genes function as dominant selectable markers. Bacteria divide rapidly and p

54、roduce large populations of cells for genetic studies. Moreover, media that select specific bacterial genotypes are selectively easy to prepare,2020/12/8,The Genetics of Microorganisms,50,2 Unidirectional Gene Transfer in Bacteria,The recombination events that occur in bacteria involve transfers of

55、genes from one bacterium to another, rather than the reciprocal exchanges of genes that occur in eukaryotes. Thus, gene transfer is unidirectional rather than bidirectional细菌间的基因转移是单向的，而不是双向的,2020/12/8,The Genetics of Microorganisms,51,Recombination events in bacteria usually occur between a fragmen

56、t of one chromosome (from donor cell) and a complete chromosome (in a recipient cell), rather than between two complete chromosome in eukaryotes. With rare exceptions, the recipient cells become partial diploids, containing a linear piece of the donor chromosome and a complete circular recipient chr

57、omosome,2020/12/8,The Genetics of Microorganisms,52,As a result, crossovers must occur in pairs and must insert a segment of the donor chromosome into the recipient chromosome. If a single crossover occurs, it will destroy the integrity of the recipient chromosome, producing a nonviable linear DNA m

58、olecule,2020/12/8,The Genetics of Microorganisms,53,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 add

59、itional metabolites for growth. Gene transfer in bacteria is unidirectional; genes from a donor cell are transferred to a recipient cell, with no transfer from recipient to donor,2020/12/8,The Genetics of Microorganisms,54,Section 4Mechanisms of Genetic Exchange in Bacteria,Bacteria exchange genetic

60、 material through three different parasexual (拟有性的) processes,2020/12/8,The Genetics of Microorganisms,55,Transformation (转化) involves the uptake of free DNA molecules released from one bacterium (the donor cell) by another bacterium (the recipient cell). Conjugation (接合) involves the direct transfe