華中農(nóng)業(yè)大學(xué)微生物英文

1、會計學(xué)1華中農(nóng)業(yè)大學(xué)微生物英文華中農(nóng)業(yè)大學(xué)微生物英文第1頁/共58頁5.1 General Properties of Viruses 5.2 General Features of Virus Reproduction5.3 Overview of Bacterial Viruses5.4 Temperate Bacteriophages: Lysogeny and Lambda5.5 Overview of Animal Viruses5.6 Pox Viruses5.7 Adcnoviruses5.8 Retroviruses5.9 Viroids and PrionsChapter o

2、utline第2頁/共58頁(1) their simple, acellular organization (2) the absence of both DNA and RNA in the same virion, (3) their inability to reproduce independently of cells and carry out cell division as prokaryotes and eukaryotes do.viruses differ from living cells in at least three ways:5.1 General Prop

3、erties of Viruses第3頁/共58頁Virion, the extracellular phase, posses few if any enzymes and can not reproduce independently of living cells. In the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components; eventually complete v

4、irus particles or virions are released. extracellular and intracellularViruses can exist in two phases第4頁/共58頁Hosts and sizeThree main classes - animal viruses, bacterial viruses (bacteriophages), and plant viruses. The particular host range of a virus is determined by the viruss requirements for it

5、s specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication.第5頁/共58頁Virus particles (virions) vary widely in size and shape. Viruses are smaller than cells, ranging in size from 0.02 to 0.3 m. A common unit of measure fo

6、r viruses is the nanometer, which is 1000 times smaller than 1 um and 1 million times smaller than 1 mm. Smallpox virus, one of the largest viruses, is about 200 nm in diameter (a bit smaller than the size of the smallest bacteria); poliovirus, one of the smallest, is only 28 nm in diameter (about t

7、he size of a ribosome).第6頁/共58頁Viruses vary considerably in size. Although most are quite a bit smaller than bacteria, some of the larger viruses (such as the smallpox virus) are about the same size as some very small bacteria (such as the mycoplasmas, rickettsias, and chlamydias). SizeViruses range

8、 from 20 to 300 nm in diameter第7頁/共58頁Genome in virionViral genomes. The genomes of viruses can be composed of either DNA or RNA, and some use both as their genomic material at different stages in their life cycle. However , only one type of nucleic acid is found in the virion of any particular type

9、 of virus. This can be single-stranded (ss), double-stranded (ds), or in the case of the hepadnaviruses, partially double-stranded.第8頁/共58頁The comparative sizes of several viruses and bacteria:第9頁/共58頁 Most viruses are too small to be seen under light microscope. All viruses consists of an RNA or DN

10、A core genome surrounded by a protein coat capsid. The combined viral genome and capsid is called the nucleocapsid. Structure of viruses第10頁/共58頁The nucleic acid of a virus is surrounded by a protein coat called the capsid Each capsid is composed of protein subunits called capsomeres. In some viruse

11、s, the capsid is covered by an envelope, which usually consists of some combination of lipids, proteins, and carbohydrates.Depending on the virus, envelopes may or may not be covered by spikes, which are carbohydrateprotein complexes that project from the surface of the envelope.第11頁/共58頁Structure o

12、f viruses Most viruses are too small to be seen under light microscope. All viruses consists of an RNA or DNA core genome surrounded by a protein coat capsid. The combined viral genome and capsid is called the nucleocapsid.第12頁/共58頁Some viruses, particularly bacterial viruses, have very complicated

13、structures and are called complex viruses. Examples of complex viruses are poxviruses, which do not contain clearly identifiable capsids but have several coats around the nucleic acid. Certain bacteriophages have capsids to which additional structures are attached. Complex viruses第13頁/共58頁A virus ca

14、n have either DNA or RNA but never both !第14頁/共58頁A virus can have either DNA or RNA but never both !第15頁/共58頁Viruses may be classified into several morphological types on the basis of their capsid architecture as revealed by electron microscopy and a technique called x-ray crystallography. GENERAL

15、MORPHOLOGY第16頁/共58頁A, Some viruses, such as tobacco mosaic virus, have a helical symmetry with the capsid surrounding an RNA genome. B, Many viruses that infect bacteria, such as the T-even bacteriophage, have a complex capsid with DNA contained within the head structure. C, Some animal viruses, suc

16、h as adenovirus, have isometric symmetry and a DNA genome. D, others, such as coronavirus(冠狀病毒）, have complex capsids and an envelope with protruding proteins surrounding an RNA genome.第17頁/共58頁Helical viruses resemble long rods that may be rigid or flexible. Surrounding the nucleic acid, their caps

17、id is a hollow cylinder with a helical structure. An example of a helical virus that is a rigid rod is the tobacco mosaic virus Helical viruses第18頁/共58頁The capsid of most polyhedral viruses is in the shape of a regular polyhedron with 20 triangular faces and 12 corners. The capsomeres of each face f

18、orm an equilateral triangle. An example of a polyhedral virus in the shape of an icosahedron is the adenovirus. Another icosahedral virus is the poliovirus. Polyhedral viruses 第19頁/共58頁The capsid of some viruses is covered by an envelope. Enveloped viruses are roughly spherical but highly variable i

19、n shape because the envelope is not rigid. When helical or polyhedral viruses are enclosed by envelopes, they are called enveloped helical and enveloped polyhedral viruses. An example of an enveloped helical virus is the influenza virus.Enveloped viruses第20頁/共58頁Some viruses, particularly bacterial

20、viruses, have very complicated structures and are called complex viruses. Examples of complex viruses are poxviruses, which do not contain clearly identifiable capsids but have several coats around the nucleic acid. Certain bacteriophages have capsids to which additional structures are attached. Com

21、plex viruses第21頁/共58頁Viral Multiplication For a virus to multiply, it must invade a host cell and take over the hosts metabolic machinery. A single virus can give rise to several or even thousands of similar viruses in a single host cell. This process drastically changes the host cell and often caus

22、es its death. 第22頁/共58頁MULTIPLICATION OF BACTERIOPHAGES The multiplication cycle of bacteriophages, like that of all viruses, can be divided into several distinct stages-attachmentpenetration biosynthesis of viral components Maturation Release第23頁/共58頁1. Attachment (adsorption) of the virion to a su

23、sceptible host cell.2. Penetration (injection) of the virion or its nucleic acid into the cell.3. Early steps in replication during which the host cell biosynthetic machinery is altered as a prelude to virus nucleic acid synthesis. Virus-specific enzymes are typically made.4. Replication of the viru

24、s nucleic acid.5. Synthesis of proteins used as structural subunits of the virus coat.6. Assembly of structural subunits (and membrane components in enveloped viruses) and packaging of nucleic acid into new virus particles.7. Release of mature virions from the cell第24頁/共58頁第25頁/共58頁Attachment of pha

25、ge to host cell :After a chance collision between phage particles and bacteria, an attachment site on the virus attaches to a complementary receptor site on the bacterial cell. 第26頁/共58頁P(yáng)enetration:During the process of penetration, the bacteriophages tail releases an enzyme, phage lysozyme, which b

26、reaks down a portion of the bacterial cell wall. then the bacteriophage injects its DNA (nucleic acid) into the bacterium. 第27頁/共58頁Biosynthesis of viral components:Any RNA transcribed in the cell is mRNA transcribed from phage DNA for biosynthesis of phage enzymes and capsid protein. The host cells

27、 ribosomes, enzymes, and amino acids are used for translation 第28頁/共58頁第29頁/共58頁Formation of mRNA after infection of cells by viruses of different types. The chemical sense of the mRNA is considered as plus (+). The sense of the various virus nucleic acids are indicated as + if the same as mRNA, as

28、if opposite, or as +- if double-stranded. Examples are indicated next to the virus nucleic acid. Although examples of viruses containing SS DNA of the - sense are known, none are discussed in the text.第30頁/共58頁Maturation and release:The phage heads and tails are separately assembled from protein sub

29、units, the head is packaged with phage DNA, and the tail is attached 第31頁/共58頁Release: Lysozyme, whose code is provided by a phage gene, is synthesized within the cell. This enzyme causes a breakdown of the bacterial cell wall, and the newly produced bactedophages are released from the host cell. Th

30、e number of newly synthesized phage particles released from a single cell usually ranges from about 50 to 200. 第32頁/共58頁There are genetic controls that regulate when different regions of phage DNA are transcribed into mRNA during the multiplication cycle. There are early messages that are translated

31、 into early phage proteins, the enzymes used in the synthesis of phage DNA. There are late messages that are translated into late phage proteins for the synthesis of capsid proteins. This control mechanism is mediated by RNA polymerase.eclipse period 第33頁/共58頁Time course of events in phage T4 infect

32、ionLatent period第34頁/共58頁one-step growth curve第35頁/共58頁第36頁/共58頁第37頁/共58頁Lysogeny Some phages can incorporate their DNA into the host cells DNA., The phage remains latent and does not cause lysis of the host cell. Such a state is called lysogeny.Such phages are called lysogenic phages or temperate p

33、hages. The participating bacterial host cells are known as lysogenic cells. 第38頁/共58頁Lysogenic cycle of bacteriophage lambda in E.coli第39頁/共58頁The consequences of infection by a temperate bacteriophage. The alternatives on infection are integration of the virus DNA into the host DNA (lysogenization)

34、 or replication and release of mature virus (lysis). The lysogenic cell can also be induced to produce mature virus and lyse. 第40頁/共58頁The consequences of infection by a temperate bacteriophage. The alternatives on infection are integration of the virus DNA into the host DNA (lysogenization) or repl

35、ication and release of mature virus (lysis). The lysogenic cell can also be induced to produce mature virus and lyse. 第41頁/共58頁1. What are the two pathways available to temperate virus?2. Describe how a single protein like the lambda represor can act both as an activator and a represor.Think and ans

36、wer following two questions !第42頁/共58頁1. Nature of the host-animal, plant, bacterial, insect, fungal2. Nucleic acid characteristics-DNA or RNA, single or double stranded, molecular weight3. Capsid symmetry-icosahedral, helical4. Presence of an envelope and ether sensitivity5. Diameter of the virion

37、or nucleocapsid6. Number of capsomers in icosahedral viruses7. Immunologic properties8. Intracellular location of viral replicationSome important characteristics for viral classification第43頁/共58頁(1)nucleic acid type(2) nucleic acid strandedness(3) presence or absence of an envelope Recently, the Int

38、ernational Committee for Taxonomy of Viruses has developed a uniform classification system and divided viruses into 50 families. The committee places greatest weight on three properties: 第44頁/共58頁Schematic representations of the main types of bacterial virusesTailed bacteriophage :Genome: DNA, doubl

39、e-stranded. Virion: complex shape, binary symmetry, variable number of capsomers. The tails of the phage are long and contractile in group A , long and noncontractile in group B, and very short in group C. Example: T-even coliphages. 第45頁/共58頁Cubic bacteriophage:Group 1 Genome:DNA, single-stranded.

40、Virion: icosahedral, cubic symmetry, 12 capsomers. Example: X174 Group 2 Genome: DNA, double-stranded. Virion: cubic symmetry, enveloped. Example: PM-2.Group 3 Genome: RNA, single-stranded. Virion: icosahedral, cubic symmetry, 32 capsomers.Group 4 Genome: RNA, double-stranded. Virion: cubic symmetry

41、, enveloped. Example: 06. 第46頁/共58頁 Filamentous bacteriophage:Genome: DNA, single-stranded. Virion: rod-shaped, helical symmetry. Example: fd.第47頁/共58頁P(yáng)lant Viruses : Tobacco mosaic virus (TMV) as an example (1) penetration by the virus of a susceptible plant cell-generally through abrasions or inse

42、ct bites, (2) tincoating of the viral nucleic acid within the plant cell,(3) assumption by the viral genome of control of the synthetic activities of the host cell, (4) expression of the viral genome so that viral nucleic acid and capsid components are synthesized, (5) assembly of the viral particle

43、s within the host cell, and, (6) release of the complete viral particles from the host plant cell. 第48頁/共58頁Animal Viruses:第49頁/共58頁Viroids and Prions:Viroids are small, circular, single-stranded RNA molecules that are the smallest known pathogens. The extracellular form of the viroid is naked RNA-t

44、here is no capsid of any kind. Prions have a distinct extracellular form, but the extracellular form seems to be entirely protein. It apparently does not contain any nucleic acid, or if it does, the molecule is not long enough to encode the single kind of protein of which the prion is composed 第50頁/

45、共58頁Maturation and release:The phage heads and tails are separately assembled from protein subunits, the head is packaged with phage DNA, and the tail is attached 第51頁/共58頁(1) their simple, acellular organization (2) the absence of both DNA and RNA in the same virion, (3) their inability to reproduce independently of