ch 13_lecture_presentation

PowerPoint® Lecture Presentations prepared byBradley W. Christian, McLennan Community College

C H A P T E R

Viruses, Viroids, and Prions

General Characteristics of Viruses

Learning Objective13-1 Differentiate a virus from a bacterium.

General Characteristics of Viruses

• Obligatory intracellular parasites• Require living host cells to multiply

• Contain DNA or RNA• Contain a protein coat• No ribosomes• No ATP-generating mechanism

Table 13.1 Viruses and Bacteria Compared

Host Range

• The spectrum of host cells a virus can infect• Most viruses infect only specific types of cells

in one host• Determined by specific host attachment sites and

cellular factors• Bacteriophages—viruses that infect bacteria• Range from 20 nm to 1000 nm in length

Figure 13.1 Virus sizes.

Bacteriophages f2, MS2

Poliovirus

Rhinovirus

Adenovirus

Rabies virus

Bacteriophage T4

Tobacco mosaic virus

Viroid

Vaccinia virus

170 × 70 nm

200 × 20 nm

225 nm

250 × 18 nm

300 × 10 nm

300 × 200 × 100 nm

Bacteriophage M13

Ebola virus

300 nm

E. coli bacterium3000 × 1000 nm

Plasma membraneof red blood cell10 nm thick

Human red blood cell10,000 nm in diameter

Chlamydia bacterium elementary body

800 × 10 nm

970 nm

Check Your Understanding How could the small size of viruses have helped

researchers detect viruses before the invention of the electron microscope?13-1

Viral Structure

Learning Objective13-2 Describe the chemical and physical structure

of both an enveloped and a nonenveloped virus.

Viral Structure

• Virion—complete, fully developed viral particle• Nucleic acid—DNA or RNA can be single- or double-

stranded; linear or circular• Capsid—protein coat made of capsomeres (subunits)• Envelope—lipid, protein, and carbohydrate coating on

some viruses• Spikes—projections from outer surface

Figure 13.2 Morphology of a nonenveloped polyhedral virus.

Nucleic acid

Capsomere Capsid

Figure 13.3 Morphology of an enveloped helical virus.

Nucleic acid

Capsomere

Spikes Envelope

General Morphology

• Helical viruses—hollow, cylindrical capsid• Polyhedral viruses—many-sided• Enveloped viruses • Complex viruses—complicated structures

Figure 13.4 Morphology of a helical virus.

Nucleic acid

CapsomereCapsid

Figure 13.5 Morphology of complex viruses.65 nm

Capsid (head)

Sheath

Tail fiber

PinBaseplate

A T-even bacteriophage

Orthopoxvirus

Check Your Understanding Diagram a nonenveloped polyhedral virus that has

spikes.13-2

Taxonomy of Viruses

Learning Objectives13-3 Define viral species.13-4 Give an example of a family, genus, and

common name for a virus.

Taxonomy of Viruses

• Genus names end in -virus• Family names end in -viridae• Order names end in -ales• Viral species: a group of viruses sharing the

same genetic information and ecological niche (host) • Descriptive common names are used for species• Subspecies are designated by a number

Check Your Understanding How does a virus species differ from a bacterial

species?13-3

Attach the proper endings to Papilloma- to show the family and genus that includes HPV, the cause of cervical cancer.13-4

Isolation, Cultivation, and Identification of Viruses

Learning Objectives13-5 Describe how bacteriophages are cultured.13-6 Describe how animal viruses are cultured.13-7 List three techniques used to identify viruses.

Growing Bacteriophages in the Laboratory

• Viruses must be grown in living cells• Bacteriophages are grown in bacteria• Bacteriophages form plaques, which are clearings on a

lawn of bacteria on the surface of agar• Each plaque corresponds to a single virus; can be

expressed as plaque-forming units (PFU)

Figure 13.6 Viral plaques formed by bacteriophages.

Plaques

Growing Animal Viruses in the Laboratory

• In living animals• In embryonated eggs• Virus injected into the egg• Viral growth is signaled by changes or death of the

embryo

Figure 13.7 Inoculation of an embryonated egg.

Air sac

ShellAmnioticcavity

Chorioallantoicmembrane Chorioallantoic

membraneinoculation

Amnioticinoculation

Allantoicinoculation

Yolk sacinoculationAllantoic

cavityAlbumin

Shellmembrane

Yolksac

Growing Animal Viruses in the Laboratory

• In cell cultures• Tissues are treated with enzymes to separate cells• Virally infected cells are detected via their deterioration,

known as the cytopathic effect (CPE)• Continuous cell lines are used

Figure 13.8 Cell cultures.

A tissue is treated with enzymesto separate the cells.

Cells are suspendedin culture medium.

Normal cells or primary cells grow in a monolayer acrossthe glass or plastic container. Transformed cells orcontinuous cell cultures do not grow in a monolayer.

Normalcells

Transformedcells

Figure 13.9 The cytopathic effect of viruses.

Viral Identification

• Cytopathic effects• Serological tests• Western blotting—reaction of the virus with antibodies

• Nucleic acids• RFLPs• PCR

Check Your Understanding What is the plaque method?

13-5 Why are continuous cell lines of more practical

use than primary cell lines for culturing viruses?13-6

What tests could you use to identify influenza virus in a patient?13-7

Viral Multiplication

Learning Objectives13-8 Describe the lytic cycle of T-even

bacteriophages.13-9 Describe the lysogenic cycle of bacteriophage

lambda.13-10 Compare and contrast the multiplication cycle

of DNA- and RNA-containing animal viruses.

Viral Multiplication

• For a virus to multiply:• It must invade a host cell• It must take over the host's metabolic machinery

• One-step growth curve

Figure 13.10 A viral one-step growth curve.

Multiplication of Bacteriophages

• Lytic cycle• Phage causes lysis and death of the host cell

• Lysogenic cycle• Phage DNA is incorporated in the host DNA• Phage conversion• Specialized transduction

PLAY Animation: Viral Replication: Virulent Bacteriophages

Viral Replication: Virulent Bacteriophages

PLAY Animation: Viral Replication: Temperate Bacteriophages

Viral Replication: Temperate Bacteriophages

T-Even Bacteriophages: The Lytic Cycle

• Attachment: phage attaches by the tail fibers to the host cell

• Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell

• Biosynthesis: production of phage DNA and proteins

• Maturation: assembly of phage particles• Release: phage lysozyme breaks the cell wall

Figure 13.11 The lytic cycle of a T-even bacteriophage.

Attachment: Phage attachesto host cell.

Penetration: Phage penetrateshost cell and injects its DNA.

Biosynthesis: Phage DNA directssynthesis of viral components by thehost cell.

Maturation: Viral components are assembled into virions.

Release: Host cell lyses, and new virions are released.

Bacterialcell wall

Bacterialchromosome

Capsid DNA

Capsid (head)Sheath

Tail fiber

Baseplate

Cell wall

Plasma membrane

Sheath contracted

Tail core

TailDNA

Capsid

Tail fibers

Bacteriophage Lambda (λ): The Lysogenic Cycle

• Lysogeny: phage remains latent• Phage DNA incorporates into host cell DNA• Inserted phage DNA is known as a prophage• When the host cell replicates its chromosome, it also

replicates prophage DNA• Results in phage conversion—the host cell exhibits

new properties

Figure 13.12 The lysogenic cycle of bacteriophage λ in E. coli.

Phage DNA(double-stranded)

Phage attachesto host cell andinjects DNA.

Bacterialchromosome

Occasionally, the prophage mayexcise from the bacterial chromosomeby another recombination event,initiating a lytic cycle.

Many celldivisions

Lysogenic bacteriumreproduces normally.

Phage DNA integrates within thebacterial chromosome by recombination,becoming a prophage.

Prophage

Phage DNA circularizes and enterslytic cycle or lysogenic cycle.

New phage DNA andproteins are synthesizedand assembled into virions.

Cell lyses, releasingphage virions.

Lysogenic cycle

Lytic cycle

Bacteriophage Lambda (λ): The Lysogenic Cycle

• Specialized transduction• Specific bacterial genes transferred to another

bacterium via a phage• Changes genetic properties of the bacteria

Figure 13.13 Specialized transduction.Prophage gal gene

Prophage exists ingalactose-using host(containing the gal gene).

Phage genome excises,carrying with it the adjacent galgene from the host.

Phage matures and cell lyses,releasing phage carrying galgene.

gal gene

Galactose-positivedonor cell

BacterialDNA

Phage infects a cell that cannotutilize galactose (lacking galgene).

Along with the prophage, thebacterial gal gene becomesintegrated into the new host'sDNA.

Lysogenic cell can nowmetabolize galactose.

Galactose-positiverecombinant cell

Galactose-negativerecipient cell

PLAY Animation: Transduction: Generalized Transduction

Transduction: Generalized Transduction

PLAY Animation: Transduction: Specialized Transduction

Transduction: Specialized Transduction

Check Your Understanding How do bacteriophages get nucleotides and

amino acids if they don't have any metabolic enzymes?13-8

Vibrio cholerae produces toxin and is capable of causing cholera only when it is lysogenic. What does this mean?13-9

Table 13.3 Bacteriophage and Animal Viral Multiplication Compared

Multiplication of Animal Viruses

• Attachment: viruses attach to the cell membrane• Entry by receptor-mediated endocytosis or fusion• Uncoating by viral or host enzymes• Biosynthesis: production of nucleic acid and

proteins• Maturation: nucleic acid and capsid proteins

assemble• Release by budding (enveloped viruses) or

rupture

Figure 13.14 The entry of viruses into host cells.

Host plasma membraneproteins at site ofreceptor-mediatedendocytosis

Fusion of viral envelopeand plasma membrane

Entry of pig retrovirus byreceptor-mediated endocytosis.

Entry of herpesvirus by fusion.

Figure 13.20 Budding of an enveloped virus.

Viral capsid

Host cell plasma membrane

Viral protein

Envelope

Release by budding

Lentivirus

PLAY Animation: Viral Replication: Overview

Viral Replication: Overview

PLAY Animation: Viral Replication: Animal Viruses

Viral Replication: Animal Viruses

The Biosynthesis of DNA Viruses

• DNA viruses replicate their DNA in the nucleus of the host using viral enzymes

• Synthesize capsid in the cytoplasm using host cell enzymes

Figure 13.15 Replication of a DNA-Containing Animal Virus.

Papovavirus

Capsid

Nucleus

Cytoplasm

Host cell

Capsid proteins

MATURATIONVirionsmature.

RELEASEVirions arereleased.

BIOSYNTHESISViral DNA is replicated,and some viral proteinsare made.

Viral DNA

Capsidproteins

ENTRY andUNCOATINGVirion enters cell, andits DNA is uncoated.

A papovavirus is a typicalDNA-containing virus thatattacks animal cells.

ATTACHMENTVirion attachesto host cell.

Late translation;capsid proteins aresynthesized.

Viral replication in animals generally follows these steps: attachment, entry,uncoating, biosynthesis of nucleic acids and proteins, maturation, and release. Knowledge of viral replication phases is important for drug development strategies,and for understanding disease pathology.

KEY CONCEPTS

A portion of viral DNA istranscribed, producingmRNA that encodes"early" viral proteins.

• Adenoviridae• Double-stranded DNA, nonenveloped

• Respiratory infections in humans• Tumors in animals

Figure 13.16a DNA-containing animal viruses.

Capsomere

Mastadenovirus

• Poxviridae• Double-stranded DNA, enveloped• Cause skin lesions

• Vaccinia and smallpox viruses (Orthopoxvirus)

• Herpesviridae• Double-stranded DNA, enveloped

• HHV-1 and HHV-2—Simplexvirus; cause cold sores• HHV-3—Varicellovirus; causes chickenpox• HHV-4—Lymphocryptovirus; causes mononucleosis• HHV-5—Cytomegalovirus• HHV-6 and HHV-7—Roseolovirus• HHV-8—Rhadinovirus; causes Kaposi's sarcoma

Figure 13.16b DNA-containing animal viruses.

Capsomeres

Simplexvirus

• Papovaviridae• Double-stranded DNA, nonenveloped

• Papillomavirus • Causes warts• Can transform cells and cause cancer

• Hepadnaviridae• Double-stranded DNA, enveloped

• Hepatitis B virus• Use reverse transcriptase to make DNA from RNA

The Biosynthesis of RNA Viruses

• Virus multiplies in the host cell's cytoplasm using RNA-dependent RNA polymerase• ssRNA; + (sense) strand

• Viral RNA serves as mRNA for protein synthesis• ssRNA; – (antisense) strand

• Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis

• dsRNA—double-stranded RNA

Figure 13.17a Pathways of multiplication used by various RNA-containing viruses.

Attachment

Capsid

RNANucleus

CytoplasmHost cell

Entryand uncoating

RNA replication by viral RNA–dependent RNA polymerase

Translation and synthesisof viral proteins

Maturationand release

+ or sensestrand ofviral genome

– or antisensestrand ofviral genome

ss = single-strandedds = double-stranded

+ strand

Capsidprotein

mRNA is transcribedfrom the – strand.

– strand is transcribedfrom + viral genome.

Uncoating releases viral RNA and proteins.

Viralgenome(RNA)

Viralprotein

ssRNA;+ or sense strand;Picornaviridae

Figure 13.17b Pathways of multiplication used by various RNA-containing viruses.

– strands areincorporatedinto capsid

Capsidprotein

Attachment

Capsid

RNANucleus

Cytoplasm

Entryand uncoating

Viralgenome(RNA)

Viralprotein

Additional – strands aretranscribed from mRNA.

The + strand (mRNA) must first betranscribed from the – viral genomebefore proteins can be synthesized.

ssRNA; – orantisense strand;Rhabdoviridae

Host cell

Figure 13.17c Pathways of multiplication used by various RNA-containing viruses.

Attachment

Capsid

RNANucleus

Cytoplasm

Entryand uncoating

Viralgenome(RNA)

Viralprotein

RNA polymerase initiates production of– strands. The mRNA and – strands form thedsRNA that is incorporated as new viral genome.

Capsid proteins and RNA-dependent RNA polymerase

mRNA is produced inside thecapsid and released into thecytoplasm of the host.

dsRNA; + or sensestrand with – or antisensestrand; Reoviridae

Host cell

• Picornaviridae• Single-stranded RNA, + strand, nonenveloped

• Enterovirus• Poliovirus and coxsackievirus

• Rhinovirus• Common cold

• Hepatitis A virus

• Togaviridae• Single-stranded RNA, + strand, enveloped

• Alphavirus• Transmitted by arthropods; includes chikungunya

• Rubivirus• Rubella

• Rhabdoviridae• Single-stranded RNA, − strand, one RNA strand

• Lyssavirus • Rabies

• Numerous animal diseases

• Reoviridae• Double-stranded RNA, nonenveloped

• Reovirus (respiratory enteric orphan)• Rotavirus (mild respiratory infections and gastroenteritis)

Biosynthesis of RNA Viruses That Use DNA

• Single-stranded RNA, produce DNA • Use reverse transcriptase to produce DNA from the

viral genome• Viral DNA integrates into the host chromosome as a

provirus• Retroviridae

• Lentivirus (HIV)• Oncoviruses

Figure 13.19 Multiplication and inheritance processes of the Retroviridae.

Reversetranscriptase

Capsid Envelope

Host cell

Two identical +strands of RNA

Retrovirus enters by fusionbetween attachment spikesand the host cell receptors.

Uncoating releases the twoviral RNA strands and theviral enzymes reversetranscriptase, integrase,and protease.

DNA of oneof the host cell'schromosomes

Mature retrovirusleaves the host cell,acquiring anenvelope andattachment spikesas it buds out. Viral

enzymes

Viral RNA

Viral DNA

Reverse transcriptasecopies viral RNA toproduce double-stranded DNA.

Viral proteins are processedby viral protease; some of theviral proteins are movedto the host plasma membrane.

Identical strandsof RNAViral

proteins

Provirus

Transcription of the provirusmay also occur, producing RNA for new retrovirusgenomes and RNA thatencodes the retroviruscapsid, enzymes, andenvelope proteins.

The new viral DNAis transported intothe host cell's nucleus,where it's integrated intoa host cell chromosomeas a provirus by viralintegrase. The provirusmay be replicated whenthe host cell replicates.

Table 13.2 Families of Viruses That Affect Humans (1 of 4)

Check Your Understanding Describe the principal events of attachment, entry,

uncoating, biosynthesis, maturation, and release of an enveloped DNA-containing virus.13-10

Viruses and Cancer

Learning Objectives13-11 Define oncogene and transformed cell.13-12 Discuss the relationship between DNA- and

RNA-containing viruses and cancer.

Viruses and Cancer

• Several types of cancer are caused by viruses• May develop long after a viral infection• Cancers caused by viruses are not contagious

• Sarcoma: cancer of connective tissue• Adenocarcinomas: cancers of glandular

epithelial tissue

The Transformation of Normal Cells into Tumor Cells

• Oncogenes transform normal cells into cancerous cells

• Oncogenic viruses become integrated into the host cell's DNA and induce tumors

• A transformed cell harbors a tumor-specific transplant antigen (TSTA) on the surface and aT antigen in the nucleus

DNA Oncogenic Viruses

• Adenoviridae• Herpesviridae• Epstein-Barr virus

• Poxviridae• Papovaviridae• Human papillomavirus

• Hepadnaviridae• Hepatitis B virus

RNA Oncogenic Viruses

• Retroviridae• Viral RNA is transcribed to DNA (using reverse

transcriptase), which can integrate into host DNA• HTLV-1 and HTLV-2 cause adult T cell leukemia and

lymphoma

Check Your Understanding What is a provirus?

13-11 How can an RNA virus cause cancer if it doesn't

have DNA to insert into a cell's genome?13-12

Latent Viral Infections and Persistent Viral Infections

Learning Objectives13-13 Provide an example of a latent viral infection.13-14 Differentiate persistent viral infections from

latent viral infections.

Latent Viral Infections and Persistent Viral Infections

• Latent virus remains in asymptomatic host cell for long periods• May reactivate due to changes in immunity

• Cold sores, shingles

• A persistent viral infection occurs gradually over a long period; is generally fatal• Subacute sclerosing panencephalitis (measles virus)

Figure 13.21 Latent and persistent viral infections.

Acute infection

Latent infection

Persistent infection

Table 13.5 Examples of Latent and Persistent Viral Infections in Humans

Check Your Understanding Is shingles a persistent or latent infection?

13-13, 13-14

Prions

Learning Objective13-15 Discuss how a protein can be infectious.

Prions

• Proteinaceous infectious particles• Inherited and transmissible by ingestion,

transplant, and surgical instruments• Spongiform encephalopathies

• "Mad cow disease"• Creutzfeldt-Jakob disease (CJD)• Gerstmann-Sträussler-Scheinker syndrome • Fatal familial insomnia • Sheep scrapie

Prions

• PrPC: normal cellular prion protein, on the cell surface

• PrPSc: scrapie protein; accumulates in brain cells, forming plaques

PLAY Animation: Prions: Overview

Prions: Overview

PLAY Animation: Prions: Characteristics

Prions: Characteristics

PLAY Animation: Prions: Diseases

Prions: Diseases

Figure 13.22 How a protein can be infectious.

PrPc produced by cellsis secreted to the cellsurface.

PrPSc may be acquired orproduced by analtered PrPc gene.

PrPSc reacts with PrPc

on the cell surface.PrPSc converts the PrPc to PrPSc.

Lysosome

Endosome

PrPSc continues to accumulateas the endosome contents aretransferred to lysosomes.The result is cell death.

PrPSc accumulates inendosomes.The new PrPSc is taken

in, possibly by receptor-mediated endocytosis.

The new PrPSc convertsmore PrPc.

Plant Viruses and Viroids

Learning Objectives13-16 Differentiate virus, viroid, and prion.13-17 Describe the lytic cycle for a plant virus.

Plant Viruses and Viroids

• Plant viruses: enter through wounds or via insects• Plant cells are generally protected from disease by an

impermeable cell wall• Viroids: short pieces of naked RNA• Cause potato spindle tuber disease

Figure 13.23 Linear and circular potato spindle tuber viroid (PSTV).

Table 13.6 Classification of Some Major Plant Viruses

Check Your Understanding Contrast viroids and prions, and for each name a

disease it causes.13-15, 13-16

How do plant viruses enter host cells?13-17

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