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VIRUSESCHAPTER 10

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What are Viruses?

Obligate intracellular parasites

Viral components– Nucleic acids

– Capsid (protein)

– Envelope(Lipid w/intg proteins)

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4 overall types of viruses

1) bacteriophages - almost always DNA with a protein capsid. Lytic and lysogenic types

2) DNA viruses of Eukaryotes - often have phospholipid envelope outside of capsid

3) RNA viruses - have RNA as genetic material. Often hypermutable

4) RNA retroviruses - have reverse transcriptase. Many can integrate into host chromosome

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Viral Shapes and Sizes Helical Icosahedral T4 and adenovirus

TMV, M13

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Some viruses

T-evenHIV lambda

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Infectious Properties

Viral Host range

Viral specificity

Viral Origins

Depends on target receptor

Depends on target receptor

Selfish DNA? Transposable elements

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Bacteriophage

Most diverse?

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Bacteriophages

Plaque counts

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Bacteriophages Replication

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Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery.

Structure of T2 phage

DNA & protein

Hershey-Chase Bacteriophage Experiment - 1953

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Fig. 2.5: Life cycle of virulent T2 phage: Lytic cyle

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1. T2 bacteriophage is composed of DNA and proteins:

2. Set-up two replicates:

• Label DNA with 32P• Label Protein with 35S

3. Infected E. coli bacteria with two types of labeled T2

4. 32P is discovered within the bacteria and progeny phages, whereas 35S is not found within the bacteria but released with phage ghosts.

Fig. 2-6: Hershey-Chase Bacteriophage Experiment - 1953

1969: Alfred Hershey

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Composition and Structure

Composition– Nucleic acid

• Genome size• Modified

bases– Protein

• Protection• Infection

Structure (T4)– Size– Head or capsid– Tail

Tail

Tail Fibers

Base Plate

Head/Capsid

Contractile Sheath

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Infection of Host Cells

Irreversible attachment

• Adsorption– LPS for T4

Nucleic acid injection

Sheath Contraction

lamB for

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Bacteriophages

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Lytic Phage Multiplication Cycle

Eclipse– Early genes– Phage DNA

synthesis– Late genes

Intracellular accumulation

Lysis and Release

Total Phage

Extracellular Phage

EclipseIntracellular accumulatio

n phase

Time after Infection

Nu

mb

er o

f In

fect

iou

s P

arti

cles

Lysis

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Assay for Lytic Phage

Plaque assay– Method– Plaque forming unit

(pfu)– Measures infectious

particles

Bacteria

Phage

+Phage

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Lytic vs Lysogenic Cycle?

Role of repressor Role of cro gene product Role of proteases

Lytic = copies and immediately lyses

Lysogenic = integrates into host chromosome

“Prophage” = the latent form of phage where viral genome is incorporated into host genome

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Bacteriophages Lysogenic

Lysogeny

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Fig. 19-6Gene designation Function

Transcribedby host RNApolymerase

Left endEarlypromoters

Promoter

Inhibits host restriction0.3

0.7 Protein kinase

11.1

T7 RNA polymeraseUnknown

Origin of DNA replication1.31.7233.5

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5

6

DNA ligaseNonessentialInactivates host RNA polymeraseEndonucleaseLysozyme

Helicase, primase

DNA polymerase

ExonucleasePromoter

Promoter

Promoter

Promoter

Virion proteinHead protein

Head assembly proteinMajor head protein

Tail proteinTail protein

Virion proteinHead protein

Head protein

Head protein

Tail protein

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910

1112

1314

15

16

17

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DNA maturation

DNA maturation

Transcribedby T7 RNApolymerase

Proteins forDNA replicationand host lysis

Phage structuralcomponents andmaturation proteins

Bacteriophage T7

1. Replication cycle requires 25 minutes

2. Genome is linear double-stranded DNA of 39,737 bp

3. T7 encodes all of its own proteins for DNA replication and transcription

4. Time to complete 100 T7 genome copies from a single copy: 5 minutes

5. Burst size (Escherichia coli host): about 300 virions/cell

6. Head size, 45 nm

7. Forms large plaques

8. T7 promoters are unique and widely used in biotechnology

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Events Leading to Lysogeny

Circularization of the phage chromosome– Cohesive ends

Lygase

Closed Circle

Cohesive Ends

Linear Double Stranded Opened Circle

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Events Leading to Lysogeny

Site-specific recombination– Phage coded

enzyme

• Repression of the phage genome

– Repressor protein– Specific– Immunity to superinfection

gal bio

gal bio

gal

bio

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Termination of Lysogeny

Induction– Adverse

conditions

Role of proteases– recA protein– Destruction of

repressor

• Excision• Lytic growth

gal

bio

gal bio

gal bio

gal bio

• Gene expression

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Significance of Lysogeny

Model for animal virus transformation Lysogenic or phage conversion

– Definition: A change in the phenotype of a bacterial cell as a consequence of lysogeny

• Modification of Salmonella O antigen• Toxin production by Corynebacterium

diphtheriae

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Types of Bacteriophage

Lysogenic or temperate phage: Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. (e.g., )– Expression of most phage genes repressed – Prophage– Lysogen

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Viruses part II - Animals and Plants

Unique challenges. Must evade immune systems and mustcross 2 lipid bilayer barriers. (ie cross into nucleus)

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RNA Viruses

Chromosomal Arrangements– + strand (directly transcribed)

– – strand

– Double strand

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RNA Virus Families

11 RNA virus families– Picornaviridae (fmdv, polio)– Togaviridae (rubella)– Flaviviridae (hep C, west nile, yellow fever)– Orthomyxoviridae (flu)

RNA viruses more prone to mutation

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Fig. 19-18

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RNA Virus Families (cont.) Retroviridae (hep B, htlv)-retrovirus reverse transcriptase

Paramyxoviridae (measles, mumps, pneumonia) - ss strand

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RNA Virus Families (cont.)

Rhabdoviridae (rabies)

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RNA Virus Families (cont.)

Orthomyxoviridae (all influenza)

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RNA Virus Families (cont.)

Filoviridae

Bunyaviridae

Arenaviridae

Reoviridae

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DNA Virus Families Adenoviridae

Herpesviridae

Poxviridae

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DNA Virus Families (cont.) Papovaviridae

Hepadnaviridae

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DNA Virus Families (cont.)

Parvoviridae

Emerging viruses

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Viral Replication Activities

– Adsorption

– Penetration (virus or chromosome)

– Synthesis

– Maturation

– Release

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Animal Viruses DNA viruses

Envelope derives from cells own plasma membrane

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Animal Viruses

RNA viruses

Latent viruses

retroviruses

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Culturing Animal Viruses

Live animals

Eggs

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Culturing Animal Viruses

Cell Culture– Primary– Continuous

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Viral Cytopathic Effects Cytopathy

Teratogenic effectsJapanese word for “little monsters” mutations that affect tissue growth

Damage to cells

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Viruslike Agents PLANTS Satellites

Viroids

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Viruslike Agents

Prions

KuruCreutzfeld-JacobBSEScrapie

Alpha helix B-pleated sheet

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Viruses and Cancer Mechanism of

cancer causation

HPV

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Viruses and Cancer

Oncogenes/proto-oncogenes V-myc V-ras

Rous Sarcoma Virus RSV

Kaposi’s sarcoma - appears when immune system depressed probably by herpes virus 8

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Viruses to know something about

•HPV (DNA) • HIV (RNA)• Flu (RNA)• Adenovirus(DNA)• Herpes(DNA)

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Herpes Simplex

After initial infection, the viruses move to sensory nerves, where they reside as life-long, latent viruses.

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HPV human papilloma virus

• Causes warts and some strains cause cervical cancer

• teratogenic

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HIV human immunodeficiency virus

• RNA retrovirus• T-cell host (CD4+ T-killer cells)• needs protease to replicate• binds to CCR5 and CD4 receptors

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Adenovirus

• Common cold• Usually affects respiratory tract.• sometimes engineered for gene therapy• DS DNA virus

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Influenza

H = hemaglutininN = neuraminidase

• RNA virus• mutates rapidly• animal reservoirs• can cross species lines