Viruses

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Virus

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Latin for poison

Can infect bacteria, fungi, plants & animals

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General Characteristics of Viruses

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- Obligate intracellular parasites

- Cannot replicate outside of host

- Very small (<0.3mm), filterable

- Requires electron microscope to see

- Contains one type of nucleic acid, either DNA or RNA

- Nucleic acid surrounded by a protein coat called the capsid

- Has very little enzymes of their own

- Use host cell enzymes for replication and packaging

- Molecular, nonliving entities (acellular: non-cellular life)

- May have lipid envelopes

- Susceptible to disinfectants

- Many have a range of host cells able to infect due to specific receptors on cell surface

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Virion

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- Complete infectious particle

- Nucleic acid surrounded by capsid +/- lipid envelope

- Size 20 - 1000m

- Pox virus = largest virus

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Genome

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- Contains a single type of nucleic acid

- Contain either DNA or RNA

- NEVER BOTH RNA & DNA

- Amount varies from a few genes to ~250 genes

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Classification of Viruses

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1. Based on nucleic acid content

2. Capsid

3. Envelope

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Nucleic Acid

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- DNA or RNA

- Can be single stranded, double stranded, linear, circular, continuous or segmented

- SS RNA genomes: have a polarity of +/-

- (+)RNA can go into the cell and directly function as mRNA

- (-)RNA must first be transcribed into a + RNA strand which then serves as the mRNA

- Retrovirus or non-retrovirus

- Retrovirus RNA transcribed into DNA inside the host

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Capsid

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- Capsid is a protein coat surrounding the nucleic acid

- Each capsid is composed of units call capsomers

- may be of one protein or several different proteins

Capsid Shape or Symmetry

- Helical

- Polyhedral

- Complex

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Helical Symmetry

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- Looks like a long rod

- Many copies of the same protein wrapped in a helix

- Nucleic acid surrounded by a hollow, helical, cylindrical capsid

- Tobacco mosaic virus (TMV)

- Rabies virus

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Polyhedral Symmetry

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- Many sides

- Icosahedral

- Regular polygon with 20 sides with 12 corners

- each side = equilateral triangle

- Pollo virus

- Adenovirus

- Herpes virus

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Complex Symmetry

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- Complicated structures

- Often bacteriophages

- Have a capsid, a tail and a sheath

- Pox virus : only one

- No capsid but several coats surrounding the nucleic acid

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Envelope

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- surrounds the nucleocapsid of some animal viruses

- enveloped viruses

- made up of host cel lipids and viral proteins

- lipid bilayer membrane surrounding the capsid

- may contain viral glycoproteins called peplomers or spikes

- may be used for attachment to host cell

- keeps segments of a segmented genome together

- antigenic - host immune system can mount an attack

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Taxonomy of Viruses

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AT FIRST : classified by host

- plant viruses

- animal viruses

- bacteriophages

THEN : classified by disease caused

- respiratory diseases

- enteric viruses

NOW: classified according to structure

- nucleic acid type

- replication strategy

- morphology

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Viral Taxonomy

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"Family" - viridae

"Species" - group of viruses

- have same nucleic acid & infect same host cells

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Cultivation of Viruses

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- Obligate intracellular parasites depends on the virus and its host cell type

- Bacteriophage: bacteria: suspension or solid media to form plaques

- Plant Viruses: whole plant or plant cell culture

- Animal viruses: whole animal, embryonated eggs or animal cell culture

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Cell Structure

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Most common way of culturing animal viruses

Three cell lines can be used

- Primary cell lines: derived from tissues by enzymes, generally die after a couple generations

-Diploid cell lines: derived from human embryos, multiply for about 50-100 generations and then die die.

-Continuous cell lines: immortal lines are derived from transformed or cancerous cells. Can multiply indefinitely in culture and are immortal. These are commonly used in propagation of virus.

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Growing Viruses

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- Animal and plant viruses may be grown in cell culture

- Continuous cell lines may be maintained indefinitely

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Viral Multiplication

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- Viral genes code for capsid (structural) proteins and some enzymes needed for replication

- Virus use host cell enzymes needed for protein synthesis , energy production & ribosomes

- Viruses ONLY replicate inside the cell

- Viruses invade host cell & controls metabolic & replicative "machinery"

- Cell now primarily makes viral proteins and nucleic acid for new viral particles.

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Virus - Host Interactions

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1. Lytic Cycle - Host cell is lysed after viral replication , T4 bacteriophage , influenza virus

2. Lysogenic or Latent Cycle - Host cell stays alive after replication, viral genome is integrated into host's genome , lambda bacteriophage, retroviruses

3. Transforming Interaction - viral genome is integrated into host's genome , integration alters host cell growth & morphology , cancer - like cell

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The Lysogenic Cycle

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The Viral Replicative Cycle of Animal Viruses

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1. Attachment or absorption

2. Penetration or entry

3. Uncoating

4. Biosynthesis

5. Maturation or assembly

6. Release

- All the steps are similar for DNA and RNA viruses except for NUCLEIC ACID SYNTHESIS

- nucleic acid synthesis is different

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A Viral One-Step Growth Curve

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1. Attachment or absorption

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•Viralparticle attaches to host cell via specific receptors on on the surface of thehost cell•Someproteins and glycoproteins on host cell membranes can function as receptors forsome viruses

•Viralparticles have attachments sites made of spikes or fibers that bind thesereceptors

•Nakedviruses - nucleocapsid

•Envelopedviruses – envelope on spikes

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2. Penetration

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•Entryof viral nucleic acid into host cell

•Canbe achieved by receptor mediated endocytosis

•Nakedand enveloped can enter this way

•Canbe achieved via fusion of envelope with host cell plasma membrane

•Thenviral nucleocapsidenters the cytoplasm

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3. Uncoating

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•Uncoating - separation of nucleic acid from theprotein capsid coat

•Canbe due to host lysosomalenzymes or configurationalchanges in the capsid

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4. Synthesis

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•Virustakes over host’s biosynthetic “machinery”

•Periodof time when new viral nucleic acid molecules, capsid proteins and other viralcomponents are produced

•Synthesisof host DNA & proteins stops

•DNAViruses

•Allreplicate in nucleus except Poxviruses, usesDNA dependent DNA polymerase

•RNAViruses

•Allreplicate in cytoplasm except Retrovirusesand Orthomyxoviruses,useRNA dependent RNA polymerase

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5. Maturation or Assembly

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•Assemblyof capsomers into capsid

•Newlysynthesized viral DNA is inserted into the newly made capsids to form new virions

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6. Release

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•Nakedviruses usually released from host cell by lysing the cell

•Hostcell dies

•50- 200 virionsare released

•Envelopedviruses usually bud out of the cell•Asthey leave the cell, they acquire their envelope from the host cell plasmamembrane

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Retroviruses

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•GENOME= RNA

•REPLICATEthrough DNA intermediate

•Beforeintegrates into host genome must make a dsDNAcopy of the viral RNA genome

•Utilizean enzyme = REVERSE TRANSCRIPTASE•RNAdependent DNA polymerase

•RNA------> DNA ----> integrates

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Retroviral Replication

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•Attachment to host by viral peplomers •Penetration by fusion with host’s membrane •Biosynthesis

•RNA ---> dsDNA •DNA enters nucleus ---> integrates

•DNA remains in the host’s genome, PROVIRUS •Integrated DNA ---> RNA ---> proteins

•Assembly

•Release ---> budding

•RNA ---> DNA ---> RNA

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Viral Latency

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•Manyviruses can remain latent in the host cell•Periodof time when there is no replication of the viral genome

•Noapparent disease occurs during this time•Viruscan be activated by different stimuli

•Nowcauses disease

•HSV-1,Simplexvirus

•Remainslatent in nerve cells

•Stimulation - cold sores or blisters

•Chickenpox virus, Varicellovirus

•Remainslatent but after activation - shingles

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Viruses and Cancer

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•HTLV- I and HTLV - 2

•Causecancer in humans

•HumanT-cell LymphotrophicVirus

•Bothretroviruses cause leukemia

•Otherviruses cause cancer in animals

•EBV :Epstein-Barr virus associated with African Burkitt’sLymphoma and nasopharyngeal carcinoma

•HBV:Hepatitis B virus associated with hepatocellular carcinoma ( liver cancer)

•HPV:human papilloma virus associated with cervical cancer

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Oncogenes

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•ONCO= tumor

•ONCOGENE= tumor gene

•Originallyderived from normal cellular genes•ONCOGENICVIRUS = a virus that promotes tumor formation

•Oncogenicviruses integrate in the host chromosome causing transformation of cells

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Prions

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•PROteinaceous INfectiousparticles•First identified by Stanley Prusiner in1982

•DISEASES: kuru, Creuztfeld-Jacobdisease, bovine spongiform encephalopathy (Mad Cow disease), Scrapie(found in sheep)•Allare neurological diseases, spongy appearance of the brain

•Allare fatal (slow viral infection)

•INFECTIVE PARTICLE = PROTEIN ONLY

•ISINFECTIOUS

•Goesto the brain

•Convertsthe normal protein (gene on chr 20) to an infectious abnormal protein

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How a protein can be infectious

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Replication Cycle Step 1

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Replication Cycle Step 2

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Replication Cycle Step 3

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Replication Cycle Step 4

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Replication Cycle Step 5

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Replication Cycle Step 6

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Replication Cycle Step 7

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Replication Cycle Step 8

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Viroids

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- cause some plant diseases

- short pieces of naked RNA

- is closed, folded molecule that protects it from cellular enzymes

- RNA does not code for any proteins