Viruses

Viruses• Viruses infect every type of cell, including bacteria, algae,

fungi, protozoa, plants, and animals

• Seawater can contain 100 million viruses per milliliter

• For many years, the cause of viral infections was unknown• Louis Pasteur postulated that a “living thing” smaller than

bacteria caused these diseases

• also proposed the term virus, which is Latin for “poison.”

When did we start to learn about viruses?• Ivanovski and Beijerinck showed that a disease in tobacco was

caused by a virus

• Loeffler and Frosch discovered an animal virus causes foot-and-mouth disease in cattle

• Filterable virus• these early researchers found that when fluids from host

organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious

• this result proved that a cell-free fluid could contain agents that could cause infection

General Characteristics of Viruses

• Are viruses alive?• better described as active or inactive

• Obligatory intracellular parasites• Not cells: no cell wall, cytoplasm or organelles• Require living host cells in order to multiply• Very small • Submicroscopic - visible only with Electron

Microscope

Role of Viruses in Evolution

• Infect cells and influence their genetic makeup

• Shape the way cells, tissues, bacteria, plants, and animals have evolved

• 10% of the human genome consists of sequences that come from viruses

• 10 – 20% of bacterial DNA contains viral sequences

How small are viruses? VERY!!

Distinctive Features of Viruses• Viruses bear no resemblance to cells and lack any of the

protein-synthesizing machinery found in cells

• Viral structure is composed of regular, repeating subunits that give rise to their crystalline appearance

• Contain only those parts needed to invade and control a host cell– external coating– core containing one or more nucleic acid strains of DNA or RNA– sometimes one or two enzymes

Distinctive Features of Viruses

Virus particle

Covering

Central core

Capsid

Envelope (not found in all viruses)

Nucleic acid molecule(s)(DNA or RNA)

Matrix proteinsEnzymes (not found inall viruses)

Viral Structure

– Nucleic acid core• DNA or RNA, single or

double stranded

– Capsid• protein coat

It’s amazing that a particle made of just a nucleic acid core and a protein coat (capsid) can kill higher level organisms

Some viruses may also have:• Envelope

• some viruses have an envelope/ some don’t

• Where does the envelope come from? The virus “steals” some of the host plasma membrane when it leaves the cell!

• Spikes• Some viruses have

carbo-protein complexes projecting from surface

• Just like organisms, viruses are very diverse!

If a virus has an envelope, what is it made of?

• A membrane from the host cell. During release (exocytosis) of enveloped animal viruses, a part of the host membrane is stolen!

• Membrane is then altered, Ex. spikes

• Envelope spikes • Added to the plasma membrane just

before the viral particle is released

What is the function of the capsid/envelope?

• Protect the nucleic acid from the host’s acid- and protein digesting enzymes

• Assist in binding and penetrating host cell

• Stimulate the host’s immune system—good for us!

General Morphology of Viruses

• Viral capsids come in many shapes:

– Helical (“like a slinky toy”)

– Icosahedral

– Complex

(Remember they can also be enveloped or not)

Helical capsid• Naked helical virus– Ex. Tobacco mosaic

virus– Nucleocapsid is rigid

and tightly wound into a cylinder-shaped package

• Enveloped helical virus– Ex. Influenza, measles,

rabies– Nucleocapsid is more

flexibleComparison between (a) naked helical plant virus and an (b) enveloped helical human virus.

Icosahedron capsid• Three-dimensional, 20-sided

with 12 evenly spaced corners (geometry in action!)

• Variation in capsomer number – Polio virus 32 capsomers– Adenovirus (cold virus) 240

capsomers

• Don’t worry about memorizing the different numbers of sides or capsomers.

• Just be amazed at how geometric in shape viruses are

Icosahedral viruses can be naked or enveloped.

Fig. 6.8 Two types of icosahedral viruses

Complex viruses• Structure is more complex than helical and

icosahedral viruses• Pox virus

• Several layers of lipoproteins• Causes smallpox, cowpox, chickenpox

• Bacteriophage• Virus that attacks bacteria• Polyhedral head with tail fibers• Looks like a spacecraft

Comparison of the morphology of a naked virus, enveloped virus and a complex virus. (Viruses are DIVERSE!)

Icosahedral virus - Hepatitis B

Filamentous Helical Virus - Ebola

Complex Virus - Pox virus

What do you find inside the viral capsid?Nucleic acid

• Viruses contain either DNA or RNA (but not both) • The Nucleic Acid can be in different forms:

• Single stranded (ss) DNA • Double stranded (ds) DNA (like us)• ssRNA• dsRNA

– Possess only the genes to invade and regulate the metabolic activity of host cells

– Ex. Hepatitis B (4 genes) and herpesviruses (100 genes) compare to E. coli (4000) and humans (40,000)

– No viral metabolic genes because the virus uses the host’s metabolic resources

Other Substances in the Virus Particle

• Enzymes for specific operations within their host cell– polymerases that synthesize DNA and RNA– replicases that copy RNA– reverse transcriptase synthesizes DNA from RNA– retroviruses: carry their own enzymes to

create DNA out of RNA

How do we classify viruses?• We look at commonalities in:– Genetic makeup (DNA, RNA, genetic sequence)– Structure (Naked, enveloped, helical, icosahedral)– Chemical composition– Host relationship– Type of disease

– Notice the characteristics in Tables 6.2 and 6.3 used to classify some medically relevant viruses. First separation based on DNA or RNA, second separation based on enveloped or nonenveloped

Three orders of viruses have been developed for classification.Even though scientists have tried to develop standardized names, the use of common names predominates (polio virus, pox virus, etc. which are usually

named for the disease they cause)

3 orders73 families283 genera

Examples of medically important DNA viruses. (Do not memorize specific viruses)

Examples of medically important RNA viruses. (Do not memorize specific viruses)

Viral Life Cycles(Can we really call it a life cycle?)

• The viral life cycle (multiplication cycle) depends on the type of virus and species infected.

• We’ll consider the following:• Animal viruses • Retroviruses• Bacterial viruses (Bacteriophages)

• Lytic vs. Lysogenic cycles

Short description of each phase in

viral multiplication

Phase Description

1.Adsorption/Attachment 

Viral particle (virion) ATTACHES to specific receptor on outside of host cell

2.Penetration 

Virion penetrates cell (membrane and/or wall) through to cytoplasm

3. Uncoating 

Uncoating of the capsid +/- envelope exposes nucleic acid of the virus for use or insertion into host DNA

4. Synthesis 

Production of virion parts (nucleic acid, capsid, spikes…)

5. Assembly Parts made in 4. synthesis are put together like a toy at Christmas

6. Release 

Virion exits the cell by lysing the cell or exocytosis and may steal some of the host membrane for its envelope

Viral Life Cycles: Animal Viruses

If you can stop just ONE of these steps you can stop

the multiplication of viruses!

How do viruses infect specific cells?

What cells does rhinovirus infect? What cells does HIV infect? How?

Viral Life Cycles: Animal VirusesAdsorption/Attachment to the host cell

Penetration and Uncoating of animal viruses occur by endocytosis or fusion between the viral envelope and the host cell membrane.

Viral Life Cycles: Animal Viruses

The multiplication of Animal Viruses:

1. Adsorption/Attachment 2. Penetration 3. Uncoating 4. Synthesis 5. Assembly 6. Release

Viral Life Cycles: Animal Viruses

Release: A mature virus can obtain an envelope by budding off the host cell.

Viral Life Cycles: Animal Viruses

Animal Viruses: Cytopathic effects(CPEs) (cell-disease effects)

• Damage to the host cell due to a viral infection may cause changes to a cell including:– Inclusion bodies

• compacted masses of viruses

– Syncytia • many cells fused into a large clump cell

– Chronic latent state • virus lays in wait until it’s reactivated

– Transformation • cancer: increased rate of growth, alterations in DNA, continuous cell

division, loss of contact inhibition

Animal Viruses: syncytia and inclusion bodies

• Retroviruses are unique since they carry their own polymerase(reverse transcriptase) and transcribe RNA-->DNA(1) Attachment/Attachment(2) Penetration(3) Uncoating(4) Biosynthesis(5) Integration(6) Biosynthesis(7) Maturation(8) Release

HIV is the most famousRetrovirus that can integrateIts DNA into our DNA and layLatent for years!

Viral Life Cycles: Retroviruses

Persistent Infections• Accumulated damage from a virus infection

kills most cells• Persistent infections– cell harbors the virus– not immediately lysed– can last from a few weeks to the remainder of

the host’s life– can remain latent in the cytoplasm

Persistent Infections• Provirus – a persistent infection in which the viral DNA is

incorporated into the DNA of the host

• Can be a inactive viral infections or a retrovirus. – In inactive viral infections the virus will not

replicate itself but through replication of its host cell.

– Endogenous retroviruses are always in the state of a provirus.

Persistent Infections

• Chronic latent state– viruses go into a period of inactivation in cells– May emerge under the influence of various stimuli

• Herpes simplex cold sore• Chickenpox Shingles (10-20% of the time)• HIV AIDS

Viruses and Cancer

• 15-20% of cancers are virus-induced

• several types of cancer are caused by viruses

• 1908-virologists transferred chicken leukemia by infecting healthy chickens with cell-free filtrates from diseased chickens

How do viruses cause cancer?• Viruses can alter our DNA when they insert their

DNA into ours.• Their viral DNA may insert into a human gene important in

controlling/inhibiting cell growth such as p53• If you don’t control cell growth you can end up with

uncontrolled cell growth = cancer

• Viruses may produce viral replication proteins that stimulate cell reproduction (mitosis)

Cancer cell characteristics• DNA is mutated,

chromosomal abnormalities

• cell shape altered

• uncontrolled growth

• loss of contact inhibition– Usually cells will stop

growing when they contact each other. In cancer, the cells keep growing and growing on top of each other to produce a tumor.

DNA & RNA Oncogenic Viruses in Humans

• DNA Oncogenic Viruses• papillomavirus--> cervical cancer• Epstein-Barr virus-->Burkitt’s lymphoma,

nasopharyngeal carcinoma• Hepatitis B-->liver cancer

• RNA Oncogenic Viruses• Human T-cell leukemia virus• Human herpesvirus 8 (HHV8)Kaposi’s Sarcoma

Viruses that Infect Bacteria

•from the Greek phage meaning “eating”

•every bacterial species is parasitized by various specific bacteriophages

•often make the bacteria they infect more pathogenic

Multiplication of Bacteriophages

Lytic Cycle (1) Absorption(2) Penetration(3) Biosynthesis(4) Maturation/Assembly(5) Release

Lysogenic Cycle(1)Absorption(2)Penetration(3) Integration of phage

DNA into host DNA(4) Binary Fission(5) Occasionally, excision of

phage DNA initiates lytic cycle at stage (3)

Bacterial virus

The lytic cycle of Viral Infection by Bacteriophages

Lytic Cycle (1) Absorption(2) Penetration(3) Biosynthesis(4) Maturation/Assembly(5) Release

Viruses can “switch” between Lytic and Lysogenic Cycles

Lysogenic Cycle(1)Absorption(2)Penetration(3) Integration of phage DNA into host DNA(4) Binary Fission(5) Occasionally, excision of phage DNA initiates lytic cycle at stage (3), biosynthesis

Lysogeny in Human Disease

• Occasionally phage genes in the bacterial chromosome cause the production of toxins or enzymes that cause pathology in the human

• Lysogenic conversion: when a bacterium acquires a new trait from its temperate phage– Corynebacterium diphtheriae – diphtheria toxin– Vibrio cholerae – cholera toxin– Clostridium botulinum – botulinum toxin

After viral multiplication inside the host cell, viral enzymes will weaken the host cell membrane, rupture the cell (lyses), and release numerous virions.

Phage Typing • Some of bacteriophages can only

infect a single strain of bacteria• Phage typing is a method used to

identify different strains of bacteria within a single species.

• The susceptible phage regions will show a circular clearing where the bacteria have been lysed, indicating a strain that is susceptible to that bacteriophage

Isolation & Cultivation of Viruses

• Can you inoculate culture media with viruses and expect them to grow? (Like we do with bacteria?)

• Viruses always need a living host to replicate.

• Bacterial viruses• Grow bacteria, then infect with virus

• Animal viruses• Infect living animals• Infect embryonated eggs• Infect animal cell cultures

The early developing bird embryo contains a protective case, providing an ideal environment for viral propagation.

Fig. 6.21 Cultivating animal viruses in a developingbird embryo

A monolayer of monkey kidney cells is a cell culture enabling the propagation of viruses.

Plaques: areas where virus-infected cells have been destroyed and show up as a clear, well-defined patches in the cell sheet

How do you identify viruses?• Not easy and expensive!

• Serological methods– Draw blood and analyze antibody-antigen

interactions (using a SPECIFIC antibody, you can identify the SPECIFC virus that binds with it)

• Examine genetic sequences

• Signs and symptoms

Are viruses susceptible to chemotherapy?

• Yes and No• It is usually difficult to treat viral infections

with drugs since the virus relies on the metabolic machinery (enzymes) of the host. In order to inhibit/kill the virus with a drug you may inhibit/kill the host cell in the process.

Treatment of Viral Infections

• Antibiotics and sulfa drugs ineffective– Antibiotics attack membrane bound structures.

Viruses lack membranes– Sulfa drugs inhibit metabolic pathways. Viruses

lack these.

Treatment of Viral Infections

• Methods– Inhibit viral penetration: Amantidine– Inhibit DNA synthesis – Inhibit viral protein synthesis: Acyclovir– Interfere with viral protein modification: Protease

inhibitors (Saquinavir)– Inhibit viral enzymes: AZT– Prevent viral production: Interferon

Prevention of Viral Infections

• Vaccination ( Smallpox, Mumps, Polio)• Interferon

• Cell protein produced by infected cells. Protects other cells by inhibiting viral replication.• Protection occurs naturally• Can be artificially produced

• Sanitation• Abstention for STDs

Viruses and Human Health•viruses with high mortality rates: rabies, AIDS, Ebola

•viruses that cause long-term debility: polio, neonatal rubella

•viruses with possible connection to chronic afflictions with an unknown cause: type 1 diabetes, multiple sclerosis, various cancers, Alzheimer’s disease, obesity

• PRIONs (pree-ons)• proteinaceous infectious particle

• 1982--Stanley Prusiner • infectious agent sensitive to proteases, but not radiation• Etiologic agent=protein? But how does it work without N.A.

• 9 animal diseases caused by PRIONs• spongiform encephalitis large vacuoles in brain• “mad-cow” disease, scrapie-sheep, kuru, Creutzfeldt-

Jakob disease

Parasitic Particles: Prions

Prion Disease is the Result of Incorrectly Folded Proteins

Proteins with theSame sequence of Amino acids folded Differently causesSpongiform encephalopathies (sponge-like braintissue)

Parasitic Particles: Prion Characteristics

– Proteinaceous infectious agents– Cellular PrP protein• Made by all mammals• Normal structure with -helices called cellular PrP

– Prion PrP• Disease-causing form with -pleated sheets called prion

PrP– Prion PrP changes shape of cellular PrP so it

becomes prion PrP-this is it’s form of replication!

– Prion diseases • Fatal neurological degeneration, fibril deposits in brain,

and loss of brain matter• Large vacuoles form in brain

– Characteristic spongy appearance• Spongiform encephalopathies

– Prions only destroyed by incineration or autoclaving in 1 N NaOH

Parasitic Particles: Prion Characteristics

Vacuole