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Latin for “poison”

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We have all gotten viruses…

from bacteria, plants to animals.Viruses cause colds, flu, warts and

diseases such as measles, AIDS and cancer.

AND not all viruses are harmful to humans.

BUT not all viruses cause diseases,

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WHAT IS A VIRUS?

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A VIRUS is a nucleic acid (DNA or RNA), enclosed by a protein coat called a CAPSID.

DNA CAPSID

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Why are some viruses harmful?

Virus invades cell

Virus forces cell to make copies of virus

Eventually so many copies are made, the cell explodes,releasing all of the new viruses

When your cells make viruses instead of operating normally, YOU get sick

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DISEASE VIRUSES

AIDS HIV

WartHerpes Simplex Virus

Flu Influenza

Measles Morbillivirus .

Cancer Hepatitis B

Examples of some viral diseases:

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Who do viruses infect?

Viruses infect Bacteria• These viruses are called bacteriophages

Viruses infect Plants• One example is the Tobacco Mosaic Virus

Viruses infect Animals• One example is the common cold

Viruses usually infect a specific host including:

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Defining Properties of Viruses

Viruses are parasites that invade cellsViruses have either DNA (Deoxyribose Nucleic Acid) or RNA (Ribonucleic Acid)Viruses direct the synthesis of new virus within a host cell.Newly made viruses infect other cells.

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How small is a virus?

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Comparative Sizes of Bacteria & Viruses

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If a cell was the size of your classroom, then an average virus would be the size of a softball.

1 nm = 0.00000004 inches

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10-9 m

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Size: 20 to 14,000 nm in length

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Structure

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Structure Nucleic acid• DNA or RNA

• Single-stranded or double-stranded

• Linear or circular

Capsid• Protects virus from the environment

• Serves as a vehicle of transmission & basis for classification

• Accounts for the mass of a virus

• Made up of capsomeres

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Structure

Envelope• Consists of lipids, proteins & carbohydrates

• May or may not be covered with spikes

Spikes• Carbohydrate-protein complexes that project

from the surface of the envelope

• Means of host cell attachment

• Project from capsids in naked viruses

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Morphological Types

Based on capsid architecture

Classified with the aid of electron microscope & x-ray crystallography

Types:• Helical

• Polyhedral

• Enveloped

• Complex

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Helical Viruses

Resemble long rodsRigid or flexibleNucleic acid found within a hollow, cylindrical capsid that has a helical structureExample: tobacco mosaic virus, bacteriophage M13

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Helical Viruses

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Helical Capsids

Helical capsids are rod-like structures with the RNA in the center of the helix. A helix is made by stacking repeating units in a spiral.

RNA

protein coat

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Tobacco Mosaic VirusTobacco Mosaic Virus (TMV) is an example of a virus with a helical structure. Protein subunits wrap around the spiraling RNA strand.

This image taken using an Electron Microscope

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Polyhedral VirusesMany-sided

Capsid shape: icosahedron (20 equilateral triangular faces & 12 corners)

Examples: adenovirus, poliovirus

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Icosahedral CapsidsSome viral protein subunits assemble to make polyhedral (many sided) structures. The most common structure is the icosahedron. An icosahedron has 20 triangular faces and has 2-fold, 3-fold and 5-fold symmetry axes.

A body with cubic symmetry possesses a number of axes about which it may be rotated to give a number of identical appearances.

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Icosahedral CapsidsThe DNA or RNA is found in the center or the core of the capsid.

DNA Capsid (Protein Coat)

The occurrence of icosahedral features in quite unrelated viruses suggests that icosahedral symmetry is preferred in virus structure.

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Images of Icosahedral Viruses

Actual images of several different icosahedral viruses.

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Enveloped Viruses

Roughly spherical

When helical or polyhedral viruses are enclosed by envelopes, they are called enveloped helical or polyhedral viruses

Examples:• Enveloped helical: influenza virus

• Enveloped polyhedral: herpes simplex virus

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Enveloped VirusesEnveloped viruses are viruses which have a membrane coat surrounding the protein coat or capsid. These viruses are common in animal viruses, but are uncommon in plant viruses.

Herpes Simplex Virus.

A membrane (made of proteins) surrounds the capsid (also made of proteins) which surrounds the viral DNA.

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Complex Viruses

Complicated structures

Example: bacteriophage, poxviruses

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BacteriophageBacteriophage is a virus that infects bacteria.

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Taxonomy

Viral species: A group of viruses sharing the same genetic information and ecological niche (host)

Family names end in –viridae

Genus names end in -virus

Common names are used for species

Subspecies are designated by a number

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Taxonomy

Based on:• Nucleic acid type (e.g. Hepadnaviridae,

Picornaviridae)

• Morphology

• Presence/absence of an envelope

• Disease it cause (e.g. Poxviridae)

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Examples

Virus Family Disease

Coronaviridae SARS

Poxviridae Smallpox, Cowpox

Herpesviridae Chickenpox

Papoviridae Warts, Tumors

Hepadnaviridae Hepatitis B

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Identification of Viruses

Cytopathic effects

Serological tests• Detect antibodies against viruses in a patient

• Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot

Nucleic acids• Restriction fragment length polymorphisms (RFLPs)

• DNA fingerprints

• Polymerase chain reaction (PCR)

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How do viruses replicate?

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Replication Phases

V - Release- Assembly of virus

DNA and protein coat into whole new viruses

- Leaving the cell

http://www.cat.cc.md.us/courses/bio141/lecguide/unit2/viruses/adlyt.html

Phase I

Phase II

Phase IV Phase V

Phase III

I, II, III - Viruses enter cell- Attachment to cell

membrane- Penetration inside cell- Losing virus protein coat

IV - Replication- Tricks cell into

making more viral DNA

- Tricks cell into making viral protein coat

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Viral Multiplication

Invasion of host cell is necessary

Types of bacteriophage multiplication• Lytic cycle - ends with the lysis & death of

host cell

• Lysogenic cycle - host cell remains alive; prophage DNA incorporated in host DNA

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Attachment:Phage attaches to host cell.

Penetration:Phage pnetrates host cell and injects its DNA.

Merozoites released into bloodsteam from liver may infect new red blood cells

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Bacterial cell wall

Bacterial chromosome

Capsid DNA

Capsid

Sheath

Tail fiber

Base platePin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

Lytic cycle of a T-even bacteriophage

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4 Maturation:Viral components are assembled into virions.

Tail

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

DNA

Capsid

Tail fibers

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Lytic cycle of a T-even bacteriophage

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The Lysogenic Cycle

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Lysogenic cells are immune to reinfection by the same phage

Host cell may exhibit new properties Specialized transduction

Process of transferring a piece of cell DNA adjacent to a prophage to another cell

3 Important Results of Lysogeny

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Specialized Transduction

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

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

Phage matures and cell lyses, releasing phage carrying gal gene.

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Prophage

gal gene

gal gene Bacterial DNA

Galactose-positive donor cell gal gene

Phage infects a cell that cannot utilize galactose (lacking gal gene).

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Galactose-negative recipient cell

Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA.

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Lysogenic cell can now metabolize galactose.

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Galactose-positive recombinant cell

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

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Multiplication of Animal Viruses

Follows basic pattern of bacteriophage multiplication but with notable differences• Mechanism of entering the host cell

• Synthesis & assembly of new viral components

• Presence of certain types of enzymes

• Mechanisms of maturation & release

• Effects on the host cell

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Multiplication of Animal VirusesAttachment Attaches to cell membrane

Penetration By endocytosis or fusion

Uncoating By viral or host enzymes

Biosynthesis Production of nucleic acid & proteins

Maturation Nucleic acid & capsid proteins assemble

Release By budding (enveloped viruses) or rupture

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Bacteriophage Vs. Viral MultiplicationStage Bacteriophage Animal Viruses

AttachmentTail fibers attach to cell wall proteins

Attachment sites are plasma membrane proteins & glycoproteins

PenetrationViral DNA injected into host cell

Capsid enters by endocytosis or fusion

Uncoating Not requiredEnzymatic removal of capsid proteins

Biosynthesis In cytoplasmIn nucleus (DNA viruses) or cytoplasm (RNA viruses)

Chronic infection LysogenyLatency; slow viral infections; cancer

Release Host cell lysedEnveloped viruses bud out; non-enveloped viruses rupture plasma membrane

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Inhibition of Virus

Phagocytosis

Neutralization by antibodies

Interaction with T-lymphocytes

Drugs

Vaccines

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Inactivation of Viruses

Physical agents – heat, UV light, X-rays

Chemical agents – halogen (chlorine & iodine), heavy metals (Hg, Ag, phenol derivatives), formaldehyde, & lipid solvents (ether, chloroform, detergents)

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Virus & Cancer

Excess tissue develops into a tumor• Malignant – cancerous

• Benign – non-cancerous

Named by the attachment of the suffix –oma to the name of the tissue from which the tumor arises• Sarcoma – cancer of the connective tissue

• Adenocarcinoma – cancer of glandular epithelial tissue

Oncovirus – cause cancer

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Cancer Cells

Undergo mitosis more rapidly

Stick together less firmly

Undergo dedifferentiation

Fail to exhibit contact inhibition

Do not adhere to one another

Overgrow to one another

Metastasize – spread to different body parts

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How Cancer Bring Illness

Interrupts normal functions

Robs the body of vital nutrients

Produces hormones & overloads the body with chemical regulators

Block air passageways

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Carcinogens

Cancer-causing substances

Radiation (UV light & X-ray)

Hydrocarbons – cigarette smoke, asbestos, nickel, certain pesticides, dyes & environmental pollutants

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Prions

Proteinaceous infectious particles

Inherited and transmissible by ingestion, transplant, & surgical instruments

Spongiform encephalopathies: mad cow disease, kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, Sheep scrapie

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The concept of a virus as an organism challenges the way we

define life:

* Viruses do not breathe.* Viruses do not metabolize.* Viruses do not grow.* However, they do reproduce.

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Are Viruses Living?

Properties of Living Organisms

Properties of Viruses

Breathes (respires) Doesn’t breathe

Metabolizes Doesn’t metabolize

Grows Doesn’t grow

Reproduces Reproduces

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Atomic Force Microscope – A tiny tip probes a surface, from which the shape of the surface can be determined

X-ray Crytallography – X-rays are directed at a sample. How those rays scatter can be used to determine the structure of that sample

Techniques to Study Viruses

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Filters – Very small holes in material filter only viruses through

Sedimentary Centrifugation – A sample is spun so fast, different elements in it are separated by density

Electron Microscope – Electrons are smaller than light wavelengths, so viruses can be “seen” by reflecting electrons off of them

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Viruses Can Help Cells, Too

- Since viruses can transport DNA and RNA into cells, scientists are exploring Gene Therapy

- In Gene Therapy, viral genetic material is replaced with new DNA

- In time, this could be used to cure genetic diseases. Currently we have no cure for these types of illnesses