VIRUSVIRUS- latin word - latin word meaning “slimy meaning “slimy

fluid”fluid”

VIRUSVIRUSinfectious particles consisting of a core of nucleic infectious particles consisting of a core of nucleic acid (DNA or RNA) enclosed in a protein shellacid (DNA or RNA) enclosed in a protein shell

acellular and able to function and reproduce only acellular and able to function and reproduce only if they can invade a living cell to use the cell’s if they can invade a living cell to use the cell’s system to replicate themselves system to replicate themselves

An Introduction to the An Introduction to the Taxonomy of VirusesTaxonomy of Viruses

Viruses can be classified into two Viruses can be classified into two ways:ways:

Based on the host they infectBased on the host they infect

The Universal System of Virus The Universal System of Virus TaxonomyTaxonomy

CLASSIFICATION ACCORDING CLASSIFICATION ACCORDING TO THE HOST THEY INFECTTO THE HOST THEY INFECT

1.1. BACTERIAL VIRUSESBACTERIAL VIRUSES

Also called bacteriophages (or Also called bacteriophages (or phage phage for short, from for short, from the Greek the Greek phagein “phagein “meaning to eat”) meaning to eat”)

CLASSIFICATION ACCORDING TO THE CLASSIFICATION ACCORDING TO THE HOST THEY INFECTHOST THEY INFECT

2. PLANT VIRUSESviruses infecting plants important in agriculture

3. FUNGI VIRUSESViruses infecting fungi

Tobacco Mosaic Virus, the cause of Leaf Mosaic Disease in tobacco plants. © Dr. Harold Fisher / Visuals Unlimited

4. ANIMAL VIRUSES : Viruses infecting animals- Virus causing

AIDS in Human

THE UNIVERSAL SYSTEM OF THE UNIVERSAL SYSTEM OF VIRUS TAXONOMYVIRUS TAXONOMY

Standardized by the International Committee on Taxonomy of Viruses (ICTV)Standardized by the International Committee on Taxonomy of Viruses (ICTV) A system to keep track of the large numbers of different viruses being isolated A system to keep track of the large numbers of different viruses being isolated

and studied through the world, a system to tie viral characteristics to virus and studied through the world, a system to tie viral characteristics to virus name.name.

It is set arbitrarily at hierarchical levels of order, family, subfamily genus and It is set arbitrarily at hierarchical levels of order, family, subfamily genus and speciesspecies

PROPERTIES OF VIRUS USED IN TAXONOMYPROPERTIES OF VIRUS USED IN TAXONOMYVirion properties:Virion properties:

MorphologyMorphology

Physicochemical and physical propertiesPhysicochemical and physical properties

ProteinsProteins

LipidsLipids

CarbohydratesCarbohydrates

Antigenic propertiesAntigenic properties

Biologic propertiesBiologic properties

THE UNIVERSAL SYSTEM OF THE UNIVERSAL SYSTEM OF VIRUS TAXONOMYVIRUS TAXONOMY

VIRUS ORDERSVIRUS ORDERS Groupings of families of viruses that share common characteristics and are distinct Groupings of families of viruses that share common characteristics and are distinct

from other orders and familiesfrom other orders and families Designated by names with the suffix – Designated by names with the suffix – viralesvirales Ex. Ex. MononegaviralesMononegavirales

VIRUS FAMILIES AND SUBFAMILIESVIRUS FAMILIES AND SUBFAMILIES Groupings of genera of viruses that share common characteristics and are distinct Groupings of genera of viruses that share common characteristics and are distinct

from the other member viruses of other familiesfrom the other member viruses of other families FAMILIES: Designated by names with the suffix – FAMILIES: Designated by names with the suffix – viridaeviridae SUBFAMILIES: SUBFAMILIES: Designated by names with the suffix – Designated by names with the suffix – virinaevirinae Ex. Family: Ex. Family: Poxviridae, Herpesviridae, Parvoviridae and ParamyxoviridaePoxviridae, Herpesviridae, Parvoviridae and Paramyxoviridae

VIRUS SPECIESVIRUS SPECIES In 1991, the ICTV accepted the definition of a virus species proposed by In 1991, the ICTV accepted the definition of a virus species proposed by

Van Regenmortel as follows: “A virus species is defined as a polythetic Van Regenmortel as follows: “A virus species is defined as a polythetic class of viruses that constitutes a replicating lineage and occupies a class of viruses that constitutes a replicating lineage and occupies a particular ecological niche.” particular ecological niche.”

Species are defined differently, with emphasis on genome properties, Species are defined differently, with emphasis on genome properties, and in others on structural, physicochemical or serological properties.and in others on structural, physicochemical or serological properties.

Species name are not italicized Species name are not italicized Ex. New Castle virus, tomato spotted wilt virus, poliovirus 1, Sindbis Ex. New Castle virus, tomato spotted wilt virus, poliovirus 1, Sindbis

virusvirus NOMENCLATURENOMENCLATUREIn formal taxonomic usage;In formal taxonomic usage; First letters of virus order, family, subfamily, and genus names are First letters of virus order, family, subfamily, and genus names are

capitalized and the terms are printed in italics capitalized and the terms are printed in italics Species designations are not capitalized (unless they are derived from a Species designations are not capitalized (unless they are derived from a

place name or a host family or genus name), nor are they italicizedplace name or a host family or genus name), nor are they italicized The name of the taxon should precede the term for the taxonomic unit ; The name of the taxon should precede the term for the taxonomic unit ;

for example: ..."the family for example: ..."the family ParamyxoviridaeParamyxoviridae" ..."the genus " ..."the genus MorbillivirusMorbillivirus." ."

DNA AND RNA VIRUSESDNA AND RNA VIRUSES

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

DEFINITIONSDEFINITIONS Host Range Host Range - This specifies both the tissue cell type and species of animal - This specifies both the tissue cell type and species of animal

that a virus can infect and in which it can multiply.that a virus can infect and in which it can multiply.

Susceptibility Susceptibility - Represents the ability of a cell or animal to become - Represents the ability of a cell or animal to become infected.infected.

Portal of Entry Portal of Entry - The site on or within the host where the virus enters. Viral - The site on or within the host where the virus enters. Viral infection of these susceptible cells may or may not have the capacity to infection of these susceptible cells may or may not have the capacity to cause a clinical disease.cause a clinical disease.

Target Cells Target Cells - The clinical manifestation of a viral disease is the result of - The clinical manifestation of a viral disease is the result of infection of the target cells by virus produced in susceptible cells at the infection of the target cells by virus produced in susceptible cells at the portal of entry. Target cells can either reside at the portal of entry (e.g., portal of entry. Target cells can either reside at the portal of entry (e.g., respiratory infections, and genital herpes simplex infections) or at some respiratory infections, and genital herpes simplex infections) or at some distant location (rabies).distant location (rabies).

VirionVirion – The complete virus particle; the nucleic acid surrounded by a – The complete virus particle; the nucleic acid surrounded by a protein coat and in some cases other material.protein coat and in some cases other material.

Routes of Entry for VirusesRoutes of Entry for Viruses

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

PHASES OF INFECTIONPHASES OF INFECTION

This schema is basically true for viruses containing either DNA or RNA This schema is basically true for viruses containing either DNA or RNA genomes, regardless of whether they reproduce in the cytoplasm or in the genomes, regardless of whether they reproduce in the cytoplasm or in the nucleus:nucleus:

1. Attachment1. Attachment2. Penetration2. Penetration3. Uncoating3. Uncoating4. Replication4. Replication5. Virus assembly and packaging5. Virus assembly and packaging6. Release 6. Release

– – direct releasedirect release- budding- budding

THE VIRUS INFECTION CYCLETHE VIRUS INFECTION CYCLE

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

1.1. ATTACHMENTATTACHMENT

To initiate infection cycle, a To initiate infection cycle, a virus must first recognize virus must first recognize and bind to a suitable host and bind to a suitable host cell.cell.

High specificity High specificity characterizes the characterizes the interaction between virus interaction between virus and host.and host.

Receptors – specific cell Receptors – specific cell surface component of the surface component of the host to which the virion host to which the virion attaches (ex. Proteins, attaches (ex. Proteins, carbohydrates, carbohydrates, glycoprotein, lipids and glycoprotein, lipids and lipoproteins)lipoproteins)

Adsorption of an Enveloped Virus to a Susceptible Host CellAdsorption of an Enveloped Virus to a Susceptible Host Cell

Adsorption of a Naked Virus to a Susceptible Host CellAdsorption of a Naked Virus to a Susceptible Host Cell

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

2.2. PENETRATION PENETRATIONThe virion enters the intracelullar The virion enters the intracelullar environment.environment.

Pinocytosis (“viropexis”)Pinocytosis (“viropexis”)Engulfment of the virus particle by the Engulfment of the virus particle by the plasma memrane and the subsequent plasma memrane and the subsequent production of an intracellular membrane- production of an intracellular membrane- bound vesicle containing the virus bound vesicle containing the virus particleparticle

Fusion of the viral envelop Fusion of the viral envelop with the host cell membranewith the host cell membrane

Not only does this method internalize the Not only does this method internalize the virus, itt can lead to fusion beween this virus, itt can lead to fusion beween this and the other host cells nearby, forming and the other host cells nearby, forming multinucleated cells called syncytia.multinucleated cells called syncytia.

Penetration of a Naked Virus by Rearrangement of Capsid Proteins

Penetration of an Enveloped Virus by Fusion of Its Envelope with the Host Cell Membrane

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

33. UNCOATING. UNCOATING

Removal of capsid from Removal of capsid from the virionthe virion

Necessary to release viral Necessary to release viral genome before the viral genome before the viral DNA or RNA is delivered to DNA or RNA is delivered to its intracellular site of its intracellular site of replication replication

THE VIRUS INFECTION CYCLETHE VIRUS INFECTION CYCLE

4. REPLICATION4. REPLICATION

Production of virally Production of virally encoded proteins and encoded proteins and replication of viral genomereplication of viral genome

Early in infection thevirus Early in infection thevirus redircts cell metabolism to redircts cell metabolism to synthesize new viral synthesize new viral nucleic acid and proteins. nucleic acid and proteins.

Late in infection, structural Late in infection, structural proteins that are subunits proteins that are subunits of the virus coat are of the virus coat are synthesized.synthesized.

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

5. VIRUS 5. VIRUS ASSEMBLY AND ASSEMBLY AND PACKAGINGPACKAGING

Construction of new Construction of new nucleocapsidsnucleocapsids

Assembly of structural Assembly of structural subunits (and membrane subunits (and membrane components in enveloped components in enveloped viruses) and packaging of viruses) and packaging of nucleic acid into new virus nucleic acid into new virus particles. particles.

THE VIRUS INFECTION THE VIRUS INFECTION CYCLECYCLE

6. RELEASE6. RELEASE

Direct release – LysisDirect release – Lysis

- mature virions or new - mature virions or new infective viruses reach the infective viruses reach the extracelullar space, killing extracelullar space, killing the host cell in some casesthe host cell in some cases

Budding Budding

- budding may also occur - budding may also occur with or without cell death with or without cell death

Bateriophage- Lytic and Bateriophage- Lytic and Lysogenic Lysogenic cyclecycle