VIRUS- latin word meaning “slimy fluid”

VIRUS infectious particles consisting of a core of nucleic acid (DNA or RNA) enclosed in a protein shell acellular and able to function and reproduce only if they can invade a living cell to use the cell’s system to replicate themselves

An Introduction to the Taxonomy of Viruses Viruses can be classified into two ways: Based on the host they infect The Universal System of Virus Taxonomy

CLASSIFICATION ACCORDING TO THE HOST THEY INFECT 1. BACTERIAL VIRUSES Also called bacteriophages (or phage for short, from the Greek phagein “meaning to eat”)

CLASSIFICATION ACCORDING TO THE HOST THEY INFECT 2. PLANT VIRUSES viruses infecting plants important in agriculture 3. FUNGI VIRUSES Viruses 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 VIRUS 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 and studied through the world, a system to tie viral characteristics to virus name. 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 name. It is set arbitrarily at hierarchical levels of order, family, subfamily genus and species It is set arbitrarily at hierarchical levels of order, family, subfamily genus and species PROPERTIES OF VIRUS USED IN TAXONOMY Virion properties: Morphology Physicochemical and physical properties ProteinsLipidsCarbohydrates Antigenic properties Biologic properties

THE UNIVERSAL SYSTEM OF VIRUS TAXONOMY VIRUS ORDERS Groupings of families of viruses that share common characteristics and are distinct from other orders and families Groupings of families of viruses that share common characteristics and are distinct from other orders and families Designated by names with the suffix – virales Designated by names with the suffix – virales Ex. Mononegavirales Ex. Mononegavirales VIRUS FAMILIES AND SUBFAMILIES Groupings of genera of viruses that share common characteristics and are distinct from the other member viruses of other families Groupings of genera of viruses that share common characteristics and are distinct from the other member viruses of other families FAMILIES: Designated by names with the suffix – viridae FAMILIES: Designated by names with the suffix – viridae SUBFAMILIES: Designated by names with the suffix – virinae SUBFAMILIES: Designated by names with the suffix – virinae Ex. Family: Poxviridae, Herpesviridae, Parvoviridae and Paramyxoviridae Ex. Family: Poxviridae, Herpesviridae, Parvoviridae and Paramyxoviridae

VIRUS SPECIES 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 class of viruses that constitutes a replicating lineage and occupies a particular ecological niche.” 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 class of viruses that constitutes a replicating lineage and occupies a particular ecological niche.” Species are defined differently, with emphasis on genome properties, and in others on structural, physicochemical or serological properties. Species are defined differently, with emphasis on genome 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 virus Ex. New Castle virus, tomato spotted wilt virus, poliovirus 1, Sindbis virus NOMENCLATURE In formal taxonomic usage; First letters of virus order, family, subfamily, and genus names are capitalized and the terms are printed in italics First letters of virus order, family, subfamily, and genus names are capitalized and the terms are printed in italics Species designations are not capitalized (unless they are derived from a place name or a host family or genus name), nor are they italicized Species designations are not capitalized (unless they are derived from a place 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 ; for example:..."the family Paramyxoviridae"..."the genus Morbillivirus." The name of the taxon should precede the term for the taxonomic unit ; for example:..."the family Paramyxoviridae"..."the genus Morbillivirus."

DNA AND RNA VIRUSES

THE VIRUS INFECTION CYCLE DEFINITIONS Host Range - This specifies both the tissue cell type and species of animal that a virus can infect and in which it can multiply. Host Range - This specifies both the tissue cell type and species of animal that a virus can infect and in which it can multiply. Susceptibility - Represents the ability of a cell or animal to become infected. Susceptibility - Represents the ability of a cell or animal to become infected. Portal of Entry - 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 cause a clinical disease. Portal of Entry - 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 cause a clinical disease. Target Cells - The clinical manifestation of a viral disease is the result of 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., respiratory infections, and genital herpes simplex infections) or at some distant location (rabies). Target Cells - The clinical manifestation of a viral disease is the result of 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., respiratory infections, and genital herpes simplex infections) or at some distant location (rabies). Virion – The complete virus particle; the nucleic acid surrounded by a protein coat and in some cases other material. Virion – The complete virus particle; the nucleic acid surrounded by a protein coat and in some cases other material.

Routes of Entry for Viruses

THE VIRUS INFECTION CYCLE PHASES OF INFECTION This schema is basically true for viruses containing either DNA or RNA genomes, regardless of whether they reproduce in the cytoplasm or in the nucleus: 1. Attachment 2. Penetration 3. Uncoating 4. Replication 5. Virus assembly and packaging 6. Release – direct release - budding

THE VIRUS INFECTION CYCLE

1. ATTACHMENT To initiate infection cycle, a virus must first recognize and bind to a suitable host cell. High specificity characterizes the interaction between virus and host. Receptors – specific cell surface component of the host to which the virion attaches (ex. Proteins, carbohydrates, glycoprotein, lipids and lipoproteins) Adsorption of an Enveloped Virus to a Susceptible Host Cell Adsorption of a Naked Virus to a Susceptible Host Cell

THE VIRUS INFECTION CYCLE 2. PENETRATION The virion enters the intracelullar environment. Pinocytosis (“viropexis”) Engulfment of the virus particle by the plasma memrane and the subsequent production of an intracellular membrane- bound vesicle containing the virus particle Fusion of the viral envelop with the host cell membrane Not only does this method internalize the virus, itt can lead to fusion beween this and the other host cells nearby, forming 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 CYCLE 3. UNCOATING Removal of capsid from the virion Necessary to release viral genome before the viral DNA or RNA is delivered to its intracellular site of replication

THE VIRUS INFECTION CYCLE 4. REPLICATION Production of virally encoded proteins and replication of viral genome Early in infection thevirus redircts cell metabolism to synthesize new viral nucleic acid and proteins. Late in infection, structural proteins that are subunits of the virus coat are synthesized.

THE VIRUS INFECTION CYCLE 5. VIRUS ASSEMBLY AND PACKAGING Construction of new nucleocapsids Assembly of structural subunits (and membrane components in enveloped viruses) and packaging of nucleic acid into new virus particles.

THE VIRUS INFECTION CYCLE 6. RELEASE Direct release – Lysis - mature virions or new infective viruses reach the extracelullar space, killing the host cell in some cases Budding - budding may also occur with or without cell death

Bateriophage- Lytic and Lysogenic cycle