Download the virus infection cycle

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
Tobacco Mosaic Virus, the cause of Leaf Mosaic Disease in tobacco plants.
© Dr. Harold Fisher / Visuals Unlimited
3. FUNGI VIRUSES
Viruses infecting fungi
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)
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
PROPERTIES OF VIRUS USED IN TAXONOMY
Virion properties:
Morphology
Physicochemical and physical properties
Proteins
Lipids
Carbohydrates
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
 Designated by names with the suffix – virales
 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
 FAMILIES: Designated by names with the suffix – viridae
 SUBFAMILIES: Designated by names with the suffix – virinae
 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.”
 Species are defined differently, with emphasis on genome properties, and in
others on structural, physicochemical or serological properties.

Species name are not italicized
 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
 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."
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.

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.

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.
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.
2.
3.
4.
5.
6.
Attachment
Penetration
Uncoating
Replication
Virus assembly and packaging
Release
– direct release
- budding
THE VIRUS INFECTION CYCLE
THE VIRUS INFECTION CYCLE
1.
ATTACHMENT
Adsorption of a Naked Virus to a Susceptible Host Cell
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
THE VIRUS INFECTION CYCLE
2. PENETRATION
Penetration of an Enveloped Virus by Fusion of Its Envelope with the
Host Cell Membrane
The virion enters the intracelullar environment.
Pinocytosis (“viropexis”)
Engulfment of the virus particle by the
plasma memrane and the subsequent
production of an intracellular membranebound vesicle containing the virus particle
Penetration of a Naked Virus by Rearrangement of Capsid Proteins
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.
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