Viruses
* Latin word meaning "Poison".
* Obligate intracellular parasites.
* They only demonstrate characteristics of life while "inside" a host cells like Bacteria, animal or plant cells.
* Outside a host cell, viruses are inert i.e. they do not show any activity.
Definition of viruses
"Viruses may be defined as a cellular organisms whose genomes consist of nucleic acid and which obligately replicate inside host cells using host metabolic machinery".
* Virology is the bioscience for study if viral nature and the relationship between and hosts.
Viral Properties
* Viruses are inert (nucleoprotein) filterable agents.
* Viruses are obligate intracellular parasites.
* Viral genome are RNA or DNA but not both.
* Viruses have a naked capsid or envelope with attached proteins.
* Viruses do not have the genetic capability to multiply by division.
Size of viruses
* Their size varies.
* A small virus has a diameter of about 20nm.
Example: Parvovirus
* A large virus have a diameter of up to 400nm.
Example : Poxviruses
Shape of Viruses
* Spherical
* Rod-shaped
* Brick-shaped
* Tadpole-shaped
* Bullet-shaped
* Filament
Rod-shaped or helical E.g. Tabacco Mosaic viru (TMV)
Brick-shaped viruses E.g. Poxviruses
Tadpole-shaped viruses E.g. Bacteriophages
Bullet-shaped viruses E.g. Rhabdoviruses
Filament shaped viruses E.g. Ebolaviruses
Structure of viruses
Virion
* The complete infections unit of virus particle.
* Structurally mature, extracellular virus particles.
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| Virion |
1. Core of genetic material (nucleic acid).
2. Protein coat or capsid.
- Protects viral genes from inactivation by adverse environmental factors.
- Core + Capsid = nucleocapsid
- In many viruses important in attachment of viruses to specific receptors on host cells.
- Composed of a large number of subunits capsomeres.
3. Many animal virus particles are surrounded by a lipoprotein envelope.
The nucleic acid genome plus the protective protein coat is called the nucleocapsid, helical or complex symmetry.
Viral core
* It contains the viral nucleic acid genome.
* In the center of the virion: Controls the viral heredity and variation, responsible for the infectivity.
Genome
*The genome of a virus can be either DNA or RNA
* DNA-double stranded (ds): linear or circular
Single stranded (ss) : linear or circular
* RNA- ss: segmented or non-segmented
ds: linear (only reovirus family)
Viral Capsid
* The protein shell, or coat, that encloses the nucleic acid genome.
* It is an outer covering of protein that protects the genetic material of a virus.
* The capsomere is a subunit of the capsid
* Capsomeres self-assemble to form the capsid.
* Subunits called protomers aggregate to form capsomeres.
Functions of capsid:
a. Protect the viral nucleic acid.
b. Participate in the viral infection.
c. Share the antigenicity.
Nucleocapsid
The core of a virus particle consisting of the genome plus a complex of proteins.
Symmetry of Nucleocapsid
* Helical
* Cubic / Icosahedral
* Complex
Helical Capsid
* The icosahedral capsid structure of adenovirus is made up of three proteins, hexon, penton base and fiber.
* Helical morphology is seen in nucleocapsids of many filamentous and pleomorphic viruses.
* Helical nucleocapsids are characterized by length, width, pitch of the helix and number of protomers per helical turn.
Icosahedral Capsid
* Icosahedral morphology is characteristic of the nucleocapsids of many "spherical" viruses.
* The icosahedral capsid structure of adenovirus is made up of three proteins hexon, penton base and fiber.
* Some proteins are associated with viral DNA, whereas others are associated with hexon and are involved in the formation of the capsid.
Envelope
* A lipid-containing membrane that surrounds some viral particles.
* It is acquired during viral maturation by a budding process through a cellular membrane
* Not all viruses have the envelope, and viruses can be divided into 2 kinds: Enveloped Virus and Naked Virus.
The Envelope
* Enveloped viruses obtain their envelope by budding through a host cell membrane.
* In some cases, the virus buds through the plasma membrane but in other cases the envelope may be derived from internal cell membranes such as those of the Golgi body of the nucleus.
* Enveloped viruses do not necessarily have to kill their host cell in order to be released, since they can bud out of the cell - a process that is not necessarily lethal to the cell - hence some budding viruses can set up persistent infections.
Functions of envelope
* Antigenicity. Some viruses possess neuraminidase
* Infectivity
* Resistance