General Characteristics of the viruses

Course: Virology

Lecturer: Dr. Weam Saad
Lecture: General Characteristics of viruses
General Characteristics:
Viruses are smaller than prokaryotic or eukaryotic cells. They are the smallest infectious agent ranging from about 20 nm to 300 nm in diameter.
They are not cells, they have a simple and stable structure.
They have no metabolic system.
They depend on the host cell for replication (obligate intracellular parasites).
They have either DNA or RNA genomes, but no ribosomes, and other factors needed for translation. Thus, they depend on the host cells for production of viral proteins.
Their genomes encode minimal information to:
Genome replication and packaging.
Production of viral proteins.
Turn cellular functions to allow the production of virions.
Some viruses (bacteriophages) infect prokaryotic cells, while others infect eukaryotic cells.
Some viruses destroy cells, producing disease; other persist in infected cells either in a latent or persistent state; and other may cause cellular malignant transformation.

Terms in Virology:

Capsid: The protein shell, or coat, which encloses the nucleic acid genome.
Defective virus: A virus particle that is functionally unable for replication.
Envelope: A lipid-containing membrane that surroundssome virus particles. It is acquired during viral maturationby a budding process through a cellular membrane. Virus encoded glycoproteins are exposed on the surface of theenvelope.
Structural units: The basic protein building blocks ofthe coat. They are usually a collection of more than one non-identical protein subunit. The structural unit is usually referredto as a protomer.
Subunit: A single folded viral polypeptide chain.
Virion: The complete virus particle. In some cases(eg, papillomaviruses, picornaviruses), the virion is identicalwith the nucleocapsid. In more complex virions (herpes viruses, orthomyxoviruses), this includes the nucleocapsid plusa surrounding envelope. This virion, function is totransfer the viral nucleic acid from one cell to another.

Viral Structure:

Viruses are composed of a nucleic acid genome (DNA or RNA) and a protein coat. Many viruses contain an external membrane called an envelope. The protein coat, or capsid, of the virion is composed of multiplecopies of one or more types of proteins. These proteins assemble, forming structural units called capsomeres (Capsomeres: Morphologic units seen in the electron microscope on the surface of icosahedral virus particles) They are clusters of polypeptides.
The nucleic acid plus the capsid shell of a virus particle is often called nucleocapsid. The simplest viruses are those devoid of envelope with single-stranded DNA or RNA. Enveloped viruses contain an external membrane surrounding the nucleocapsid. The viral envelope is derived from host cell membranes (nuclear, Golgi apparatus, endoplasmic reticulum or plasma membrane). It is composed by a lipid bilayer, with proteins inserted on it encoded by the virus. Some viruses, such as bacteriophages, have complex protein tails that are required for attachment and/or penetration of viral DNA into susceptible host cells.
Viruses are different in size, poxviruses are 300 nm, and they are large enough to be seen under the light microscope, unlike the other viruses that require an electron microscope to be visualized (seen) like circoviruses 17 - 22 nm in diameter. To examine the structure of a virus, several techniques are used. X-ray crystallography is a method of determining the physical structure, dimensions of the individual proteins and components of the virus. The obtained information is then used to build the structure of the virus particle. Electron microscopy is used to get information about the shape of the virus; it is also used in clinical diagnosis through detection of virus in clinical specimens.