Microbiology and Immunology

Virology, viral classification, types of viruses WORLD OF MICROBIOLOGY AND IMMUNOLOGY

580

•

In the viral classification scheme, all families end in the

suffix viridae, for example Picornaviridae. Genera have the

suffix virus. For example, in the family Picornaviridae there

are five genera: enterovirus, cardiovirus, rhinovirus,

apthovirus, and hepatovirus. The names of the genera typically

derive from the preferred location of the virus in the body (for

those viral genera that infect humans). As examples, rhi-

novirus is localized in the nasal and throat passages, and hepa-

tovirus is localized in the liver. Finally, within each genera

there can be several species.

As noted above, there are a number of criteria by which

members of one grouping of viruses can be distinguished from

those in another group. For the purposes of classification,

however, three criteria are paramount. These criteria are the

host organism or organisms that the virus utilizes, the shape of

the virus particle, and the type and arrangement of the viral

nucleic acid.

An important means of classifying viruses concerns the

type and arrangement of nucleic acid in the virus particle.

Some viruses have two strands of DNA, analogous to the dou-

ble helix of DNA that is present in prokaryotes such as bacte-

ria and in eukaryotic cells. Some viruses, such as the

Adenoviruses, replicate in the nucleusof the host using the

replication machinery of the host. Other viruses, such as the

poxviruses, do not integrate in the host genome, but replicate

in the cytoplasmof the host. Another example of a double-

stranded DNA virus are the Herpesviruses.

Other viruses only have a single strand of DNA. An

example is the Parvoviruses. Viruses such as the Parvoviruses

replicate their DNA in the host’s nucleus. The replication

involves the formation of what is termed a negative-sense strand

of DNA, which is a blueprint for the subsequent formation of

the RNA and DNA used to manufacture the new virus particles.

The genome of other viruses, such as Reoviruses and

Birnaviruses, is comprised of double-stranded RNA. Portions

of the RNA function independently in the production of a

number of so-called messenger RNAs, each of which pro-

duces a protein that is used in the production of new viruses.

Still other viruses contain a single strand of RNA. In

some of the single-stranded RNA viruses, such as

Picornaviruses, Togaviruses, and the HepatitisA virus, the

RNA is read in a direction that is termed “+ sense.” The sense

strand is used to make the protein products that form the new

virus particles. Other single-stranded RNA viruses contain

what is termed a negative-sense strand. Examples are the

Orthomyxoviruses and the Rhabdoviruses. The negative strand

is the blueprint for the formation of the messenger RNAs that

are required for production of the various viral proteins.

Still another group of viruses have + sense RNA that is

used to make a DNA intermediate. The intermediate is used to

Thin section electron micrograph of adenoviruses.

womi_V 5/7/03 11:00 AM Page 580