Classification of Viruses
Viruses are not usually classified into conventional taxonomic
groups but are usually grouped according to such properties
as size, the type of nucleic acid they contain, the structure
of the capsid and the number of protein subunits in it, host
species, and immunological characteristics. It also means that
when a new species of known virus family or genus is investigated
it can be done in the context of the information that is available
for other members of that group. Without a classification scheme
each newly discovered virus would be like a black box, everything
would have to be discovered and rediscovered. The development
of a classification scheme is therefore an important and inevitable
consequence. The current classification scheme allows most newly
described viruses to be labeled. In the best cases much can
be assumed about the biology of the virus. Even in the worse
case a framework for investigation would be suggested. Because
there are so few virus discoveries now being made which do not
fit into the existing classification scheme we can state with
a degree of confidence that most of the major groupings of viruses
infecting humans and domesticated animals have been identified.
How are viruses
Two classification systems exist: The Hierarchical virus classification
system and the Baltimore Classification System.
virus classification system
In 1962 Lwoff, R. W. Horne, and P. Tournier advanced a comprehensive
for the classification of all viruses consisting of phylum -
class - order - family - subfamily - genus - species - strain/type.
The subsequently formed internat. commitee on the nomenclature
of viruses accepted many principles of this system. The most
imortant principle embodied in this system was that viruses
should be grouped according to their shared properties rather
than the protperties of the cells or organisms theyinfect.
Four main characteristics are used:
- Nature of the nucleic acid: RNA or DNA
- Symmetry of the capsid
- Presence or absence of an envelope
- Dimensions of the virion and capsid
At the moment classification is really only important from
the level of families down. Members within a virus family are
ordered with Genomics, the elucidation of evolutionary relationships
ba analyses of nucleic acid and protein sequence similarities.
All Families have the
e.g. Caliciviridae, Picornaviridae, Reoviridae. Genera
have the suffix -virus.
Within the Picornaviridae there are 5 genera: enterovirus, cardiovirus,
rhinovirus, apthovirus and hepatovirus.
The definition of `species'
is the most important but difficult assignment to make with
viruses. There is an element of subjectivity about it.
Another graphic by www.biosmart.ch:
The Baltimore system of virus classification provides a useful
guide with regard to the various mechanisms of viral genome
replication. The central theme here is that all viruses must
generate positive strand mRNAs from their genomes, in order
to produce proteins and replicate themselves. The
precise mechanisms whereby this is achieved differ for each
virus family. These various types of virus genomes can be broken
down into seven fundamentally
different groups, which obviously require different basic strategies
for their replication. David Baltimore, who originated
the scheme, has given his name to the so-called "Baltimore
Classification" of virus genomes. By convention the top
strand of coding DNA written in the 5' - 3' direction is + sense.
mRNA sequence is also + sense.The replication strategy of the
virus depends on the nature of its genome. Viruses can be classified
into seven (arbitrary) groups:
I: Double-stranded DNA (Adenoviruses;
Herpesviruses; Poxviruses, etc)
Some replicate in the nucleus e.g adenoviruses using cellular
proteins. Poxviruses replicate in the cytoplasm and make their
own enzymes for nucleic acid replication.
II: Single-stranded (+)sense DNA
Replication occurs in the nucleus, involving the formation of
a (-)sense strand, which serves as a template for (+)strand
RNA and DNA synthesis.
III: Double-stranded RNA
These viruses have segmented genomes. Each genome segment is
transcribed separately to produce monocistronic mRNAs.
IV: Single-stranded (+)sense RNA
(Picornaviruses; Togaviruses, etc)
a) Polycistronic mRNA e.g. Picornaviruses; Hepatitis A. Genome
RNA = mRNA. Means naked RNA is infectious, no virion particle
associated polymerase. Translation results in the formation
of a polyprotein product, which is subsequently cleaved to form
the mature proteins.
b) Complex Transcription e.g. Togaviruses. Two or more rounds
of translation are necessary to produce the genomic RNA.
V: Single-stranded (-)sense RNA
(Orthomyxoviruses, Rhabdoviruses, etc)
Must have a virion particle RNA directed RNA polymerase.
a) Segmented e.g. Orthomyxoviruses. First step in replication
is transcription of the (-)sense RNA genome by the virion RNA-dependent
RNA polymerase to produce monocistronic mRNAs, which also serve
as the template for genome replication.
b) Non-segmented e.g. Rhabdoviruses. Replication occurs as above
and monocistronic mRNAs are produced.
VI: Single-stranded (+)sense RNA with DNA
intermediate in life-cycle (Retroviruses)
Genome is (+)sense but unique among viruses in that it is DIPLOID,
and does not serve as mRNA, but as a template for reverse transcription.
VII: Double-stranded DNA with RNA intermediate
This group of viruses also relies on reverse transcription,
but unlike the Retroviruses, this occurs inside the virus particle
on maturation. On infection of a new cell, the first event to
occur is repair of the gapped genome, followed by transcription.