Today's topic on Baltimore Classification will also be the last that we're covering for this blog. Along with our previous ones, we hope that the little information we have shared thus far is able to paint a clearer picture in everyones' minds of what virology is all about. So without delaying, let's get started on today's main focus!
How are viruses classified ?- HIV-1, Human Immunodeficiency Virus 1
Hierarchical virus classification:
order - family - subfamily - genus - species - strain/type
At the moment classification is really only important from the level of families down. All families have the suffix viridae e.g.
Members of the family Picornaviridae are generally transmitted via the faecal/oral and airborne routes.
Genera have the suffix virus. Within the Picornaviridae there are 5 genera:
- enterovirus (alimentary tract) species e.g. poliovirus 1, 2, 3
- cardiovirus (neurotropic) species e.g. mengovirus
- rhinovirus (nasopharyngeal region) species e.g. Rhinovirus 1a
- apthovirus (cloven footed animals ) species e.g. FMDV-C
- hepatovirus (liver) species e.g. Hepatitits A virus
The definition of `species' is the most important but difficult assignment to make with viruses. There is an element of subjectivity about it. Consider the lentivirus genus, it is known to contain many different species including the following:
- HIV-2, Human Immunodeficiency Virus 2
- SIV, Simian Immunodeficiency Virus
- FIV, Feline Immunodeficiency Virus
- BIV, Bovine Immunodeficiency Virus
- Visna (sheep)
- EIAV (horses)
- CAEV (goats)
But there are viruses that are intermediate between HIV and SIV. Should these be a different species or included with HIV or SIV and if so which ?
HIV-1 has many different strains with different properties, some infect brain cells, others infect macrophage cells. When do strains diverge far enough to become separate species ? HIV and similar retroviruses have been called quasispecies because any one individual although infected with a particular strain of HIV actually carries an enormous number (1015) of different viral genome sequences. Such variation is extreme but is a feature of viruses with an RNA genome.
Basis of Taxonomic Classification.
Features such as morphology (size, shape, enveloped/unenveloped), physicochemical properties (molecular mass, buoyant density, pH, thermal, ionic stability), genome (RNA, DNA , segmented sequence, restriction map, modifications etc), macromolecules (protein composition and function), antigenic properties, biological properties (host range, transmission tropism etc) are all considered.
The Baltimore Classification
Baltimore classification is a classification system which places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar.
In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research.
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 (Parvoviruses)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 (Reoviruses; Birnaviruses)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 (Hepadnaviruses)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.
http://www.expasy.ch/viralzone/all_by_protein/254.htmlFinally to sum it all up, that concludes the whole series of microbiology topics that we are sharing. Hope everyone had a wonderful time browsing through our blog & learnt something new!Thank You & Have a Nice Day!(: