The Baltimore classification is a virus classification system which groups viruses into families depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds) etc. David Baltimore (born March 7, 1938) is an American biologist Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Virus classification involves naming and placing Viruses into a taxonomic system A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units ) and their method of replication. DNA replication is the process of copying a double-stranded DNA molecule to form two double-stranded molecules
Classifications
Classifying viruses according to their genome means that those in a given category will all behave in much the same way, which offers some indication of how to proceed with further research. In short:
- I: dsDNA viruses (e. A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. g. Adenoviruses, Herpesviruses, Poxviruses)
- II: ssDNA viruses (+)sense DNA (e. Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Poxviruses (members of the family Poxviridae) are viruses that can as a family infect both Vertebrate and Invertebrate animals A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. g. Parvoviruses)
- III: dsRNA viruses (e. Parvovirus, commonly abbreviated to parvo, is a Genus of the Parvoviridae family linear non-segmented single stranded DNA viruses Double-stranded ( ds) RNA viruses are a diverse group of Viruses that vary widely in host range (humans animals plants Fungi, and g. Reoviruses)
- IV: (+)ssRNA viruses (+)sense RNA (e. Reoviridae is a family of Viruses that can affect the Gastrointestinal system (such as Rotavirus) and Respiratory tract An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. g. Picornaviruses, Togaviruses)
- V: (-)ssRNA viruses (-)sense RNA (e. A Picornavirus is a Virus belonging to the family Picornaviridae. The Togaviridae are a family of Viruses including the following genera Genus Alphavirus; type species Sindbis virus An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. g. Orthomyxoviruses, Rhabdoviruses)
- VI: ssRNA-RT viruses (+)sense RNA with DNA intermediate in life-cycle (e. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that Rhabdoviruses are Viruses belonging to the family Rhabdoviridae, which is in the order Mononegavirales. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template g. Retroviruses)
- VII: dsDNA-RT viruses (e. A retrovirus is any Virus belonging to the viral family Retroviridae. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template g. Hepadnaviruses)
Class I: Double stranded DNA viruses
This type of virus usually must enter the host nucleus before it is able to replicate. Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. Furthermore, these viruses require host cell polymerases to replicate the viral genome and hence are highly dependent on the cell cycle. A DNA Polymerase is an Enzyme that assists in DNA replication. In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication Proper infection and production of progeny requires that the cell be in replication as that is when the cell's polymerases are active. The virus may induce the cell to forcefully undergo cell division, and chronically, this may lead to transformation of the cell and ultimately, cancer. Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. Malignant transformation is the process by which cells acquire the properties of Cancer. Cancer (medical term Malignant Neoplasm) is a class of Diseases in which a group of cells display uncontrolled Examples include Herpesviridae, Adenoviridae and Papovaviridae. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA A Papovaviridae is a member of the Papovaviridae family of Viruses Papovaviridae includes two genera: Papillomavirus and
There is only one well studied example in which a class 1 virus is not replicating within the nucleus, that is the Poxvirus family, a highly pathogenic virus that infects vertebrates and includes the smallpox virus. Poxviruses (members of the family Poxviridae) are viruses that can as a family infect both Vertebrate and Invertebrate animals Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor.
Class II: Single stranded DNA viruses
Viruses that fall under this category includes ones that are not as well studied, but still do pertain highly to vertebrates. A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. Two examples include the Circoviridae and Parvoviridae. The Circoviridae are a family of Viruses including the following genera Genus Anellovirus; type species Transfusion The Parvoviridae family includes the smallest known Viruses and some of the most environmentally resistant They replicate within the nucleus, and form a double stranded DNA intermediate during replication. A human Circovirus called TTV is included within this classification and is found in almost all humans, infecting them asymptomatically in nearly every major organ. In Biology, an organ ( Latin: organum, "instrument tool" from Greek όργανον - organon "organ instrument
Class III: Double stranded RNA viruses
As with most RNA viruses, this class replicates in the cytoplasm, not having to use the host replication polymerases to as much a degree as DNA viruses. Double-stranded ( ds) RNA viruses are a diverse group of Viruses that vary widely in host range (humans animals plants Fungi, and Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known This family is also not as well studied as the rest and includes 2 major families, the Reoviridae and Birnaviridae. Reoviridae is a family of Viruses that can affect the Gastrointestinal system (such as Rotavirus) and Respiratory tract The birnaviridae are a family of Viruses including the following genera Genus Aquabirnavirus; type species Infectious Replication is monocistronic and includes individual, segmented genomes, meaning that each of the genes code for only one protein, unlike other viruses which exhibit more complex translation. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product
Class IV & V: Single stranded RNA viruses
These viruses consist of two types, however both share the fact that replication is primarily in the cytoplasm, and that replication is not as dependent on the cell cycle as other DNA viruses. This class of viruses are also one of the best well studied, alongside the double stranded DNA viruses.
Class IV: Single stranded RNA viruses - Positive (+) sense
The positive sense RNA viruses and indeed all genes defined as positive sense can be directly accessed by host polymerases to immediately form proteins. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Sense, when applied in a Molecular biology context is a general concept used to compare the polarity of Nucleic acid molecules such as DNA or RNA These can be divided into two groups, both of which reproduce in the cytoplasm:
- Viruses with polycistronic mRNA where the genome RNA forms the mRNA and is translated into a polyprotein product that is subsequently cleaved to form the mature proteins. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product This means that the gene can utilize a few methods in which to produce proteins from the same strand of RNA, all in the sake of reducing the size of its gene.
- Viruses with complex transcription, for which subgenomic mRNAs, ribosomal frameshifting and proteolytic processing of polyproteins may be used. Subgenomic mRNA' s are essentially smaller sections of the original transcribed Template strand. Translational frameshifting or ribosomal frameshifting refers to an alternate process of Protein Translation. Proteolysis is the directed degradation ( digestion) of Proteins by cellular Enzymes called Proteases or by intramolecular digestion All of which are different mechanisms with which to produce proteins from the same strand of RNA.
Examples of this class include the families Astroviridae, Caliciviridae, Coronaviridae, Flaviviridae, Picornaviridae, Arteriviridae and Togaviridae. Astrovirus is a type of Virus that infects Mammals and Birds. The Caliciviridae family are a family of Viruses, members of Class IV of the Baltimore scheme. Coronaviruses are enveloped single stranded positive-sense RNA viruses (27-31kb with club-shaped surface about 120-160 nm in diameter that resemble a “corona” The Flaviviridae are a family of Viruses that are primarily spread through arthropod vectors (mainly ticks and mosquitoes A Picornavirus is a Virus belonging to the family Picornaviridae. Arterivirus is a genus of Virus, with type species Equine arteritis virus. The Togaviridae are a family of Viruses including the following genera Genus Alphavirus; type species Sindbis virus
Class V: Single stranded RNA viruses - Negative (-) sense
The negative sense RNA viruses and indeed all genes defined as negative sense cannot be directly accessed by host polymerases to immediately form proteins. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Sense, when applied in a Molecular biology context is a general concept used to compare the polarity of Nucleic acid molecules such as DNA or RNA Instead, they must be transcripted by viral polymerases into a "readable" form, which is the positive sense reciprocal. These can also be divided into two groups:
- Viruses containing non segmented genomes for which the first step in replication is transcription from the (-) stranded genome by the viral RNA-dependent RNA polymerase to yield monocistronic mRNAs that code for the various viral proteins. In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby A (+) sense genome copy is then produced that serves as template for production of the (-) strand genome. Replication is within the cytoplasm.
- Viruses with segmented genomes for which replication occurs in the nucleus and for which the viral RNA-dependent RNA polymerase produces monocistronic mRNAs from each genome segment. The largest difference between the two is the location of replication.
Examples in this class include the families Arenaviridae, Orthomyxoviridae, Paramyxoviridae, Bunyaviridae, Filoviridae and Rhabdoviridae (the latter which includes rabies). Arenavirus is a genus of Virus. The type species is Lymphocytic choriomeningitis virus (LCMV; it also includes the species responsible for The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that Paramyxoviruses are Viruses of the Paramyxoviridae family of the Mononegavirales order they are negative-sense single-stranded Bunyaviridae is a family of negative-stranded RNA viruses Though generally found in Arthropods or rodents certain viruses in this family occasionally infect Filoviridae is the family of Viruses that belong to the order Mononegavirales. Rhabdoviruses are Viruses belonging to the family Rhabdoviridae, which is in the order Mononegavirales. Rabies (from rabies “madness rage fury” Also known as “ hydrophobia ” is a viral Zoonotic neuroinvasive disease that
Class VI: Positive (+) sense single stranded RNA viruses that replicate through a DNA intermediate
A well studied family of this class of viruses include the retroviruses. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template A retrovirus is any Virus belonging to the viral family Retroviridae. One defining feature is the use of reverse transcriptase to convert the positive sense RNA into DNA. In Biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase Enzyme that transcribes Instead of using the RNA for templates of proteins, they use DNA to create the templates, which is spliced into the host genome using integrase. Integrase is an enzyme produced by a Retrovirus (including HIV) that enables its genetic material to be integrated into the DNA of the infected cell Replication can then commence with the help of the host cell's polymerases. A well studied example includes HIV. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome
Class VII: Double stranded DNA viruses that replicate though a single stranded RNA intermediate
This small group of viruses, exemplified by the Hepatitis B virus (which is in the Hepadnaviridae family), have a double-stranded, gapped genome that is subsequently filled in to form a covalently closed circle (ccc DNA) that serves as a template for production of viral mRNAs and a subgenomic RNA. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Subgenomic mRNA' s are essentially smaller sections of the original transcribed Template strand. The pregenome RNA serves as template for the viral reverse transcriptase and for production of the DNA genome.
See also
References
"Virus Taxonomy Portal." (Website. ) Viral Bioinformatics Resource Center & Viral Bioinformatics - Canada. Retrieved on 2007-09-27. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 489 - Odoacer attacks Theodoric at the Battle of Verona and is defeated again
- ^ Baltimore D (1971). "Expression of animal virus genomes". Bacteriol Rev 35 (3): 235–41. PMID 4329869.
External links
- Family Groups - The Baltimore Method
- The Universal Virus Database of the International Committee on Taxonomy of Viruses
- The taxonomy portal of the Genbank database
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