Baltimore classification
Encyclopedia
David Baltimore
David Baltimore is an American biologist, university administrator, and Nobel laureate in Physiology or Medicine. He served as president of the California Institute of Technology from 1997 to 2006, and is currently the Robert A. Millikan Professor of Biology at Caltech...
, is a virus classification
Virus classification
Virus classification is the process of naming viruses and placing them into a taxonomic system. Similar to the classification systems used for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is mainly due to the pseudo-living nature of viruses, which...
system that groups virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es into families, depending on their type of genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
(DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, single-stranded (ss), double-stranded (ds), etc.) and their method of replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
.
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:Class I: Double-stranded DNA viruses
This type of virus usually must enter the host nucleusCell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
before it is able to replicate. Furthermore, these viruses require host cell polymerases
DNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
to replicate the viral genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
and, hence, are highly dependent on the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
. Proper infection and production of progeny requires that the cell be in replication, as it is during replication that the cell's polymerases are active. The virus may induce the cell to forcefully undergo cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
, which may lead to transformation of the cell and, ultimately, cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. Examples include Herpesviridae
Herpesviridae
The Herpesviridae are a large family of DNA viruses that cause diseases in animals, including humans. The members of this family are also known as herpesviruses. The family name is derived from the Greek word herpein , referring to the latent, recurring infections typical of this group of viruses...
, Adenoviridae
Adenoviridae
Adenoviruses are medium-sized , nonenveloped icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA genome...
, and Papovaviridae.
There is only one well-studied example in which a class 1 virus is not replicating within the nucleus: the Poxvirus family, a highly pathogenic virus that infects vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s and includes the smallpox
Smallpox
Smallpox was an infectious disease unique to humans, caused by either of two virus variants, Variola major and Variola minor. The disease is also known by the Latin names Variola or Variola vera, which is a derivative of the Latin varius, meaning "spotted", or varus, meaning "pimple"...
virus.
Class II: Single-stranded DNA viruses
Viruses in this category include the Anelloviridae, CircoviridaeCircoviridae
The Circoviridae are a family of viruses. These are small, relatively poorly-studied viruses, with circular, single-stranded DNA genomes of approximately one to four kilobases-Virology:...
, and Parvoviridae
Parvoviridae
The Parvoviridae family includes the smallest known viruses, and some of the most environmentally resistant. They were discovered during the 1960s and affect vertebrates and insects...
(which infect vertebrates), the Geminiviridae
Geminiviridae
Geminiviruses are plant viruses which have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication . According to the Baltimore classification they are considered class II viruses...
and Nanoviridae
Nanoviridae
The Nanoviridae are a family of viruses, including the following genera:*Genus Nanovirus; type species: Subterranean clover stunt virus*Genus Babuvirus; type species: Banana bunchy top virus-Virus structure and genome:...
(which infect plants), and the Microviridae
Microviridae
The Microviridae are a family of bacteriophages with a single stranded DNA genome. The name of this family is derived from the Greek word 'micro' meaning small. This refers to the size of their genomes which are among the smallest of the DNA viruses....
(which infect prokaryotes). Most of them have circular genomes (the parvoviruses are the only known exception). Eukaryote-infecting viruses replicate mostly within the nucleus - usually via a rolling circle mechanism, forming double-stranded DNA intermediate in the process. A prevalent but asymptomatic
Asymptomatic
In medicine, a disease is considered asymptomatic if a patient is a carrier for a disease or infection but experiences no symptoms. A condition might be asymptomatic if it fails to show the noticeable symptoms with which it is usually associated. Asymptomatic infections are also called subclinical...
human Anellovirus, called Transfusion Transmitted Virus
Transfusion Transmitted Virus
TT virus was the first member of the new family Anelloviridae to be discovered.- Initial discovery :TTV, for Transfusion Transmitted Virus or Torque teno virus was first reported in a Japanese patient in 1997 by the research scientist T. Nishizawa...
(TTV), is included within this classification.
Class III: Double-stranded RNA viruses
As with most RNARNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
viruses, this class replicates in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
, not having to use the host replication polymerases to as much a degree as DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
viruses. This family is also not as well-studied as the rest and includes 2 major families, the Reoviridae
Reoviridae
Reoviridae is a family of viruses that can affect the gastrointestinal system and respiratory tract. Viruses in the family Reoviridae have genomes consisting of segmented, double-stranded RNA...
and Birnaviridae
Birnaviridae
The birnaviridae are a family of viruses, including the following genera:*Genus Aquabirnavirus; type species: Infectious pancreatic necrosis virus*Genus Avibirnavirus; type species: Infectious bursal disease virus...
. Replication is monocistronic and includes individual, segmented genomes, meaning that each of the genes codes for only one protein, unlike other viruses that exhibit more complex translation.
Class IV & V: Single-stranded RNA viruses
Class IV and V ssRNA viruses do not depend as heavily as DNA viruses on the cell cycle.Class IV: Single-stranded RNA viruses - Positive-sense
The positive-sense RNA viruses and indeed all RNA defined as positive-senseSense (molecular biology)
In molecular biology and genetics, sense is a concept used to compare the polarity of nucleic acid molecules, such as DNA or RNA, to other nucleic acid molecules...
can be directly accessed by host ribosomes to immediately form proteins. 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. 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. 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
Caliciviridae
The Caliciviridae family are a family of viruses, members of Class IV of the Baltimore scheme. They are positive-sense, single stranded RNA which is non-segmented. The caliciviruses have been found in a number of organisms such as humans, cattle, pigs, cats, chickens, reptiles, dolphins and...
, Coronaviridae
Coronaviridae
Coronaviruses are enveloped, single-stranded, positive-sense RNA viruses with club-shaped surface about 120-160 nm in diameter that resemble a “corona”.-Virology:...
, Flaviviridae
Flaviviridae
The Flaviviridae are a family of viruses that are primarily spread through arthropod vectors . The family gets its name from Yellow Fever virus, a type virus of Flaviviridae; flavus means yellow in Latin...
, Picornaviridae, Arteriviridae, and Togaviridae
Togaviridae
The Togaviridae are a family of viruses, including the following genera:* Genus Alphavirus; type species: Sindbis virus, Eastern equine encephalitis virus, Western equine encephalitis virus, Venezuelan equine encephalitis virus, Ross River virus, O'nyong'nyong virus, Chikungunya* Genus Rubivirus;...
.
Class V: Single-stranded RNA viruses - Negative-sense
The negative-sense RNA viruses and indeed all genes defined as negative-senseSense (molecular biology)
In molecular biology and genetics, sense is a concept used to compare the polarity of nucleic acid molecules, such as DNA or RNA, to other nucleic acid molecules...
cannot be directly accessed by host ribosomes to immediately form proteins. Instead, they must be transcribed
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
by viral polymerases into a "readable" form, which is the positive-sense reciprocal. These can also be divided into two groups:
- Viruses containing nonsegmented 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. A positive-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 nucleusCell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
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
Orthomyxoviridae
The Orthomyxoviridae are a family of RNA viruses that includes five genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus and Thogotovirus. A sixth has recently been described...
, Paramyxoviridae, Bunyaviridae
Bunyaviridae
Bunyaviridae is a family of negative-stranded RNA viruses. Though generally found in arthropods or rodents, certain viruses in this family occasionally infect humans. Some of them also infect plants....
, Filoviridae
Filoviridae
The family Filoviridae is the taxonomic home of several related viruses that form filamentous virions. Two members of the family that are commonly known are Ebola virus and Marburg virus. Both viruses, and some of their lesser known relatives, cause severe disease in humans and nonhuman primates in...
, and Rhabdoviridae
Rhabdoviridae
Rhabdoviruses are viruses belonging to the family Rhabdoviridae, which is in the order Mononegavirales. The name is derived from the Greek rhabdos meaning rod referring to the shape of the viral particles. Rhabdoviruses infect a broad range of hosts throughout the animal and plant kingdoms...
(the latter which includes rabies
Rabies
Rabies is a viral disease that causes acute encephalitis in warm-blooded animals. It is zoonotic , most commonly by a bite from an infected animal. For a human, rabies is almost invariably fatal if post-exposure prophylaxis is not administered prior to the onset of severe symptoms...
).
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. One defining feature is the use of reverse transcriptaseReverse transcriptase
In the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
to convert the positive-sense RNA into DNA. 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
Retroviral integrase is an enzyme produced by a retrovirus 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
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
.
Class VII: Double-stranded DNA viruses that replicate through a single-stranded RNA intermediate
This small group of viruses, exemplified by the Hepatitis B virus (which is in the HepadnaviridaeHepadnaviridae
Hepadnaviruses are a family of viruses which can cause liver infections in humans and animals. There are two recognized genera:*Genus Orthohepadnavirus; type species: Hepatitis B virus...
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. The pregenome RNA serves as template for the viral reverse transcriptase and for production of the DNA genome.