Baltimore Classification
The Baltimore Classification is a system used to categorize viruses based on their genome type and replication strategy. It was developed by David Baltimore in 1971 and classifies viruses into seven groups (I-VII) based on the type of nucleic acid they contain and how that nucleic acid is replicated. This classification system provides a framework for understanding the diversity of viruses.
6 Key excerpts on "Baltimore Classification"
eBook - ePubTaxonomic Guide to Infectious Diseases
Understanding the Biologic Classes of Pathogenic Organisms
- Jules J. Berman(Author)
- 2019(Publication Date)
- Academic Press(Publisher)
...The Baltimore Classification divides viruses into seven groups based on whether their genome is DNA, RNA, single stranded, or double stranded, the sense of the single strand, and the presence or absence of a reverse transcriptase. Here are the classes of the pathogenic viruses. This classification, though nonphylogenetic in concept, has the great advantage of being comprehensive: every known virus can be assigned to a group within the Baltimore Classification. Group I, double-stranded DNA Group II, single-stranded DNA Group III, double-stranded RNA Group IV, positive sense single-stranded RNA Group V, negative sense single-stranded RNA ssRNA Group VI, single-stranded RNA with a reverse transcriptase Group VII, double-stranded DNA with a reverse transcriptase It is worth repeating that when we use the Baltimore Classification (or any alternate viral classification, for that matter) we must grudgingly accept the fact that biologically relevant features of grouped viruses will cross taxonomic boundaries. Consider the arboviruses. Arbovirus is a shortened name for Arthropod borne virus. The arboviruses fall into several different groups of viruses. The principle vectors of the arboviruses are mosquitoes, ticks. Mosquito-borne arboviruses are members of Class Bunyaviridae (Group V), Flaviviridae (Group IV), or Togaviridae (Group IV). Tick-borne arboviruses are members of Class Bunyarviridae (Group V), Flaviviridae (Group IV), or Reoviridae (Group III). Over 500 arboviruses, infecting a variety of animals, have been described [47]. The arboviruses, organized by their transmission vectors, as shown below, cross multiple viral groups. Mosquito-borne viruses. Bunyaviridae (Group V) La Crosse encephalitis virus California encephalitis virus Rift Valley fever virus Flaviviridae (Group IV) Japanese encephalitis virus Australian encephalitis virus St...
eBook - ePub- Jane Flint, Vincent R. Racaniello, Glenn F. Rall, Theodora Hatziioannou, Anna Marie Skalka(Authors)
- 2020(Publication Date)
- ASM Press(Publisher)
...When the gapped DNA genome of hepadnaviruses (e.g., hepatitis B virus) was discovered, these viruses were assigned to a seventh class (VII). The DNA and RNA descriptors for the viral classes [single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), (+) RNA, or (–) RNA, etc.], but not the Roman numeral designations, have been adopted universally and are a valuable complement to classical taxonomy. The information embodied in classification by genome type provides virologists with immediate insight into the steps that must take place to initiate the replication and expression of any viral genome. Because the viral genome carries the entire blueprint for virus propagation, molecular virologists have long considered it the most important characteristic for classification purposes. Although individual virus families are known by their classical designations, they are commonly grouped according to their genome type. In the ICTV compilation, all viral families are assigned to one of the seven classes described in the Baltimore system (Fig. 1.13). A Common Strategy for Viral Propagation The basic thesis of this textbook is that all viral propagation can be described in the context of three fundamental properties. Viral genomes are packaged inside particles that mediate their transmission from host to host. The viral genome contains the information for initiating and completing an infectious cycle within a susceptible, permissive cell. An infectious cycle includes attachment and entry, decoding of genome information, genome replication, and assembly and release of particles containing the genome. Viral propagation is ensured by establishment in a host population. Perspectives The study of viruses has increased our understanding of the importance and ubiquitous existence of these diverse agents and, in many cases, yielded new and unexpected insight into the molecular biology of host cells and organisms...
eBook - ePubPrinciples of Virology, Volume 1
Molecular Biology
- Jane Flint, Vincent R. Racaniello, Glenn F. Rall, Theodora Hatziioannou, Anna Marie Skalka(Authors)
- 2020(Publication Date)
- ASM Press(Publisher)
...However, none of these genomes encode the complete machinery needed to carry out protein synthesis. Consequently, one important principle is that all viral genomes must be copied to produce messenger RNAs (mRNAs) that can be read by host ribosomes. Literally, all viruses are parasites of their host cells’ translation system. A second principle is that there is unity in diversity: evolution has led to the formation of only seven major types of viral genome. The Baltimore Classification system integrates these two principles to construct an elegant molecular algorithm for virologists (Fig. 3.1). When the bewildering array of viruses is classified by this system, we find seven pathways to mRNA. The value of the Baltimore system is that by knowing only the nature of the viral genome, one can deduce the basic steps that must take place to produce mRNA. Perhaps more pragmatically, the system simplifies comprehension of the extraordinary reproduction cycles of viruses. The Baltimore system omits the second universal function of viral genomes, to serve as a template for synthesis of progeny genomes. Nevertheless, there is also a finite number of nucleic acid-copying strategies, each with unique primer, template, and termination requirements. We shall combine this principle with that embodied in the Baltimore system to define seven strategies based on mRNA synthesis and genome replication. The Baltimore system has stood the test of time: despite the discovery of multitudes of viral genome sequences, they all fall into one of the seven classes. Replication and mRNA synthesis present no obvious challenges for most viruses with DNA genomes, as all cells use DNA-based mechanisms. In contrast, animal cells possess no known systems to copy viral RNA templates and to produce mRNA from them...
eBook - ePub- Britannica Educational Publishing, Kara Rogers(Authors)
- 2010(Publication Date)
- Britannica Educational Publishing(Publisher)
...The icosahedral viruses are further subdivided into families based on the number of capsomeres making up the capsids. Finally, all viruses fall into two classes depending on whether the nucleocapsid is surrounded by a lipoprotein envelope. Some virologists adhere to a division of viruses into those that infect bacteria, plants, or animals; these classifications have some validity, particularly for the unique bacterial viruses with tails, but there is otherwise so much overlap that taxonomy based on hosts seems unworkable. Classification based on diseases caused by viruses also is not tenable, because closely related viruses frequently do not cause the same disease. Eventually, it is likely that the classification of viruses will be based on their nucleotide sequences and their mode of replication, rather than on structural components, as is now the case. The basic taxonomic group is called a family, designated by the suffix -viridae. The major taxonomic disagreement among virologists is whether to segregate viruses within a family into a specific genus and further subdivide them into species names. In the first decade of the 21st century, there occurred a shift toward the use of binomial nomenclature—as used for bacteria—dividing viruses into italicized genera and species. This move was prompted in large part by the International Committee on Taxonomy of Viruses (ICTV), a member group of the International Union of Microbiological Societies. The ICTV oversees the ongoing process of devising and maintaining a universal classification scheme for viruses. In the virus classification hierarchy, the ICTV recognizes orders, families, subfamilies, genera, and species. The placement of viruses in these groups is based on information provided by study groups comprised of experts on specific types of viruses. In the ICTV system, each species of virus is generally recognized as representing a group of isolates, or viruses with distinct nucleic acid sequences...
eBook - ePub- David P. Clark(Author)
- 2009(Publication Date)
- Academic Cell(Publisher)
...Chapter Seventeen Viruses Viruses are Infectious Packages of Genetic Information Life Cycle of a Virus Bacterial Viruses are Known as Bacteriophage Lysogeny or Latency by Integration The Great Diversity of Viruses Small Single-Stranded DNA Viruses of Bacteria Complex Bacterial Viruses with Double Stranded DNA DNA Viruses of Higher Organisms Viruses with RNA Genomes Have Very Few Genes Bacterial RNA Viruses Double Stranded RNA Viruses of Animals Positive-Stranded RNA Viruses Make Polyproteins Strategy of Negative-Strand RNA Viruses Plant RNA Viruses Retroviruses Use both RNA and DNA Genome of the Retrovirus Subviral Infectious Agents Satellite Viruses Viroids are Naked Molecules of Infectious RNA Prions are Infectious Proteins Viruses are Infectious Packages of Genetic Information Viruses are packages of genes inside protective shells of protein. Viruses cannot grow or divide alone. In order to replicate, a virus must first infect a host cell. Only then are the virus genes expressed and the virus components manufactured using the host cell machinery. Viruses are not merely pieces of nucleic acid like plasmids or transposons and neither are they true living cells with the ability to generate energy and make protein. They lie in the gray area between. Viruses cannot make their own proteins or generate their own energy. They can only multiply when they have entered a suitable host cell and taken over the cellular machinery. Despite this a virus is certainly not inert; it does replicate if it can subvert a host cell. Viruses are sub-cellular parasites that rely on a cell to provide energy and raw material. Virus particles contain proteins plus genetic information in the form of DNA or RNA (Fig. 17.01). The virus particle, or virion, consists of a protein shell, known as a capsid, surrounding a length of nucleic acid, either RNA or DNA, which carries the virus genes and is often referred to as the viral genome...
eBook - ePub- Richard L. Hodinka, Stephen A. Young, Benjamin A. Pinksy, Richard L. Hodinka, Stephen A. Young, Benjamin A. Pinksy(Authors)
- 2016(Publication Date)
- ASM Press(Publisher)
...32 Parvoviruses RICHARD S. BULLER VIRAL CLASSIFICATION AND BIOLOGY The parvoviruses are a large group of DNA viruses capable of infecting a wide variety of both invertebrate and vertebrate hosts, including humans and their companion animals. Depending on the virus and the immune status of the host, human infection can range from overt disease to persistent asymptomatic infection. Current parvovirus taxonomy dates from 2004; however, in the intervening 10 years, as new viruses have been identified and more taxonomic data have become available, the current taxonomy is now considered outdated. A new taxonomic classification of the family Parvoviridae has therefore been proposed and is currently under review and likely to be instituted in the near future (1). The proposed taxonomy, based on the amino acid sequence of the NS1 and viral capsid proteins, maintains the family Parvoviridae and subfamily Parvovirinae for the vertebrate parvoviruses, but adds three new genera and a nomenclature change that impact the taxonomy of the human parvoviruses. A member of a proposed genus must now have >30% amino acid sequence identity to other members of the same genus but <30% identity to members of other genera (1). Criteria for inclusion in a viral species have also changed under the proposed taxonomy, with members of the same species having >85% amino acid sequence identity to other members of the same species and >15% amino acid diversity from members of other species (1). The proposed genera and the human viruses they contain are Bocaparvovirus : human bocaviruses 1–4 (HBoV1-4); Dependoparvovirus : human adeno-associated viruses (AAVs); Erythroparvovirus : human parvovirus B19-related viruses; Protoparvovirus : human bufaviruses; and Tetraparvovirus : human parvovirus 4–related viruses (Par4) (1). The parvoviruses are small (~25 nm) non-enveloped, icosahedral viruses...
