43 lines
6.8 KiB
Markdown
43 lines
6.8 KiB
Markdown
---
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title: "Baltimore classification"
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chunk: 2/7
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source: "https://en.wikipedia.org/wiki/Baltimore_classification"
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category: "reference"
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tags: "science, encyclopedia"
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date_saved: "2026-05-05T09:06:57.533227+00:00"
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instance: "kb-cron"
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---
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The first Baltimore group contains viruses that have a double-stranded DNA (dsDNA) genome. All dsDNA viruses have their mRNA synthesized in a three-step process. First, a transcription preinitiation complex binds to the DNA upstream of the transcription site, recruiting a host RNA polymerase enzyme. Once the RNA polymerase is recruited, it uses the negative-sense strand as a template for synthesizing mRNA strands, which are positive sense. The RNA polymerase then terminates transcription upon reaching a specific signal, such as a polyadenylation site.
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dsDNA viruses make use of several mechanisms to replicate their genome. A widely used method is bidirectional replication, in which two replication forks are established at a replication origin site and move in opposite directions on a DNA molecule. A rolling circle mechanism that produces linear strands while progressing in a loop around a circular genome is also common. Many dsDNA viruses use a strand displacement method whereby one strand is synthesized from a template strand, and a complementary strand is then synthesized from the previously synthesized strand to form a dsDNA genome. Lastly, some dsDNA viruses are replicated as part of a process called replicative transposition, whereby a viral genome that is integrated into a host cell's genome is replicated to another part of the host cell's genome.
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dsDNA viruses can be divided informally into those that replicate in the nucleus, and as such are relatively dependent on host cell machinery for transcription and replication, and those that replicate in cytoplasm, in which case they have obtained their own means of transcription and replication. dsDNA viruses are also sometimes divided between tailed dsDNA viruses, which refers to members of the realm Duplodnaviria, specifically the head-tail of the class Caudoviricetes, and tailless or non-tailed (icosahedral) dsDNA viruses, which refers to viruses in the realms Singelaviria and Varidnaviria.
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dsDNA viruses are classified into six realms and many taxa that are unassigned to a realm:
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Viruses in the realms Adnaviria, Duplodnaviria, and Singelaviria are dsDNA viruses.
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In the realm Floreoviria, members of the class Papovaviricetes are dsDNA viruses.
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The realm Pleomoviria contains both dsDNA viruses and ssDNA viruses.
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Viruses in the realm Varidnaviria are dsDNA viruses except for the class Ainoaviricetes, which are ssDNA viruses.
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The following taxa that are unassigned to a realm exclusively contain dsDNA viruses:
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Classes: Naldaviricetes
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Families: Ampullaviridae, Basaltiviridae, Bicaudaviridae, Clavaviridae, Eurekaviridae, Fuselloviridae, Globuloviridae, Guttaviridae, Halspiviridae, Huangdiviridae, Itzamnaviridae, Lomiviridae, Nipumfusiviridae, Ovaliviridae, Plasmaviridae, Portogloboviridae, Thaspiviridae, Xigoviridae, Yamazakiviridae
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Genera: Dinodnavirus
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=== Group II: single-stranded DNA viruses ===
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The second Baltimore group contains viruses that have a single-stranded DNA (ssDNA) genome. ssDNA viruses have the same manner of transcription as dsDNA viruses. Because the genome is single-stranded, however, it is first made into a double-stranded form by a DNA polymerase enzyme upon entering a host cell. mRNA is then synthesized from the double-stranded form. The double-stranded form of ssDNA viruses may be produced either directly after entry into a cell or as a consequence of replicating the viral genome.
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Most ssDNA viruses contain circular genomes that are replicated by rolling circle replication (RCR). ssDNA RCR is initiated by an endonuclease enzyme that bonds to and cleaves the positive-sense strand, which allows a DNA polymerase to use the negative-sense strand as a template for replication. Replication progresses in a loop around the genome by extending the 3′-end ("three prime end") of the positive-sense strand, which displaces the prior positive-sense strand. The endonuclease then cleaves the positive-sense strand again to create a standalone genome that is joined (ligated) into a circular loop. The new ssDNA genome may be packaged into virions or replicated by a DNA polymerase to create a double-stranded form for transcription or additional rounds of replication.
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Parvoviruses and bidnaviruses have linear ssDNA genomes that are replicated by rolling hairpin replication (RHR), which is similar to RCR. Their genomes have hairpin loops at each end of the genome that repeatedly unfold and refold during replication to change the direction of DNA synthesis to move back and forth along the linear genome, which produces numerous copies of the genome in a continuous process. Individual genomes are then excised from this molecule by the endonuclease.
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Nearly all ssDNA viruses have positive-sense genomes, but a few exceptions and peculiarities exist. Anelloviruses are the only ssDNA viruses that have negative-sense genomes. Parvoviruses may package either the positive- or negative-sense strand into capsids. Lastly, bidnaviruses may package both the positive- and negative-sense strands of their bipartite genome. In any case, the sense of ssDNA viruses, unlike that of ssRNA viruses, is not sufficient to separate ssDNA viruses into two Baltimore groups since all ssDNA viral genomes are converted to dsDNA forms before transcription and replication.
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ssDNA viruses are classified into five realms and a couple families that are unassigned to realms:
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Viruses in the realms Efunaviria and Volvereviria are ssDNA viruses.
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In the realm Floreoviria, members are ssDNA viruses except for viruses in the class Papovaviricetes, which are dsDNA viruses.
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The realm Pleomoviria contains both dsDNA viruses and ssDNA viruses.
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In the realm Varidnaviria, viruses of the class Ainoaviricetes are ssDNA viruses.
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The unassigned families Obscuriviridae and Spiraviridae are ssDNA virus families.
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=== Group III: double-stranded RNA viruses ===
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The third Baltimore group contains viruses that have a double-stranded RNA (dsRNA) genome. After entering a host cell, the viral RNA-dependent RNA polymerase (RdRp) synthesizes a positive-sense strand from the negative-sense strand of the dsRNA genome. This positive-sense strand may be used either as mRNA for translation or as a template for replication to form the dsRNA genome.
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dsRNA is not a molecule made by cells, so eukaryotes have evolved antiviral systems to detect and inactivate viral dsRNA. To counter this, many dsRNA viruses replicate their genomes inside of capsids, thereby avoiding detection inside of the host cell's cytoplasm. Positive-sense strands are then forced out from the capsid to be translated or translocated from the mature capsid to a progeny capsid.
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dsRNA viruses are classified into two phyla within the kingdom Orthornavirae, realm Riboviria: |