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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Baltimore classification | 5/7 | https://en.wikipedia.org/wiki/Baltimore_classification | reference | science, encyclopedia | 2026-05-05T09:06:57.533227+00:00 | kb-cron |
=== Genome structure === Viral genomes may be either linear with ends or circular in a loop. Whether a virus has a linear or circular genome varies from group to group. Most dsDNA and –ssRNA viruses have linear genomes, ssDNA viruses mainly have circular genomes, dsRNA, +ssRNA, and ssRNA-RT viruses have linear genomes, and dsDNA-RT viruses have circular genomes. As all +ssRNA viruses have genomes that can act as mRNA, and circular mRNA does not exist in cellular life, all +ssRNA viruses have linear genomes. Among –ssRNA viruses, those of the phylum Negarnaviricota have linear genomes, and those of the realm Ribozyviria have circular genomes.
=== Genome size ===
The size, or length, of a genome varies by Baltimore group. dsDNA viruses have genomes ranging from about 5 to 2,500 kilobases (kb) in length, ssDNA viruses 1–25 kb, dsRNA viruses 4–30 kb, +ssRNA viruses 3.5–40 kb, –ssRNA viruses 1.7–20 kb, ssRNA-RT viruses 5–13 kb, and dsDNA-RT viruses 3–10 kb in length. The relatively small genomes of viruses that are not group I are likely due to physical limitations. For example, ssDNA has the potential to form extensive secondary structures, and ssRNA is relatively chemically unstable. dsDNA viruses have much more varied genome sizes, likely because they have the same genomic organization as cells. This enables them to either exploit cellular machinery or encode their own machinery. As such, dsDNA seems to be the only genomic organization that can support genomes that exceed about 50 kb.
=== Host range === Different Baltimore groups tend to be found within different branches of cellular life. The vast majority of dsDNA viruses infect prokaryotes, but they also infect protists, animals, and rarely fungi. ssDNA viruses infect bacteria and most eukaryotes but are rare in archaea. dsRNA viruses infect plants, protists, and animals, are rare in bacteria, but are not found in archaea. +ssRNA viruses are found in all eukaryotes, infect many bacteria, but do not infect archaea. –ssRNA viruses infect animals and plants, are rare in fungi, but are not found in prokaryotes. ssRNA-RT viruses infect all eukaryotes but do not infect prokaryotes. Lastly, dsDNA-RT viruses infect animals and plants but not prokaryotes. Whether dsDNA-RT viruses infect protists is unknown. By host, a large majority of prokaryotic viruses are dsDNA viruses, but a significant minority are ssDNA and +ssRNA viruses. There are a relatively small number of prokaryotic dsRNA viruses and no prokaryotic –ssRNA or RT viruses. Eukaryotic viruses, in contrast, are predominantly RNA viruses, though eukaryotic DNA viruses are common. Well-characterized eukaryotic viromes contain mostly +ssRNA viruses and, in some lineages such as fungi, dsRNA viruses. ssRNA-RT viruses are also common in eukaryotes, especially in animals. Biological factors influence host range. For example, dsDNA viruses do not infect plants because large dsDNA molecules are unable to pass through plasmodesmata, intercellular channels that connect plant cells. The dominance of DNA viruses in prokaryotes may be because they outcompete RNA viruses. In eukaryotic cells, however, the nucleus is a barrier that requires adaptation by DNA viruses. They either have to evolve means to enter the nucleus to replicate or obtain their own replication and transcription machinery to replicate in virus factories in the cytosol. In contrast, the endomembranes of eukaryotic cells appear to be a beneficial environment for RNA virus replication.
=== Packaging of replication and transcription machinery ===
Viruses often package into the virion machineries necessary for replication and transcription, varying by Baltimore group. dsDNA viruses sometimes package transcription machinery, ssDNA and +ssRNA viruses do not package either replication or transcription machinery, dsRNA and +ssRNA-RT viruses package both, –ssRNA viruses package almost everything, and dsDNA-RT viruses package most components of their replication and transcription machinery. dsDNA viruses encode a broad range of proteins involved in replication and transcription. In some cases, they encode nearly complete systems that grant the virus partial autonomy from cells during genome expression and replication. Most ssDNA viruses encode an endonuclease that initiates RCR or RHR while relying on host cell machinery for the rest of replication and transcription. The endonuclease has to be encoded by these viruses because they use a DNA replication method not normally used by cells. Anelloviruses and bidnaviruses are the exceptions: anelloviruses encode proteins that aren't homologous to known proteins, and bidnaviruses encode a protein-primed DNA polymerase. RNA replication and reverse transcription are usually discouraged by cells, which necessitates that all RNA and RT viruses encode their own RNA-dependent polymerase. Satellite viruses, such as the viruses of Ribozyviria, are the only exception because they depend on other viruses for replication. Almost all RNA and RT viruses incorporate their RNA-dependent polymerase into the virion because the enzyme is required to synthesize viral mRNA in infected cells. The exceptions are +ssRNA viruses and caulimoviruses, which are dsDNA-RT viruses. +ssRNA viruses do not do so because their genomes function as mRNA and are translated upon cell entry. For caulimoviruses, the host enzyme RNA polymerase II transcribes the genome.
=== Translation ===
Translation is the process by which proteins are synthesized from mRNA by ribosomes. Baltimore groups do not directly pertain to the translation of viral mRNA to proteins, but atypical types of translation used by viruses are usually found within specific Baltimore groups. For example: