DNA viruses have genomes made of deoxyribonucleic acid (DNA) and are organized into two groups: double-stranded DNA (dsDNA) viruses, and single-stranded DNA (ssDNA) viruses. , dsRNA is not a molecule made by cells, so cellular life has evolved antiviral systems to detect and inactivate viral dsRNA. mRNA, which is positive sense, is transcribed directly from the negative sense genome. Cann, A. Lostroh, P. (2019). Furthermore, in addition to alternative splicing, because cellular unspliced RNA cannot be transported out of the nucleus, hepadnaviruses and retroviruses contain their own proteins for exporting their unspliced genomic RNA out of the nucleus. RHR is similar to RCR but each end of the linear genome contains an inverted terminal repeat in a hairpin loop structure. , Translation is the process by which proteins are synthesized from mRNA by ribosomes. For +ssRNA viruses, the genome functions as mRNA, so no transcription is required for translation. This acts as both a transcription and a replication process since the replicated RNA is also mRNA.  dsDNA viruses are also commonly divided between tailed dsDNA viruses, referring to members of the realm Duplodnaviria, usually the tailed bacteriophages of the order Caudovirales, and tailless or non-tailed dsDNA viruses of the realm Varidnaviria..  Because of the utility of Baltimore classification, it has come to be used alongside standard virus taxonomy, which is based on evolutionary relationships and governed by the International Committee on Taxonomy of Viruses (ICTV).  Negarnaviricota is divided into two subphyla: Haploviricotina, whose members synthesize a cap structure on viral mRNA required for protein synthesis, and Polyploviricotina, whose members instead obtain caps on mRNA via cap snatching.. Retrieved 6 August 2020. , ssRNA-RT viruses are all included in the class Revtraviricetes, phylum Arterviricota, kingdom Pararnavirae of the realm Riboviria.  Viruses in the ssDNA plant virus family Geminiviridae likewise vary between being monopartite and bipartite.  Instead of replicating both strands at once, some 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 prior synthesized strand, forming a dsDNA genome. Replication progresses in a loop around the genome by means of extending the 3'-end of the positive strand, displacing the prior positive strand, and the endonuclease cleaves the positive strand again to create a standalone genome that is ligated into a circular loop. , Because the process of replicating the viral genome produces intermediate dsRNA molecules, +ssRNA viruses can be targeted by the host cell's immune system. , From the 1990s to the 2010s, virus taxonomy used a 5-rank system ranging from order to species with Baltimore classification used in conjunction. Most eukaryotic viruses, including most human, animal, and plant viruses, are RNA viruses, although eukaryotic DNA viruses are also common. The 5'-end may be naked, capped, or covalently bound to a viral protein, and the 3'-end may be naked or polyadenylated. By organizing viruses based on their manner of mRNA production, it is possible to study viruses that behave similarly as a distinct group. デビッド・ボルティモア（David Baltimore、1938年3月7日 - ）はアメリカ合衆国の分子生物学者。1975年度ノーベル生理学医学賞受賞者の1人である。現在カリフォルニア工科大学教授で、1997年から2006年まで学長を務めた。またアメリカ科学振興協会副会長を務める。 Molecular and Cellular Biology of Viruses. The family Anelloviridae is the only ssDNA family whose members have negative sense genomes, which are circular.  A rolling circle mechanism that produces linear strands while progressing in a loop around the circular genome is also used that likewise replicates both strands simultaneously. There are seven Baltimore groups numbered with Roman numerals, listed hereafter. For certain viruses, including the families Orthomyxoviridae and Papillomaviridae, alternative splicing acts as a way to regulate early and late gene expression during different stages of infection. The sixth Baltimore group contains viruses that have a (positive-sense) single-stranded RNA genome that has a DNA intermediate ((+)ssRNA-RT) in its replication cycle. While not the direct focus of Baltimore classification, groups are organized in such a manner that viruses in each group also typically have the same mechanisms of replicating the viral genome. David Baltimore (New York, 7 marzo 1938) è un biologo statunitense, amministratore universitario e vincitore del Premio Nobel. Baltimore classification was proposed in 1971 by virologist David Baltimore in a paper titled Expression of Animal Virus Genomes.  The second manner is similar but instead of synthesizing a cap, RdRp may make use of cap snatching, whereby a short sequence of host cell mRNA is taken and used as the 5' cap of the viral mRNA. The dsDNA genome is then cleaved in two, and the hairpin loops at both ends of both strands are formed. The first Baltimore group contains viruses that have a double-stranded DNA (dsDNA) genome.  As part of this, the Baltimore groups for RNA viruses and RT viruses were incorporated into formal taxa.  Lastly, bidnaviruses package both the positive and negative linear strands. In 2018 and 2019, Baltimore classification was partially integrated into virus taxonomy based on evidence that certain groups were descended from common ancestors.  Additionally, some -ssRNA viruses are ambisense, as both the positive and negative strands separately encode viral proteins, and these viruses produce two separate mRNA strands: one directly from the genome and one from a complementary strand. David Baltimore, (born March 7, 1938, New York, New York, U.S.), American virologist who shared the Nobel Prize for Physiology or Medicine in 1975 with Howard M. Temin and Renato Dulbecco. Third, the RNA polymerase terminates transcription upon reaching a specific signal, such as a polyadenylation site. Nonsegmented -ssRNA viruses replicate in the cytoplasm, and segmented -ssRNA viruses replicate in the nucleus.  All -ssRNA viruses are classified in the phylum Negarnaviricota in the kingdom Orthornavirae in the realm Riboviria. In 2018, the realm Riboviria was established and initially included the three RNA virus groups. Monopartite viruses are found in all Baltimore groups, whereas multipartite viruses are usually RNA viruses. sgRNA transcription may occur by commencing RNA synthesis within the genome rather than from the 5'-end, by stopping RNA synthesis at specific sequences in the genome, or by, as a part of both prior methods, synthesizing leader sequences from the viral RNA that are then attached to sgRNA strands. Alternative splicing is a mechanism by which different proteins can be produced from a single gene by means of using alternative splicing sites to produce different mRNAs. Garland Science. , Various -ssRNA viruses use special mechanisms for transcription. Furthermore, the three Baltimore groups for RNA viruses are used as defining characteristics of the phyla in Orthornavirae. ISBN 978-0128112571. The following taxa that are unassigned to a realm exclusively contain dsDNA viruses: Viral initiation of translation: used primarily by +ssRNA and ssRNA-RT viruses, various viruses have evolved mechanisms to initiate translation, such as having internal ribosomal entry sites to allow for cap-independent translation, having downstream hairpin loops that allow for cap-dependent translation in the absence of an, Termination-reinitiation: used by some dsRNA and +ssRNA viruses, ribosomes may translate an ORF, but following termination of translation of that ORF, a proportion of. pp. After replication, the dsDNA genome may be packed or sent to the nucleus for further rounds of transcription.  More specifically, the vast majority of dsDNA viruses infect prokaryotes, ssDNA viruses are found in all three domains of life, dsRNA and +ssRNA viruses are primarily found in eukaryotes but also in bacteria, and -ssRNA and reverse transcribing viruses are only found in eukaryotes.
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