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Introduction to Cells, DNA, and Viruses
Published in Patricia G. Melloy, Viruses and Society, 2023
Viruses are obligate intracellular parasites that can be found all over the natural world. Viruses are typically considered nonliving, consisting of some genetic material and protein, which may or may not be covered by a membrane. Viruses enter cells and use all the host cell’s machinery, like an unwanted houseguest, to make many copies of itself and eventually leave the cell. Many viruses known as pathogens cause disease and many factors govern the impact of the pathogen on a host cell. Viruses come in many different types, and can be classified in numerous ways, including based on their genome using a method known as the Baltimore classification system. Viruses may have a DNA or an RNA genome. Viruses can evolve, one of the reasons that they can be so difficult to stop if they are pathogens. Virologists study viruses, using tools like microscopy and other assays used by other cell and molecular biologists.
Viral Pathogens: A General Account
Published in Jagriti Narang, Manika Khanuja, Small Bite, Big Threat, 2020
Vinod Joshi, Bennet Angel, Annette Angel, Neelam Yadav, Jagriti Narang
According to the Baltimore classification of viruses, Togaviruses fall under Group IV, which consists of single-stranded (+ve) RNA viruses. The family Togaviridae includes two genera: Alphaviruses and Rubiviruses. Their genome is linear and non-segmented and approximately 10,000–12,000 nucleotides long. The virus has an envelope and a nucleocapsid. It is icosahedral in shape constituted by 240 monomers. Glycoprotein spikes are found throughout the surface, which help to attach efficiently to the host cell surface (Fig. 3.2).
Introduction to virus structure, classification, replication, and hosts
Published in Avindra Nath, Joseph R. Berger, Clinical Neurovirology, 2020
Philippe Simon, Kevin M. Coombs
Although the Baltimore classification scheme is based on the transcription strategies of viruses, the various classes usually can be distinguished by the manner in which the viral genomes are replicated. Class I viruses use their dsDNA genomes as a template to synthesize more dsDNA during genome replication. As in transcription, this replication is usually carried out by host enzymes. Class VII Hepadnaviridae are an exception to this generalization; replication takes place through an RNA intermediate that is longer than the genome. Class II viruses go through a replicative intermediate to copy their genomes. For example, (+)ssDNA viruses must make a (−)ssDNA copy to act as a template for more (+)ssDNA. In the case of class III viruses, the mRNA that was used for protein synthesis is copied by viral enzymes into a (−) sense RNA that remains associated with the mRNA template to form the progeny dsRNA. Class IV viruses have (+)ssRNA genomes. A (−)ssRNA intermediate is produced, which then serves as the template for more (+)ssRNA. Class V viruses use the same replication strategy as class IV viruses, except that they start with a (−)ssRNA genome and use a (+)ssRNA strand as an intermediate. In class VI, (+)ssRNA genomes are replicated by a unique mechanism. The (+)ssRNA is used as a template to synthesize (−)ssDNA, which in turn serves as a template for the synthesis of a (+)ssDNA strand. The resulting dsDNA molecule is transcribed into mRNA or used to synthesize progeny (+)ssRNA genomes.
Virus-like particle vaccines: immunology and formulation for clinical translation
Published in Expert Review of Vaccines, 2018
Braeden Donaldson, Zabeen Lateef, Greg F. Walker, Sarah L. Young, Vernon K. Ward
VLP possess a variety of shapes and structures, representative of the inherently vast diversity of the virus taxon. Examples of VLP can be identified within each of the seven groups defined by the Baltimore classification [17], including VLP derived from double-stranded DNA viruses such as Epstein-Barr virus [18], positive-sense RNA viruses such as Chikungunya virus [19], and negative-sense RNA viruses such as Human parainfluenza virus type 3 [20]. Variety can be observed in VLP size, ranging from MS2 bacteriophage VLP at around 27.5 nm in diameter [21], to HPV VLP at around 60 nm [22], and influenza VLP at around 100 nm [23,24]. VLP also vary in structural complexity, as illustrated in Figure 1, including mono-layer VLP such as HBV VLP formed from HBV surface antigen (HBsAg) or HBV core antigen (HBcAg) [25–27], and multi-layer VLP such as rotavirus VLP [28,29]. VLP can be encapsulated within a phospholipid bilayer envelope to resemble their parent virus, such as HIV [30] or Sendai virus VLP [31]. The envelope itself can also form the primary particle structure of some VLP, with recombinantly expressed virus envelope-stabilizing proteins embedded within the membrane, such as with IAV virosomes [32].