An icosahedral capsid is 20-sided, while there are also helical viruses like Ebola and the tobacco mosaic virus. Polyhedral viruses include rhinovirus and poliovirus subtypes. Bacteriophages have tail fibers, a polyhedral head, along with tail pins that attach the virus to the host cell. Poxviruses are shaped like bricks and have unique surface features. Viruses are also classified according to the viral genome they have. There can be positively stranded or negatively stranded RNA viruses, DNA viruses, and further distinction based on whether or not the virus is double-stranded or single-stranded.
VIRAL LIFE CYCLE As mentioned, viruses do not have an adequate genome to sustain themselves unless they infect a host cell and capture the host cell’s reproductive capacities. DNA viruses in most eukaryotic cells will need to be present in the nucleus for replication, while bacteriophages replicate in the cytoplasm. Large poxviruses are DNA viruses that can replicate in the cytoplasm. RNA viruses replicate primarily in the cytoplasm. Bacteriophage life cycles have been studied extensively. There are virulent phages that automatically kill the cell and temperate phages that become part of the host genome. These lead to latent infections that ultimately get activated to make progeny viruses or virions that have been newly assembled. Virulent phages basically take over the cell, reproduce to make new phages, and destroy the cell. There are five stages associated with the bacteriophage lytic cycle. The first stage is attachment, which involves association of the bacterial surface receptors with the virus particle. The second stage is penetration, with injection of the nucleic acids into the host. The virus itself remains outside. Then comes biosynthesis, which is replication of the viral proteins. After this is maturation, in which new virus particles are assembled. Lysis only happens with virulent phages that kill the cell. In what’s called the lysogenic cycle, the phage genome participates in attachment and penetration, just as with the lytic cycle. The difference is that the phage genome instead is integrated into the host genome and does not immediately kill the cell. In such cases, the integrated genome is referred to as the prophage and the bacterial host along with the prophage is called a lysogen. The entire process is known as lysogeny.
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