Madical brain Notes 10

  What are viruses? All viruses are obligate intracellular parasites; they inhabit a no-man’s-land between the living and the non-living worlds, and possess characteristics of both. They are now known to differ radically from the simplest true organisms, bacteria, in a number of respects: ·                they cannot be observed using a light microscope ·                they have no internal cellular structure they contain either DNA or RNA, but not both* ·                they are incapable of replication unless occupying an appropriate living host cell ·                they are incapable of metabolism ·                individuals show no...

  Viral structure The demonstration by Wendel Stanley in 1935 that a preparation of tobacco mosaic virus could be crystallised was an indication of the relative chemical homogeneity of viruses, and meant that they could not be thought of in the same terms as other living things. Compared to even the most primitive cellular organism, viruses have a very simple structure (Figure 10.1). An intact viral particle, or  virion , has in essence just two components: a core of nucleic acid, surrounded and protected by a protein coat or capsid , the combination of the two being known as the  nucleocapsid . In certain virustypes, the nucleocapsid is further surrounded by a membranous  envelope , partly derived from host cell material. Most viruses are smaller than even the smallest bacterial cells; Figure 10.2 shows the size of some viruses compared to that of typical bacterial and eucaryotic cells. The viral genome The genetic material of a virus may be either RNA or DNA, and e...

  Classification of viruses As we saw, viruses are not considered to be strictly living, and their classification is a complex issue. As with true organisms we have species, genera, families and orders of viruses, but none of the higher groupings (class, phylum, kingdom). Latin binomials (e.g.  Homo sapiens, Escherichia coli ), familiar from conventional biological taxonomy, are not used for viruses; however, a proposal for non-Latinised viral bi-nomials has been proposed. Originally, no attempt was made to draw up any sort of phylogenetic relationship between the viruses, but more recent developments in sequencing of viral genomes has meant that insights are being gained in this area. Factors taken into account in the classification of viruses include:   ·                host range (vertebrate/invertebrate, plant, al-gae/fungi, bacteria) ·           ...

  Viral replication cycles One characteristic viruses share in common with true living organisms is the need to reproduce themselves ∗  . As we have seen, all viruses are obligate intracellular parasites, and so in order to replicate, a host cell must be successfully entered. It is the host cell that provides much of the ‘machinery’ necessary for viral replication. All viral growth cycles follow the same general sequence of events (Figure 10.7), with some differences from one type to another, determined by viral structure and the nature of the host cell. Replication cycles in bacteriophages Viruses that infect bacterial cells are called  bacteriophages  (phages for short), which means, literally, ‘bacteria eaters’. Perhaps the best understood of all viral replication cycles are those of a class of bacteriophages which infect  E. coli , known as the  T-evenphages . These are large, complex viruses, with a characteristic head and tail structure(Figure 10.8). ...

  Replication cycles in bacteriophages Viruses that infect bacterial cells are called  bacteriophages  (phages for short), which means, literally, ‘bacteria eaters’. Perhaps the best understood of all viral replication cycles are those of a class of bacteriophages which infect  E. coli , known as the  T-evenphages . These are large, complex viruses, with a characteristic head and tail structure(Figure 10.8). The double-stranded, linear DNA genome contains over 100 genes, and is contained within the icosahedral head. The growth cycle is said to be  lytic , because itculminates in the lysis ( = bursting) of the host cell. Figure 10.9 shows the lytic cycle of phage T4, and the main stages are described below.   1. Adsorption  ( attachment ): T4 attaches by means ofspecific tail fibre proteins to complementary recep-tors on the host cell’s surface. The nature of these receptors is one of the main factors in determining a virus’s host specificity. ...