E 1.3 Describe the difficulties in the classification of viruses

Revision – what is a virus? Very small obligate intracellular parasites Contain either an RNA or DNA genome surrounded by a protective protein coat May be viewed as mobile genetic elements Most probably of cellular origin Characterised by a long co-evolution of virus and host For reproduction viruses rely on host cells to supply the complex requirements A complete virus particle is called a virion

Structure and function The main function of the virion is to deliver its RNA or DNA genome into the host cell so that it can expressed by the host cell The viral genome, often with associated basic proteins, is packaged inside a symmetric protein capsid The nucleic acid associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid In enveloped viruses the nucleocapsid is surrounded by a lipid bilayer and studded with an outer layer of virus envelope glycoproteins

Virology Virology is the study of viruses – sub microscopic, parasitic particles of genetic material contained in a protein coat Virology focuses on virus structure, classification and evolution, modes of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them and their use in research and therapy Virology is considered to be a subfield of microbiology or medicine

Virus structure and classification Viruses can be classified according to the host cell they infect Animal virus Plant virus Fungal virus Bacteriophage (infecting bacteria) Viruses can be classified according to the geometrical shape of their capsid Helix icosahedron Viruses can be classified according to their structure Presence or absence of a lipid envelope

Virus structure and classification The most widely used system distinguishes viruses according to the type of nucleic acid they use as genetic material and the viral replication method they use: DNA viruses (double or single stranded) RNA viruses (positive sense single; negative sense single and double stranded) Reverse transcribing viruses (double stranded and single stranded including retroviruses) The latest report by the International Committee on Taxonomy of Viruses lists 5450 viruses organised in over 2000 species, 287 genera, 73 families and 3 orders

Virologists Virologists also study subviral particles which are infectious entities that are smaller and simpler than viruses These include: Viroids (naked circular RNA molecules infecting plants) Satellites (nucleic acid molecules with or without a capsid that require a helper virus for infection and reproduction) Prions (proteins that can exist in a pathological conformation that induces other prion molecules to assume the same formation)

Origin of viruses Three hypotheses exist: 1. Viruses arose from non-living matter, separately from, but in parallel to cells 2. Viruses arose by genomic reduction form earlier more competent cellular life forms that became parasites to host cells and subsequently lost most of their functionality 3. Viruses arose form mobile genetic elements of cells (such as transposons, retrotransposons or plasmids) that became encapsulated in protein capsids, developed the ability to break free from the host cell and infect other cells

Alive or not? They DO reproduce and evolve They DO NOT engage in metabolism They DO NOT move They need a host cell for reproduction BUT if they are alive or not doesn’t really matter! The question does not affect the biological reality of viruses This is a matter of definition – if we change the definition of ‘live’ it wont change what viruses are, or what they do 

Viral pathogenesis Viruses cause many important infections diseases Influenza, rabies, common cold, measles, hepatitis, polio, smallpox Some viruses (oncoviruses) contribute to the development of certain forms of cancer As in the association between human papillomavirus and cervical cancer Hepatitis B and C are associated with liver cancer Some subviral particles also cause disease Prions cause the transmissible spongiform encephalopathies (mad cow disease) Hepatitis D is due to a satellite virus

Immunity

Why would a virus kill its host?

Molecular biology research and viral therapy