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- 2. Viruses are the smallest known infective agents and perhaps the simplest form of life. Viruses do not posses cellular organization and they do not fall strictly in to the category of unicellular microorganism.
- 3. They do not have cellular organization. They contain only one type of nucleic acid either DNA or RNA but never both. They are obligate intracellular parasites. They lack the enzymes necessary for protein and nucleic acid synthesis and are dependent for replication on the machinery of host cells. They multiply by a complex process and not by binary fission. They are unaffected by antibacterial antibiotics.
- 5. Extracellular infectious viral particle is called ‘Virion’ Viruses are much smaller than bacteria For a time, they were known as ‘filterable agents’ as they can pass through filters that can hold back bacteria. They can not be seen under light microscope hence called as ‘ultramicroscopic’. The virus particles seen in this manner are known as ‘elementary bodies’. Size range: 20-300 nm Parvovirus: 20 nm (smallest virus) Pox virus: 300 nm (biggest virus and can be seen under light microscope)
- 6. Passing them through collodion membrane Electron microscopy Sedimentation in the ultracentrifuge Comparatative measurements Shape of the Virus The overall shape of the virus particle varies in different groups of viruses. Most of the animal virus are roughly spherical, irregular and pleomorphic. Eg. Pox virus are brick shaped, rabies virus are bullet shaped, tobacco mosiac virus is rod shaped
- 8. A. Viral Capsid Viruses consists of nucleic acid core surrounded by a protein called capsid. Capsid is composed of large number of capsomer which is made up of polypeptide molecules. The capsid with the enclosed nucleic acid is known as nucleocapsid.
- 9. It protects the viral genome from physical destruction and enzymatic inactivation by nucleases in biological material. It provides the binding site which enable the virus to attach to specific site on the host cell. It facilitates the assembly and packaging of viral genetic informataion. It serves as a vehicle of transmission from host to another. It is antigenic and specific for each viruses It provides the structural symmetry to the virus particle.
- 10. Viral architecture can be grouped into three types based on the arrangement of morphological subunits. 1. Icosahedral Symmetry An icosahedral (icosa, meaning 20 in greek) is a polgon with 12 vertices or corners and 20 facets or sides. Each facet is in the shape of an equilateral triangle. Pentagonal capsomers at the vertices (pentons) and hexagonal capsomers making up the facets (hexons) Eg. Adeno viruses
- 11. 2. Helical Symmetry The nucleic acid and capsomers are wound together in the form of helix or spiral. Eg. Influenza virus, parainfluenz virus, rabies virus. 3. Complex Symmetry Viruses which don not show either icoshedral or helical symmetry due to complexity of their structure are referred to have complex symmetry. Eg. Pox viruses
- 12. Virions may be enveloped and nonenveloped (naked). 1. Enveloped Virus The envelop or outer covering of virus containing lipid is derived from the plasma membrane of the host cell during the release by budding from the cell surface. The envelop is glycoprotein in nature. Enveloped viruses are susceptible to the action of lipid solvent such as ether, chloroform and detergent. Eg. Herpes virus, Hepatitis B virus, HIV virus
- 13. 2. Non enveloped virus Viruses which does not have outer covering. Naked viruses are more likely to be resistant to lipid solvents like ether, chloroform and detergent. Peplomers In mature virus particle, the glycoproteins often appear as projecting spikes on the outer surface of the envelop which are known as peplomers. A virus may have more than one type of peplomers. E.g the influenza virus carries two types of peplomers, the hemagglutinin and neuraminidase
- 14. It helps for attachment of virus to the host cell receptors to initiate the entrance of the virion into the cell. It attach to receptors on red blood cells, causing these cell to agglutinate. It has enzymatic activity like neuraminidase which cleave neuraminic acid from host cell glycoproteins. It has antigenic properties.
- 16. Viruses contain a single kind of nucleic acid either DNA or RNA which encodes the genetic information necessary for replication of the virus. The genome may be single stranded or double stranded, circular or linear, segmented or nonsegmented. According to nucleic acid present, viruses can be classified in to DNA viruses and RNA viruses.