Virus PDF

VIRUS Virus are the simplest form of life known, they do not possess a cellular organization. Virus are obligate intracellular parasites, they contain only one type of nucleic acid, either DNA or RNA but never both, they lack the enzymes necessary for protein and nucleic acid synthesis and are depen...

VIRUS Virus are the simplest form of life known, they do not possess a cellular organization. Virus are obligate intracellular parasites, they contain only one type of nucleic acid, either DNA or RNA but never both, they lack the enzymes necessary for protein and nucleic acid synthesis and are dependent for replication on the synthetic machinery of host cells and they are unaffected by antibacterial antibiotics. MORPHOLOGY OF VIRUSES Size: Viruses are much smaller than bacteria. The extracellular infectious virus particle is called the virion. They were for a time known as ‘filtera¬ble viruses’ due to their ability to pass through filters that can hold back bacteria as a result their small size and that led to their recognition as a separate class of infectious agents. They were also called ‘ultramicro¬scopic’ as they were too small to be seen under the light microscope. Some of the larger viruses, such as poxviruses can be seen under the light microscope when suitably stained. The virus particles seen in this manner are known as ‘elementary bodies’. The unit for measurement of virion size is nanometers (nm). Viruses vary widely in size from 20 nm to 400 nm. The largest among them is pox virus (300 nm) and is as large as the smallest bacteria. The smallest viruses are the parvovirus and are nearly as small as the largest protein molecules such as hemocyanin MEASURING THE SIZE OF VIRUSES 1. Passing them through Collodion Membrane The earliest method of estimating the size of virus particles was by passing them through collodion membrane filters of graded porosity (gradocol membranes). 2. Electron Microscopy Electron microscopy is the most widely used method for estimating particle size. 3. Sedimentation in the Ultracentrifuge The virus size could be calculated from the rate of sedimentation of virus in the ultracentrifuge with the development of ultracentrifuge. 4. Comparative Measurements It can be done with reference to: Staphylococcus, bacterial viruses (bacteriophages) and representative protein molecules. SHAPE OF VIRUS The overall shape of the virus particle varies in different groups of viruses. Most of the animal viruses are roughly spherical, some are irregular and pleomorphic. Poxviruses are brick-shaped, rabies virus is bullet-shaped, tobacco mosaic virus is rod-shaped. Bacteriophages have a complex morphology. The extracellular infectious virus particle is known as virion. STRUCTURE OF A VIRUS STRUCTURE AND CHEMICAL COMPOSITION OF THE VIRUSES A. Viral Capsid B. Virus Symmetry C. Viral Envelope D. Viral Nucleic Acids A. VIRAL CAPSID Viruses consist of nucleic acid core surrounded by a protein coat called capsid. The capsid with the enclosed nucleic acid is known as nucleocapsid. The capsid is composed of a large number of capsomers which form its morphological units. The chemical units of the capsid are polypeptide molecules which are arranged symmetrically to form molecules to form an impenetrable shell around the nucleic acid core. Functions of Capsid  i. Protection: It protects the viral genome from physical destruction and enzymatic inactivation by nucleases in biological material. ii. Binding sites: It provides binding sites which enable the virus to attach to specific receptor sites on the host cell. iii. It facilitates the assembly and packaging of viral genetic information. iv. Vehicle of transmission: It serves as a vehicle of transmission from one host to another. v. Host’s defense: It is of paramount importance in the host’s defense to virus infection. vi. It provides the structural symmetry to the virus particle. B. VIRUS SYMMETRY Viral architecture can be grouped into three types based on the arrangement of morphologic subunits: (1) Icosahedral symmetry (2) Helical symmetry (3) Complex structures. 1. Icosahedral Symmetry An icosahedral is a polygon with 12 vertices or corners and 20 facets or sides. Each facet is in the shape of an equilateral triangle. Two types of capsomers constitute the icosahedral capsid. They are the pentagonal capsomers at the vertices (pentons) and the hexagonal capsomers making up the facets (hexons). There are always 12 pentons but the number of hexons varies with the virus group, e.g., adenoviruses. 2. Helical Symmetry The nucleic acid and the capsomers are wound together in the form of a helix or spiral. Examples: Single-stranded RNA viruses such as influenza, the parainfluenza viruses, and rabies. 3. Complex Symmetry Viruses (e.g. poxviruses) which do not show either icosahedral or helical symmetry due to complexity of their structure are referred to have complex symmetry. C. VIRAL ENVELOPE Virions may be enveloped or nonenveloped (naked). Enveloped Virus The envelope or outer covering of virus containing lipid is derived from the plasma membrane of the host cell during their release by budding from the cell surface. The envelope is glycoprotein in nature. The lipid is largely of host cell origin while the protein is virus encoded. Enveloped viruses are susceptible to the action of lipid solvents such as ether, chloroform and detergents, whereas most viruses existing as naked capsids are more likely to be resistant to them. PEPLOMERS In mature virus particle, the glycoproteins often appear as projecting spikes on the outer surface of the envelope. These are known as peplomers (from peplos, meaning envelope). A virus may have more than one type of peplomers, e.g., the influenza virus carries two kinds of peplomers, the hemagglutinin which is a triangular spike and the neuraminidase which is a mushroom shaped structure. Envelope confer chemical, antigenic and biological properties on viruses. Functions of Peplomers i. Mediate attachment: Many peplomers mediate attachment of the virus to the host-cell receptors to initiate the entrance of the virion into the cell ii. ii. Attach to receptors: Some viral glycoproteins also attach to receptors on red blood cells, causing these cells to agglutinate (hemagglutination). iii. iii. Enzymatic acti¬vity: Other glycoproteins possess enzymatic activity like neuraminidase which cleave neuraminic acid from host cell glycoproteins. iv. iv. Major anti¬gens: Glycoproteins are also major antigens for protective immunity. D. VIRAL NUCLEIC ACIDS Viruses contain a single kind of nucleic acid which could either DNA or RNA that encodes the genetic information necessary for replication of the virus. The genome may be single-stranded or double-stranded, circular or linear and segmented or non-segmented. The type of nucleic acid, its strandedness, and its size are major characteristics used for classifying viruses into families