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MM 526-541

Table of Contents



1.    To familiarize you with the structural components of the virus, which can act as antigens during the infection process.

2.    To emphasize the unique nature of viral nucleic acid and its role in the infection process.

3.    To familiarize you with the morphological types of virus in order that this information can be used in making a diagnosis.

4.    To develop an understanding of the virus replication cycle in order to appreciate how the physician can interrupt this cycle.

Specific educational objectives (terms and concepts upon which you will be tested)

General Properties of Viruses


Modifications in viral nucleic acid
1. Nucleic acid -contains 3-400 genes 2. Capsid -The capsid accounts for most of the virion mass. It is the protein coat of the virus. It is a complex and highly
           organized entity which gives form to the virus. Subunits called protomeres aggregate to form capsomeres
            which in turn aggregate to form the capsid.

3. Envelope -this is an amorphous structure composed of lipid, protein and carbohydrate which lies to the outside of the capsid.
        It contains a mosaic of antigens from the host and the virus. A naked virus is one without an envelope.

4. Spikes. These are glycoprotein projections which have enzymatic and/or adsorption and/or hemagglutinating activity. They
        arise from the envelope and are highly antigenic.

Morphology (Symmetry)

1. Icosahedral -The protomeres aggregate in groups of five or six to form the capsomere. In electron micrographs,
        capsomeres are recognized as regularly spaced rings with a central hole. The shape and dimensions of the
        icosahedron depends on characteristics of its protomeres. All icosahedral capsids have 12 corners each occupied by a penton
        capsomere and 20 triangular faces, each containing the same number of hexon capsomeres.  Icosahedral symmetry is
        identical to cubic symmetry.

2. Helical -The protomeres are not grouped in capsomeres, but are bound to each other so as to form a ribbon-like structure.
        This structure folds into a helix because the protomeres are thicker at one end than at the other. The diameter of the helical
        capsid is determined by characteristics of its protomeres, while its length is determined by the length of the nucleic acid it


3. Complex -e.g., that exhibited by poxvirus and rhabdovirus. This group  comprises all those viruses which do not fit into either
        of  the above two  groups.


Replication Cycle

1.    Adsorption -Viruses can enter cells via phagocytosis, viropexis or adsorption.  Adsorption is the most common process and
        the most highly specific process.  It requires the interaction of a unique protein on the surface of the virus with a
       highly specific receptor site on the surface of the cell.

2.    Penetration -This occurs by one or more processes.

3.    Uncoating -During this stage cellular proteolytic enzymes digest the capsid away from the nucleic acid. This always occurs in
        the cytoplasm of the host cell. The period of the replication cycle between the end of the uncoating stage and maturation of
        new viral particles is termed the eclipse. Thus during the eclipse stage, no complete viral particles can be viewed within the

4.    Replication of nucleic acid. Replication of viral nucleic acid is a complex and variable process. The specific process depends
        on the nucleic acid type.

Replication of nucleic acid
5.    Maturation and Release


1.    Viruses contain either DNA or RNA as their genetic material, but not both. This nucleic acid usually has unique chemical
        and/or physical features which makes it distinguishable from human nucleic acid.

2.    Viral nucleic acid is enclosed in a capsid made up of protein subunits called protomeres.

3.    Some species of viruses have a membrane, the envelope, surrounding the capsid; other species do not have an envelope, i.e.,
        they are naked.  Enveloped viruses have glyco-protein spikes arising from their envelope.  These spikes have enzymatic,
        absorptive, hemagglutinating and/or antigenic activity.

4.    The morphology of a virus is determined by the arrangement of the protomeres. When protomeres aggregate into units of five or
        six (capsomeres) and then condense to form a geometric figure having 20 equal triangular faces and 12 apices, the virus is
        said to have icosahedral (cubic) morphology. When protomeres aggregate to form a capped tube, they are said to have helical
        morphology. Any other arrangement of the protomeres results in a complex morphology.

5.    All viruses undergo a replication cycle in their human host cell consisting of adsorption, penetration, uncoating, nucleic acid
        replication, maturation and release stages.

6.    During the viral replication cycle, an accumulation of mature viruses, incomplete viruses and viral parts occurs within the cell.
        This accumulation is the inclusion body. The size, shape, location and chemical properties of the inclusion body are used
        by the pathologist to diagnose viral infectious disease.

7.    A virally-infected cell generally presents three signals that it is infected. The first is the production of double-stranded RNA,
       which induces interferon; the second is the expression of viral protein on the surface of the plasma membrane, thus causing
       activation of cytotoxic T-cells, natural killer cells and sometimes induction of antibody synthesis. The third is the formation of an
        inclusion body either within the cytoplasm or the nucleus or very rarely within both the cytoplasm and nucleus.

8.    In general, all DNA-containing viruses replicate in the host cell nucleus. The exceptions to the rule are the poxviruses.

9.    In general, all RNA-containing viruses replicate in the host cell cytoplasm. The exceptions to the rule are the retroviruses and
        the orthomyxoviruses.

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