Chapter 21 Viral Diversity in Brock Biology of Microorganisms (13th edition)


Origin of Viruses

General Characteristics

Host Range

Viral Structure

Naming and Taxonomy

1. International Committee on Taxonomy of Viruses (ICTV) established in 1973 oversees this function. A number of guidelines are set down by the ICTV for naming and taxonomy of viruses. Please refer to these links if you want in order to obtain details. Below I have outlined the summary of these guidelnes.

2. Family suffix -VIRIDAE.

3.Viruses are never named after people but geographical locations are okay. For example, Bunyaviridae (Bunyamwere in Uganda)

4. Currently 2828 viruses belonging to 103 families, 22 subfamilies and 455 genera are known (2014).

5. Taxonomy is based on a variety of criteria:

Methods of cultivating viruses

Animal viruses can be cultured in a variety of ways.

Viral Multiplication

Viral multiplication can be divided into 5 phases. These phases are dependent on the structure and the type on nucleic acid present in the viron.
    (a) Attachment
    (b) Penetration
    (c) Uncoating
    (d) Biosynthesis
    (e) Maturation and release

The following provides an overview of these different phases citing examples of various common viruses.

(a) Adsorption and Attachment:
Attachment site of the viron specifically "connect" to a complimentory receptor sites of a host cell.

Receptor sites are an inherited trait and can vary from one individual to another. Therefore susceptibility to a viral infection (and indeed to other infections)  can vary from one person to another.

    Bacteriophage (eg lamda phage): A chance collision followed by weak chemical bonds forming between a specific appendage, end of tail fibres and receptors on the bacterial cell wall.

    Animal viruses: Attachement appendages are unlike lamda phage as these are
        distributed all over an animal viron and vary from viron to viron. The animal
        receptors are glycoproteins found on plasma membrane.
            Non-enveloped dsDNA icosahedral adenovirus: fibres at corners of
            Enveloped, 8 segmented linear ssRNA Influenza virus: Hemagglutinin spikes
                attach to sialic acid of RBC plasma membrane (cause
                hemagglutination, a hallmark of family Orthomyxoviridae (Influenza virus
                A, B, C).
            Enveloped retrovirus (dsRNA) Human Immunodeficiency Virus (HIV I & II):
                causes Acquired Immune Deficiencies (AIDS): Envelope contains
                glycoprotein spikes (gp120) which attach to CD4 receptors of T cells and
                macrophages (CD4 absent in most other cells)
(b) Penetration:

Viruses in Australia

1. All virues with the exception of members of the family Parvoviridae have genetic
    information for viral multiplication. Parvovirus require "helper" viruses to supply
    necessary components to produce more virons.

2. Viroids are much smaller than viruses, contain RNA only and this RNA does not
     code for proteins. Discovered in 1971.

3. A plant pathologist discovered the first viroid called Potato Tuber Spindle Viroid
        (PTLV), which causes the disease Potato Tuber Spindle Disease.

4. There are 6 ways in which viroids are different to viruses:
        Contain single circular RNA of low molecular weight (300 to 400 nucleotides).
        RNA is often internally paired (3D) and is therefore protected against cellular
        Contain no capsids or envelopes
        There is no requirement for helper viruses
        Viroids do not produce proteins
        RNA is always copied in the host cell nucleus
        Viroids acn only be identified by nucleotide sequencing techniques

5. Viroids have so far been shown to infect plants only. Viroids like plant viruses can
        cause serious plant diseases eg potatoe spindle disease.

6. Plant cells are protected from diseases by an impermeable cell wall. Viroids like
        plant viruses enter through wounds or via other parasites eg. plant sucking
        parasites, nematodes, fungi.  Cell death is perhaps due to RNA interference in
        cell metabolism.

7. Do viroids have an old precellular origin or are they a modern day example of a
        new and extreme example of parasitism. Recently, base sequence homology has
        been shown between viroids and introns (introns also do not code for
        polypeptides) which has led to the speculation that viroids evolved from introns

1. Term proposed by Stanley Prusiner. Proteinaceous infectious particles (should be
        pronounced "Proin" but "Prion" sounds better)

2. Prions cause 8 diseases in human and animal all of which are neurological
        disorders progressive (dementing illnesses):
            Creutzfeldt-Jacob Disease (CJD) - humans
            Kuru Disease (PNG) - spread by canabilism in humans. Spread has ceased
                due to lack of this practice.
            Scrapie Disease in sheep
            Bovine Spongiform Encephalopathy (BSE) or mad cows disease.
            A variant of Creutzfeldt-Jacob Disease (vCJD) in humans from eating tainted
                meat originating from cows suffering mad cows disease

3. Prions are infectious and infective proteins and not DNA, RNA or toxins. Why?
            Ressist inactivation by heat at 90oC.
            Insensitive to radiation  (damages DNA)
            Insensitive to nucleic acid digesting enzymes (RNAse and DNAse)
            Sensitive to protein denaturing agenst (phenol, urea)

4. There are 3 hypothesis for the infectious nature of prions:
       (a) PrP (Prion proteins) codes for normal proteins. PrP gene is located on
            chromosome 20 in humans. Prion proteins  are membrane proteins.
            Abnormal mutant prion proteins PrPSC fold incorrectly, and form fibres or
            fibrils which stick together in cells ie they are not organised in the plasma
            membrane properly killing the cells.
         (b) The prion protein might contain a small but so far undetectable nucleic acid
             that directs the synthesis of the abnormal PrPSC protein.

        (c) The infectious agent might be an unculturable or an unculture bacterium
             which can pass through filters. A Spiroplasma-like bacterium has been seen
             in brain tissues from CJD patients.

5. Transmission:
        (a) Largely unknown but it is thought that prion proteins cause normal proteins
            to fold incorrectly.
        (b) Spread to cows via scrapie infected sheep that was used to supplement cow
            feed in 1990.
        (c) Spread to humans by the consuming the milk of affected cows has been
            ruled out.
        (d) Transmitted to mice by injection of prion proteins and disease symptoms
         (e) Some of the prion diseases are spread only in families ie it is inheritable

Please direct comments, suggestions & errors to: Professor Bharat Patel

[Created 06 Sept 1995]
[Modified 07 April 2014]