VIRUSES
Five kingdom classification of RH Whittaker has not
classified viruses, viroids and lichens in any group.
Lichens are already described earlier in this chapter.
The viruses did not find a place in any classification system
since they are not truly living, as living organisms are those
that have a cellular structure. The viruses are non-cellular
organisms that are characterised by having an inert
crystalline structure outside the living cell. An inert virus
outside the cell is called virion. These are obligate parasites.
They do not have a biosynthetic machinery. Once they infect
a living host cell, they take over the machinery of the host cell
to replicate themselves, killing the host.
Discoveries Related to Virus
(i) The name virus was given by Pasteur (virus means
venom or poisonous fluid). DJ Ivanowsky (1892)
recognised certain microbes as causative organism of
the mosaic disease of tobacco.
(ii) MW Beijerinck (1898) demonstrated that the
extract of the infected plants of tobacco could cause
infection in healthy plants. He called the fluid as
contagium vivum fluidum (infectious living fluid).
(iii) WM Stanley (1935) showed that viruses could be
crystallised and crystals consist largely of proteins.
Virus is a ultramicroscopic nucleoprotein entity. Size of
virus ranges from about 10 nm (foot and mouth virus of
cattle), 17 nm (alfalfa mosaic virus) to 1250 x 40 nm
(beet yellow virus), T MV (Tobacco Mosaic Virus) is
300 x 15 nm, 400 nm (parrot fever virus), x 15 nm
( Pseudomonas).
The architecture of viruses is mainly of following types
(i) Spherical, e.g., Alfalfa mosaic virus,
(ii) Cubical, e.g., Vaccinia
(iii) Helical (elongate body) e.g., T M V.
(iv) Polyhedral (short, broad body, rounded, polyhedral
shape), e.g., Poliomyelitis virus.
(v) Binal (cuboidal and helical parts both), e.g., many
bacteriophages like T 2 and T

Structure
A virus structure has mainly four parts
i. Envelope
It is the outer covering present in certain viruses. It is made
of protein Of viral origin, lipid and carbohydrate of host.
Spikes or outgrowths may or may not be present. Some
common enveloped viruses are HIV, Herpes virus, Vaccinia
virus.
ii. Capsid
It is a protein covering around the genetic material. Capsid
has protein subunits called capsomeres. T MV has
2130 capsomeres. The capsomeres are arranged helically or
in geometric forms.
iii. Nucleoid
It contains genetic material which is either DNA or RNA
but never both. Most of the animal viruses have
double-stranded DNA as genetic material, while majority
of plant viruses have single-stranded RNA as genetic
material. Some animal viruses may even have single or
double-stranded RNA.
iv. Enzymes
They are rarely present. Lysozyme is found in
bacteriophages. In some RNA viruses called retroviruses,
RNA polymerase, reverse transcriptase enzymes are present.
Classification
Based on the presence of DNA or RNA, viruses are divided
in two main groups
i. Deoxyvira or DNA viruses
These are generally animal viruses with a few important
ones having RNA. e.g., rabies virus, polio virus, retroviruses
including HIV or AIDS virus. The structural form of
deoxyvira are deoxyhelica, deoxycubica and deoxybinala.
ii. Ribovira or RNA viruses
These are generally plant viruses with a few containing
DNA. e.g., cauliflower mosaic virus. The structural form of
ribovira are ribohelica and ribocubica.

Types of Viruses
Viruses are classified by Holmes (1948) into three groups
based on their host types
i. Plants Viruses (Phytophagineae)
These viruses cause disease in plants. Examples of plant
viruses are Tobacco Mosaic Virus (T M V), Potato Mosaic
Virus (P M V), tomato leaf curl virus, etc.
ii. Animal Viruses (Zoophagineae)
These viruses cause disease in humans.
e.g., influenza virus, smallpox virus, poliomyelitis virus,
hepatitis virus, mumps virus, rhino viruses.
iii. Bacterial Viruses (Phagineae)
These are also known as bacteriophages and mainly infect
lower organisms (phagineae bacteria). Bacteriophages were
discovered by Edward Twort (1915) and d’ Herelle (1917)
independently.
For gxample, T 2 , T 4 , lambda, coliphages (bacteriophag«
of E.coli), cyanophages (blue-green algal viruses,
e.g., LPP-I, SM-I, N-1), phycophages (algal viruses),
mycophages (fungal viruses), zymophages (mycophages of
yeast).
The shape of bacteriophages is tadpole-like consisting of
head (icosahadral) and a tail. Head has a protein cover
capsid. Internally, the head encloses a highly folded
double-stranded DNA (approx. 50 in length).
A connector is present in between the head and the
The connector contains a neck and a collar. Collar possesses
several whiskers. The tail is the narrow cylindrical part.
has central hollow core or tube through which viral DNA’
injected into the host.
The core contains enzymes like lysozyme, etc. and
(formed
surrounded by
sheath proteins
2000 capsomeres). The tail ends in a basal plate or e
plate. The basal plate includes tail pins and tail
(spikes) that are generally involved in attachment to the
host cell.
Viral Reproduction
Viruses multiply after entering into living cells. The two
types of viral reproduction are lytic and lysogenic cycle.