Viruses are microscopic organisms that exist almost everywhere on earth. They can infect animals, plants, fungi, and even bacteria.Viruses vary in complexity. They consist of genetic material, RNA or DNA, surrounded by a coat of protein, lipid (fat), or glycoprotein. Viruses cannot replicate without a host, so they are classified as parasitic.They are considered the most abundant biological entity on the planet. Here we discuss the general properties of viruses in detail.

1. GENERAL PROPERTIES
OF VIRUSES
Prasanna R Kovath
Assistant Professor
Department of Biotechnology
St. Mary's College Thrissur

2. • Virology is the study of viruses – submicroscopic, parasitic particles of
genetic material contained in a protein coat – and virus-like agents.
• It focuses on the following aspects of viruses: their structure, classification
and evolution, their ways to infect and exploit host cells for reproduction,
their interaction with host organism physiology and immunity, the diseases
they cause, the techniques to isolate and culture them, and their use in
research and therapy.
• Virology is a subfield of microbiology.

3. VIRUS HISTORY
• The history of viruses and the field of virology are broadly divided
into three phases,
• Discovery
• Early
• Modern

4. THE DISCOVERY PHASE (1886-1913)
• Adolf Mayer, a German scientist first observed the dark and light
spot on infected leaves of tobacco plant and named it tobacco mosaic
disease .
• He showed the infectious nature of the disease after inoculating the
juice extract of diseased plant to a healthy one.
• The next step was taken by a Russian scientist Dimitri Ivanovsky in
1890, who demonstrated that sap of the leaves infected with tobacco
mosaic disease retains its infectious property even after its filtration
through a Chamberland filter.
• Martinus Beijerinck showed that filterable agent form the infectious
sap could be diluted and further regains its strength after replicating in
the living host; he called it as “contagium vivum fluidum”.

5. Adolf Mayer Dimitri Ivanovsky
Martinus Beijerinck
Father of virology

6. The tobacco mosaic virus seen under 160,000× magnification
Progression of tobacco mosaic disease

7. • Loeffler and Frosch discovered the first animal virus, the foot and
mouth disease virus in 1898.
• Walter Reed and his team discovered the yellow fever virus, the first
human virus from Cuba in1901.
• Poliovirus was discovered by Landsteiner and Popper in 1909 and
two years later Rous discovered the solid tumor virus which he called
Rous sarcoma virus.

8. THE EARLY PHASE (1915-1955)
• Frederick W. Twort discovered the phenomenon of transformation
while working with the variants of vaccinia viruses
• Felix d’Herelle discovered bacteriophage
• Wendell Stanley (1935) first crystallized the TMV and the first
electron micrograph of the tobacco mosaic virus (TMV)
• In 1933 Shope described the first papillomavirus in rabbits.
• The vaccine against yellow fever was made in 1938 by Thieler
• Polio virus vaccine was made by Salk in 1954.

9. THE MODERN PHASE (1960-PRESENT)
• The superhelical nature of polyoma virus DNA was first described by
Weil and Vinograd
• Dulbecco and Vogt showed its closed circular nature
• Blumberg discovered the hepatitis B virus
• Temin and Baltimore discovered the retroviral reverse transcriptase
• The first human immunodeficiency virus (HIV) was reported in 1983
by Gallo and Montagnier
• The phenomenon of RNA splicing was discovered in Adenoviruses by
Roberts, Sharp, Chow and Broker

10. • In the year 2005 the complete genome sequence of 1918 influenza
virus was done
• In the same year hepatitis C virus was successfully propagated into the
tissue culture.
• Many discoveries are done using viruses as a model.

11. VIRUS DIVERSITY
• In 1966 when international committee on the taxonomy of viruses
(ICTV) was formed with the aim to classify the viruses. The ICTV has
adopted a norm for the description of the viruses.
• Name for genera, subfamilies, families, and orders must all be a single
word, ending with the suffixes -virus, -virinae, -viridae, and -virales
respectively. In written usage, the name should be capitalized and
italicized.
• Viruses are obligate parasite which means their absolute dependence
on living host system. This property of virus made it a valuable tool to
study cell functions and its biology.

12. NOMENCLATURE OF VIRUSES
• Cryptogram is a code adopted for describing a virus consisting four
pairs of symbols. It was proposed by Gibbs and Harrison (1968).
• 1st pair – Type of nucleic acid / number of strands in nucleic acid.
• 2nd pair – Represent molecular weight of nucleic acid in millions /
percentage of nucleic acid.
• 3rd pair – Shape of virion / shape of nucleocapsid.
• (S – represent spherical, E – Elongated with parallel sides, ends not
rounded; U – for elongated with parallel sides, end rounded and X- for
complex or none of the above).
• 4th pair-Type of host infected / nature of vector.
• (Symbols : A – actinomycetes; B – bacterium; F – fungus; I –
invertebrates; S – seed plants; V- vertebrate and so on).
• For example – Cryptogram of TMV: R/l: 2/5: E/E: S/A
• Polio virus: R/l: 2.5/30: S/S: V/O
• T4 bacteriophage: D/2:130/40: X/X: B/O

13. VIRUS SHAPES
• A major advancement in determining the morphology of virus was the
development of negative stain electron microscopy.
• Another modification of classical electron microscopy is cryo-
electron microscopy where the virus containing samples were rapidly
frozen and examined at a very low temperature; this allows us to
preserve the native structure of the viruses.

14. (A) Negative stain EM image of orthoreovirus particles, with a
stain-penetrated particle (arrow) showing the double capsid
layers; (B) Negative stain EM image of rotavirus particles.
Bars, 100 nm. (Figure B, courtesy of Charles D. Humphrey,
Centers for Disease Control and Prevention, Atlanta, GA,
USA.)
Cryo-electron microscope image of
a Bluetongue virus.
viral infection cattle in Europe

15. • A virion is an entire virus particle consisting of an outer protein shell
called a capsid and an inner core of nucleic acid (either ribonucleic or
deoxyribonucleic acid—RNA or DNA).
• Sometime structure without nucleic acid can be visible under the
electron microscope those structures are called as empty capsids.
• In some of the viruses like paramyxoviruses the nucleic acid is
surrounded by the capsid proteins and the composite structures are
referred as nucleocapsid.
• Some of the viruses contain the lipid envelope which surrounds the
nucleocapsids.
• The envelopes are derived from the host cell membrane during the
budding process.
• As the envelopes are derived from host cell membrane they contain
many of the surface proteins present in the host cells.

16. KINDS OF SYMMETRY
• There are two kinds of symmetry found among the viruses:
Icosahedral and Helical.
• Icosahedral symmetry may sometime referred as spherical based on
the external morphology.
• Icosahedral symmetry has 12 vertices, 30 edges, and 20 faces. They
also have two, three, or five fold symmetry based on the rotation
through axes passing through their edges, faces, and vertices
respectively .
• The viruses of this kind look spherical in shape.

17. • In Helical symmetry the genomic RNA forms a spiral within the core
of the nucleocapsids.
• The viruses of this kind look rodlike or filamentous.
• The viruses which contain large DNA genomes are more complex in
structure, for example- poxviruses and herpesviruses.

18. • .
Virus structure with helical symmetry
An icosahedral virion structure showing
two, three, and fivefold symmetry

19. FAMILY SHAPE
Poxviridae Pleomorphic
Herpesviridae Icosahedral
Adenoviridae Icosahedral
Retroviridae Spherical
Rhabdoviridae Bullet shaped
Coronaviridae Spherical
Filoviridae Filamentous

20. VIRUS SIZE
• Viruses are generally much smaller than the bacteria and its average
size varies from 25-300 nm in diameter.
• Smallest viruses belong to the families Circoviridae, Parvoviridae and
Picornaviridae which measure about 20 - 30 nm .
• The largest one belongs to Poxviridae that measures around 250-300
nm in diameter.
• Recently, scientists isolated a new form of virus that infects amoeba
and grouped it under a separate family Mimiviridae. The members of
the family Mimiviridae range from 400-800 nm in diameter.

21. FAMILY SIZE (NM)
Poxviridae 300
Herpesviridae 150
Retroviridae 80-100
Rhabdoviridae 100
Adenoviridae 80-100
Filoviridae 80
Coronaviridae 120-150

22. COMPONENTS OF GENOMES
• In general the viruses are made up of nucleic acids (genome), proteins
(capsid), and lipids (envelope).
• Viral genomes can be either DNA or RNA, when once inside a host
cell it directs synthesis of new viral proteins, and replication of new
viral genomes.
• Capsid is a protein covering that surrounds and protects the viral
genome.
• It is made up of many small subunits called as capsomeres which
determine the shape of the virus.

23. • Envelopes are the lipid bilayer membranes that are derived from the
host cell membrane when virus “buds” out from the plasma membrane
or passes through a membrane-bound organelle (such as the Golgi
body or endoplasmic reticulum).
• The envelope contains sometimes glycoprotein (protein with
carbohydrate) in the form of spikes which helps them in the
attachment during the time of infection to the host cell surface (gp120
in HIV).
• In non-enveloped viruses, grooves present in the capsid and specific
capsid proteins may bind to the cell surface receptor.

25. REPLICATION
• Different families of viruses have their genome made of either double
stranded (ds) DNA or single stranded (ss) DNA or RNA.
• Among the families of viruses that infect animals and human, those
containing RNA genome outnumber those containing DNA genome. This
disparity is even more in case of plant viruses (no double stranded DNA
virus that infect plant is known).
• Almost all viruses encodes unique proteins and enzymes, moreover they
follow unique pathways to transfer their genetic information.
• This phenomenon is more pronounced in case of RNA viruses, they
either use RNA dependent RNA polymerase or in case of retrovirus
(HIV) RNA dependent DNA polymerase to complete their replication
cycle.
• Both of these processes requires unique enzymes that are encoded by the
virus following infection to the host cells and are generally absent
elsewhere.

26. • Presence of many different subspecies of virus particle in a population
is also called as quasispecies nature of RNA viruses.
• The RNA dependent RNA polymerase and reverse transcriptase have
minimal proofreading ability, as a result their error rate is very high (1
in 10,000) as compared to the DNA replication.
• This means that an RNA virus particle will contain 1 or more mutation
from its parental wild type virus.
• The error prone activity of RNA virus polymerase restricts the upper
size limit of the genome above which they cannot survive.
• As a result of this phenomenon most of the RNA virus have their
genome size in the range of 5-15kb (coronavirus 30kb).

28. VIRAL ENZYMES
• Earlier, it was believed that viruses lack enzymes. But now, many
viruses are known to contain enzymes.
• DNA beta-glucosyltransferase
• Glucosyl-DNA beta-glucosyltransferase
• OBPgp279
• Lysin
• Integrase
• Reverse transcriptase
• Thymidine kinase from herpesvirus
• Viral neuraminidase
• Hemagglutinin esterase
• Integrase
• Lysin

29. • The spikes of enveloped viruses like influenza, measles and mumps
contain the enzyme neuraminidase which helps in penetrating the host
cell.
• In some cases the spikes also contain haemoglutinin that allow
clumping of RBCs and help in adsorption to specific host cell, e.g.,
polioviruses, adenoviruses, influenza, measles and mumps etc.
• The tips of bacteriophage tails contain enzyme lysozyme which
facilitates the penetration into host cell.
• In retroviruses like HIV, Rous Sarcoma Virus, an RNA-dependant
DNA polymerase called reverse transcriptase found associated with
the genome.
• This enzyme synthesizes DNA from viral RNA and the process is
called reverse transcription.

30. VIRAL TRANSMISSION
• The transmission of viruses from diseased to healthy host occurs
through various agencies or vectors.
• Plant viruses are Known to be transmitted through soil (e.g. wheat
mosaic viruses), seed (e.g., mosaics of beans, cowpea, lettuce etc.),
pollen (e.g., mosaics of beans), weeds (e.g., Cuscuta, a total parasite),
nematodes (e.g., tobacco rattle, Colocasia mosaic, early browning of
pea), fungi (e.g., tobacco necrosis, lettuce big veins etc.), by grafting a.
healthy scion to an infected stock or vice versa.
• Potato, oat and wheat mosaic viruses are usually transmitted by hands,
tools and other agricultural implements.

31. TRANSMISSION OF SOME ANIMAL VIRAL DISEASES
• Chicken pox is transmitted through close contact, vomits etc.
• Smallpox spreads through close contact, sputum, vomits, scales
• Polio is transmitted through sputum and faces. Cockroaches also act as
vector of polio virus.
• Influenza and cold spread through close contact and nasal discharge.
• Yellow fever is transmitted through cockroaches.
• Certain viral diseases, such as Rift valley fever, are hereditary.
• Equine encephalitis, yellow fever and dengue are mostly transmitted
through mosquito.
• Foot and mouth disease is mostly transmitted through the milk of
infected cattle. Birds may also act its vector.

32. VIRAL REPLICATION OR LIFE CYCLE
• A generalized viral replication cycle involves following steps:
• (a) Entry of viruses into host cells, either through breaches in cell wall
(in plant viruses) or by adsorption (animal and bacterial viruses).
• (b) Eclipse or biosynthesis phase which includes replication of viral
genome and synthesis of viral proteins.
• (c) Assembly of virions
• (d) Release of progeny virions

33. • Eclipse period : The time interval between viral infection (i.e. entry of
viral genome into the cell) and the appearance of the first intracellular
virus particle
• Latent period : The time taken between viral infection and the first
release of progeny viruses. For bacteriophages, it is 15-30 minutes and
for animal viruses it is 15-30 hours.
• Abortive infection : the viral infection does not produce any progeny
virions or if produced are not infectious.
• Defective, Incomplete or Satellite viruses : Some viruses are
genetically defective and, therefore, incapable of producing infectious
daughter virions without the assistance of helper virus

34. WHAT IS A PRION?
• "'Prion' is a term first used to describe the mysterious infectious agent
responsible for several neurodegenerative diseases found in mammals,
including Creutzfeldt-Jakob disease (CJD) in humans.
• The word itself derives from 'proteinaceous infectious particle'; it
refers to the initially heretical hypothesis that the infectious agent
causing those diseases consists only of protein, with no nucleic acid
genome. (All previously known pathogens, such as bacteria and
viruses, contain nucleic acids, which enable them to reproduce.)

35. Microscopic "holes" are characteristic in prion-affected tissue sections, causing the tissue to develop a
"spongy" architecture. This causes deterioration of the sponge-like tissue in the brain.

36. REFERENCE
• https://www.scientificamerican.com/article/what-is-a-prion-specifica/
• www.ncbi.nlm.nih.gov › books › NBK8174
• Biotechnology - General Virology – NPTEL nptel.ac.in › courses
• Ananthanarayan and Paniker's Textbook of Microbiology,7th edition,
Orient longman publications.
• Microbiology Pelczar,Tata McGraw-Hill Education.
• Prescott's Microbiology, Johanne M. Willey, Linda M. Sherwood,
Christopher J. Woolverton, McGraw Hill Education