This Power point presentation intends to explore a general structure and classification of virus, a biologist puzzle.

1. Viruses Discovery, Physiochemical and Biological
Characteristics; Classification (Baltimore)
By
N. Sannigrahi, Associate Professor,
Deptt. Of Botany,
Nistarini College, Purulia (W.B) India

2. VIRUS DISCOVERY
 WHAT IS VIRUS?
 A virus is an infectious agent that can only replicate within a
host organism. Viruses can infect a variety of living organisms,
including bacteria, plants, and animals. Viruses are so small
that a microscope is necessary to visualize them, and they have
a very simple structure. When a virus particle is independent
from its host, it consists of a viral genome, or genetic material,
contained within a protein shell called a capsid. In some
viruses, the protein shell is enclosed in a membrane called an
envelope. Viral genomes are very diverse, since they can be
DNA or RNA, single- or double-stranded, linear or circular,
and vary in length and in the number of DNA or RNA
molecules.

3. VIRUS IMAGE

4. VIRUS-AT A GLANCE
 The viral replication process begins when a virus infects its
host by attaching to the host cell and penetrating the cell wall
or membrane. The virus's genome is uncoated from the protein
and injected into the host cell. Then the viral genome hijacks
the host cell's machinery, forcing it to replicate the viral
genome and produce viral proteins to make new capsids. Next,
the viral particles are assembled into new viruses. The new
viruses burst out of the host cell during a process called lysis,
which kills the host cell. Some viruses take a portion of the
host's membrane during the lysis process to form an envelope
around the capsid.
 Following viral replication, the new viruses may go on to infect
new hosts. Many viruses cause diseases in humans, such as
influenza, chicken pox, AIDS, the common cold, and rabies.

5. GENERAL CHARACTERS
 Viruses are ultramicroscopic, acellular, infectious,
nucleoproteinous, obligate parasites which can cross the
bacterial filter.
 The extracellular forms of viruses are inert particles and are
known as virions. Sometimes the terms ‘viruses’ and ‘virions’
are interchangeably used.
 On the basis of the host cell they infect, viruses are called as
zoophages (animal viruses) phytophages (plant viruses),
phycophages (algal viruses), mycophages (fungal viruses),
zymophages (yeast viruses), bacteriophages (bacterial viruses),
cyanophages (infecting cyanobacteria), coliphages (infecting E.
coli bacteria).

6. BACTERIOPHAGE

7. GENERAL CHARACTERS
 1.Viruses are acellular and can easily be crystallized.
 2. They are ultramicroscopic and filterable (pass through
bacteria proof filter).3. Viruses are obligate intracellular
parasites and infect all the major biological groups-animal,
plants and bacteria. 4.They are host specific i.e. a specific virus
infects a specific host.
 5. Each virus particle or virion is nucleoprotein in nature.
 6. Nucleic acid is either DNA or RNA but not the both as in
any cell.7. They use enzymes and other machinery of host cell
for replication and protein synthesis.
 8. They are resistant to chemicals, alcohols and environmental
changes.
 9. They are easily transmitted from infected host to healthy one
through vectors like insects, nematodes etc.

8. CHEMISTRY OF VIRUS
 Most of the virus consists of two major components- Capsid &
Nucleic acid along with presence of a third component
covering the Capsid. But if the three components present, is
called virion.
 CAPSID: Protein coat or shell encloses the nucleic acid
 Capsid contains similar protein units called Capsomere,
 Each Capsomere is an association of few protein molecules,
 Proteins are identical in structure and the sequence of amino
acids forming short chain of polypeptide,
 Nature of amino acids and the nature of the polypeptide formed
out of them differ from virus to virus,
 Capsid physiologically inert –protect the viral genome from
adverse effect and helps in recognizing the host and attachment
to the host surface and penetration

9. INFLUENZA VIRUS

10. CHEMISTRY OF VIRUS
 Besides capsomeres, few enzyme proteins, mucolytic enzyme,
neuramindase, lysozyme etc help in penetration.
 NUCLEIC ACID
The nucleic acid have normal composition containing ribose or
deoxy ribose sugar along with the desire Nitrogen bases as far
as the nucleic acid is concerned,
Most of the plant virus contain RNA except few and most of the
animal virus are DNA except few,
The strandedness of the nucleic acid differ from virus to virus;
RNA and DNA found either in single stranded condition in
some whereas some contain ds DNA & ds RNA ,
The nucleic acid occupies a definite place in the protein coat and
partially attached to it through salt linkages, hydrogen bonds
and other linkages.

11. VIRUS STRUCTURE

12. ENVELOPE
 External to Capsid, lipoprotein layer with embedded proteins in
the phospholipids bilayers,
 Spike like structure composed of glycoprotein found in the
envelope help in hemagglutination activities,
 Flexible envelope enabled no fixed shape and soze-
pleomorphic in nature,
 Envelope sometimes arise from nuclear or plasma membrane
of the host cell,
 Proteins components of envelope viral in origin or mixed origin
with host proteins,
 Enzymes like RNAase, reverse transcriptase in retrovirus,
protein kinase in herpes virus, DNA dependent RNA
polymerase in poxviruses are found in envelope. Virus may be
enveloped or naked virus

13. MORPHOLOGY OF VIRUS
 The arrangement of capsomeres units forming capsid gives the
morphological shapes of virus. Crick & Watson ( 1956) put
forth a hypothesis that the shell’s assembly is based on the
prini9ciple of minimization of energy so that the sub units
would fall into place spontaneously to form a stable structure.
Goodheart proposed three symmetry-Cubical, Helical and
Complex symmetry. According to Mathews, all plant viruses
can be divided under five major categories-Rod shaped,
Icosahedral, Alfalfa mosaic virus, Viruses with ds DNA and
viruses with membranes.
 Cubical ( Icosahedral) Symmetry: 20 sides polyhedron, each
face with equilateral triangular shape, 25-30 nm in small virus
& 70-100 nm in large virus, TYMV (Turnip Yellow mosaic),
BBMV ( Broad Bean Motile), Herpes, Adenovirus, Polyoma,
Polio, many bacteriophages etc ---

14. MORPHOLOGY OF VIRUS
 HELICAL : Mostly elongated virus, more in length(100-1250
nm) and less in diameter(10-25 nm),
 Screw like arrangement in capsomeres giving helical
appearance,
 Helical arrangement occurs around in imaginary rotational
axis,
 Helical capsid may be naked as in TMV or surrounded by
envelope as in mumps virus & Influenza virus,
 Capsomeres curve into helix because of their thickness at one
end than the other end.
 COMPLEX SYMMETRY: Exhibit complex or uncertain
symmetry , combination of both helical and cubical symmetry,
 Divided into two groups-combination of helical & cubical but
other with identifiable symmetry

15. DIFFERENT SYMMETRY

16. MORPHOLOGY OF VIRUS
 The mixed symmetry found in bacteriophage T4 whose head
has an icosaahedral structure with the rod like tail containing a
hollow tube surrounded by helical contractile protein sheath.
 However, the shape and assembly of the capsomeres that
occurs in the host cell is a repeated gene controlled process by
viral genome.
 EVOLUTION AND ORIGIN :
 One possible hypothesis, called devolution or the regressive
hypothesis, proposes to explain the origin of viruses by
suggesting that viruses evolved from free-living cells.
However, many components of how this process might have
occurred are a mystery. A second hypothesis (called escapist or
the progressive hypothesis) accounts for viruses having either
an RNA or a DNA genome.

17. EVOLUTION AND ORIGIN
 and suggests that viruses originated from RNA and DNA
molecules that escaped from a host cell A third hypothesis
posits a system of self-replication similar to that of other self-
replicating molecules, probably evolving alongside the cells
they rely on as hosts; studies of some plant pathogens support
this hypothesis.
 As technology advances, scientists may develop and refine
further hypotheses to explain the origin of viruses. The
emerging field called virus molecular systematics attempts to
do just that through comparisons of sequenced genetic material.
These researchers hope to one day better understand the origin
of viruses, a discovery that could lead to advances in the
treatments for the ailments they produce.

18. CLASSIFICATION OF VIRUS
 Virus classification is the process of naming viruses and
placing them into a taxonomic system. Much like the
classification systems used for cellular organisms, virus
classification is the subject of ongoing debate and proposals.
This is mainly due to the pseudo-living nature of viruses,
which is to say they are non-living particles with some
chemical characteristics similar to those of life. As such, they
do not fit neatly into the established biological classification
system in place for cellular organisms.
 Baltimore classification (first defined in 1971) is a
classification system that places viruses into one of seven
groups depending on a combination of their nucleic acid (DNA
or RNA), strandedness (single-stranded or double-stranded),
Sense, and method of replication. Named after David

19. BALTIMORE CLASSIFICATION
 Baltimore, a Nobel Prize-winning biologist, these groups are
designated by Roman numerals and discriminate viruses
depending on their mode of replication and genome type. Other
classifications are determined by the disease caused by the
virus or its morphology, neither of which are satisfactory due to
different viruses either causing the same disease or looking
very similar. In addition, viral structures are often difficult to
determine under the microscope.
 Classifying viruses according to their genome means that those
in a given category will all behave in a similar fashion, offering
some indication of how to proceed with further research.
Viruses can be placed in one of the seven following groups:

20. BALTIMORE CLASSIFICATION
 I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses,
Poxviruses),replicate in the nucleus
 II: ssDNA viruses (+)sense DNA (e.g. Parvoviruses)
 III: dsRNA viruses (e.g. Reoviruses), segmented genome
 IV: (+)ssRNA viruses (+)sense RNA (e.g. Picornaviruses,
Togaviruses), complex transcription
 V: (−)ssRNA viruses (−)sense RNA (e.g. Orthomyxoviruses,
Rhabdoviruses), Possess a virion particle RNA dependent RNA
polymerase
 VI: ssRNA-RT viruses (+)sense RNA with DNA intermediate
in life-cycle (e.g. Retroviruses), segmented
 VII: dsDNA-RT viruses (e.g. Hepadnaviruses), Non-segmented

21. BALTIMORE CLASSIFICATION

22. CONCLUSION
 In addition to the above virus general structure, some other
agents virus like are also exists in nature. Virusoids – small
circular RNAs which are similar viroids linked with larger
molecules of the viral RNA, Prions-infectious agents different
from viruses and viroids mainly protenaceous infectious
particle responsible for causing disease –Kuru are some of the
agents deserve mentioning in this regard. Rhizobiophages are
kind of virus that reduce the rhizobial population in soil and
negatively affect the nitrogen fixing abilities of those bacteria
with the host legume plants.
 Virus still is a biologist puzzle and the recent pandemic of
COVID19 due to virus, Sars2cov2 has again drawn the
attention of the researchers along with mass destruction
potentiality and to sort out preventive measures in this regard.

23. THANKS FOR YOUR PLEASURE
 ACKNOWLEDGEMENT
 Google for different images
 Different websites for information and contents,
 A text book of Microbiology- Dubey & Maheswari
 Microbiology & Phycology – Mishra & Dash
 Brock Microbiology of Microorganisms
 A textbook of Microbiology- Chakraborty
 Disclaimer: The presenter acknowledges the different persons
for using images in this content. This is being prepared as open
source without any financial interest .