Viruses are infectious intracellular obligate parasites with subcellular level of organisation and without protoplasm, cell, cell organells and the molecular machineries for energy metabolism and protein synthesis,Grouped on the basis of size and shape, chemical composition and structure of the genome and mode of replication most viruses have a specific shape that is determined by the capsomeres or the envelope. Capsid symmetry - Three types (1) HELICAL CAPSID (2) ICOSAHEDRALCAPSID (3) COMPLEX CAPSID Capsid - large macromolecular structures. Made up of proteins called capsomers. Chemical unit of capsomers are polypeptide chain. Capsid - surrounded by lipoprotein layer called envelop Envelop is made up of proteins and glycoproteins Presence of lipid -envelope seems flexible and loose. Envelope is composed of both the host viral components. projections on the envelope known as spikes/peplomers which are arranged into distinct units.

1. CLASSIFICATION OF
VIRUS
Hanna Lawrence Pereira
1st MSc Marine Microbiology

2. • Viruses are infectious intracellular obligate parasites with
subcellular level of organisation and without protoplasm,
cell, cell organells and the molecular machineries for
energy metabolism and protein synthesis.
• They are extremely simple, ultramicroscopic organisms,
formed of only viral proteins and DNA or RNA.

3. • Most viruses are too small when compared with bacteria
• Ranges from 20 nm to 300 nm
• Parvovirus is 20 nm - smallest viruse
• Pox viruse is 300 nm - largest viruse
• Size of viruse is measured by using ultrafiltration,
ultracentrifugation, electron microscopy
SIZE OF VIRUSES

4. STRUCTURE OF VIRUSES
• The complete assembly of the infectious particle is known
as virion.
• Virion constructed around a nucleocapsid core –
composed of nucleic acid
• Held within a protein coat called Capsid - protect viral
genetic material and aids in its transfer between host
cells.

5. • Capsid - large macromolecular structures.
• Made up of proteins called capsomers.
• Chemical unit of capsomers are polypeptide chain.
• Capsid - surrounded by lipoprotein layer called envelop
• Envelop is made up of proteins and glycoproteins
• Presence of lipid -envelope seems flexible and loose.
• Envelope is composed of both the host viral components.
• projections on the envelope known as
spikes/peplomers which are arranged into distinct units.

6. • Having an envelope - Enveloped viruses
• Lack envelope – naked viruses

8. • GENOME virus contains either DNA or RNA as genetic
material but not both.
• DNA as genetic material are called DNA virus
• RNA as genetic material are called RNA virus.
• A viral genome may consists of linear or circular ds DNA,
single stranded DNA, ss linear RNA or ds linear RNA.
Examples; Reo virus is a RNA –ds RNA genome.
*Parvovirus - ss DNA
*Papovavirus - ds circular DNA as genetic materials.

9. • Grouped on the basis of size and shape, chemical
composition and structure of the genome and mode
of replication
• most viruses have a specific shape that is determined
by the capsomeres or the envelope.
• Capsid symmetry - Three types
(1) HELICAL CAPSID
(2) ICOSAHEDRALCAPSID
(3) COMPLEX CAPSID

10. • HELICAL CAPSIDS
• The capsomere and nucleic acid are wined together to
form helical or spiral tube like structure
• Most of the helical viruses are enveloped and all are RNA
viruses.
• The size of helical capsid influenced by both its
capsomers and the nucleic acid enclosed within the
capsid. The diameter of the capsid is a function of the
size, shape, and interactions of the capsomers.
• The nucleic acid - determine helical capsid length

12. • TOBACCO MOSAIC VIRUS
• Rigid, rod shaped and helical RNA viruse.
• About 300 nm long and 15-18 nm in diameter.
• Formed of RNA core, encased by a hollow and cylindrical
capsid of proteins.
• Only a single RNA molecule which is single strandes, helically
coiled, and spirally twisted.
• The capsid is formed of about 2130 identical protein sub units,
called capsomers, which assemble around the viral ssRNA to form
a helical structure, with a hollow central cavity of 4 nm diameter.

14. • ICOSAHEDRAL CAPSID
Regular polygon with 20 equilateral triangular faces
and 12 vertices. Eg- Adenoviruses, Herpes virus

15. • Adenoviruses
• Medium-sized (90–100 nm)
• Nonenveloped viruses with an icosahedral nucleocapsid -
double stranded DNA genome
• Resemble a space vehicle
• Capsid made up of 252 structural capsomers
• Of the 252 capsomers, 240 have Six neighbours and are called
hexons
• 12 capsomer at the vertices have 5 neighbours and are called pentons

16. • Each penton unit –
consist of a penton
base anchored in the
capsid –a projection
or fibre consist of rod
like portion with a
knob attached at the
distal end –virions has
the appearance of
space vehicle

17. • They are able to be transported through the endosome
• The virion has a unique "spike" or fiber associated with
each penton base of the capsid.
• That aids in attachment to the host cell via the receptor
on the surface of the host cell.
• simple virus and is heavily reliant on the host cell for
survival and replication

18. • HERPES VIRUS
• belong to the Herpesviridae family
• Herpesviruses have a unique four-layered structure:
• core containing the large, double-stranded DNA genome is enclosed
by an icosapentahedral capsid which is composed of capsomers
• Capsid consists of 162 capsomers - is surrounded by an
amorphous protein coat called the tegument.
• It is encased in a glycoprotein-bearing lipid bilayer envelope.

20. • COMPLEX CAPSID
• A complex capsid is a combination of helical and
icosahedral shapes
• Eg. Bacteriophages and large pox viruse

21. • BACTERIOPHAGE
• Tadpole-shaped
• The bacteriophage consists of a polyhedral head
• Short collar and a helical tail.
• Head- consists of 2000 capsomeres
• With double stranded DNA enclosed within.
• Tail- consists of an inner hollow tube which is surrounded by a contractile
sheath with 24 annular rings.

23. • RHABIDOVIRUSE
• They have complex bacilliform or bullet-shaped.
• The virions are about 75 nm wide and 180 nm
long.
• Genome of virus –unsegmented Linear –ve sense
RNA.
• Five proteins—the nucleoprotein (N), polymerase-
associated phosphoprotein (P), matrix protein (M),
glycoprotein (G) and RNA-dependent RNA polymerase (L)
• Two major structural components: a helical
ribonucleoprotein core (RNP) and a surrounding
envelope.

25. • REFERENCE
• Prescott Harley and kleins microbiology 7 th edition –
• Joanne M. Willey
• Linda M. Sherwood
• Christopher J. Woolverton
• Microbiology, phycology, mycology, lichenology and plant pathology
• K. K Bhaskaran
• A. R Ajith Kumar Ramesh