classification of virus

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 –
Nucleocapsid composed of nucleic acid
• Nucleic acid Held within a protein coat called Capsid -
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
• Due to the 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. • Eg 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 (non envelope ), Herpes
virus(enveloped)

15. • Eg - 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. • Eg - 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. • Eg - - 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.capsid
• 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