A brief account of viruses is given here.

1. - Keshav Narayan Pai
General Characteristics of Virus

2.  The term ‘virus’ is derived from Latin which
means poison or venom.
 In 1886, Adolf Meyer observed viruses in
Tobacco plants and the viruses were TMV
( Tobacco mosaic virus).
 Virus is an ultra microscopic, infectious agent
that are metabolically inert and which multiply
only within the living cells or host.

3. General Characters
 Viruses are metabolically inert.
 Viruses are obligate intracellular parasites.
 Viruses cannot make energy or proteins independent
of a host cell.
 Viruses multiply inside the living cells using host cell
machinery.

4. VIRUS AS A LIVING THING
 Virus occur in different strains.
 Virus have their own genetic material.
 Virus can undergo Mutation.
 They get destroyed by Ultra violet Rays.

5.  They lack a cellular structure.
 They do not respire.
 They do not move, grow.
 They can be crystallised and stored in bottles
like chemicals.
 They lack enzyme system and donot have
metabolic activity of their own.

6.  Viruses are not able to survive without a
host cell, so active viruses reside inside a
host body.
 They are present either in bacterial cell,
animal cell or plant cell.
 Example: HerpesVirus,TMV,
Bacteriophages.

7.  Viruses are much smaller than Bacteria.
 Viral Size ranges from 20 – 1400nm.
 Poliovirus is of 30nm.
 Giant mimi viruses are upto 800 nm.

9. Different shapes of Viruses
• Viruses are of
different shapes.
• They are rod shaped,
bullet shaped,
filament shaped,
icosahedral in shape
and tadpole shaped.

10. Viral Structure
 Virus consists of nucleic acid and a protein.
 Genome or the nucleic acid is covered by
protein coat called capsid.
 Some viruses have envelope outside the
capsid.
 Capsid without the envelope is called Naked.

11. VIRUS STRUCTURE
Capsid
Viral core
Envelope

12.  It consists of either deoxyribonucleic
acid(DNA) or ribonucleic acid ( RNA).
 It may be single stranded form or
double stranded.
 Either circular or linear.

13. Viral Capsid
 Viral nucleic acids surrounded by Protein
coat called Capsid.
 Viral capsid is of 3 types.
 Helical
 Icosahedral
 Complex.
Capsid proteins
DNA or RN
inside tu

14.  Filamentous Capsid.
 Long tube of protein
With genome inside.
example: Influenza virus,
Rabies virus.

15. Rabies virus Influenza Vi

16. Cubic or Icosahedral Symmetry
 Made up of Icosahedral Capsid.
 It has 20 triangular sides.
Example: HIV, Hepatitis B Virus,
AdenoVirus.

17. Hepatitis – B Virus

18. STRUCTURE OF HIV

19. COMPLEX SYMMETRY
 The head of the virus is
cubic and tail is helical.
Example: Pox viruses,
Bacteriophage.

20. REPLICATION
 Viral Replication includes 2 types.
 Lytic cycle
 Lysogenic cycle.
Lytic cycle includes 5 steps,
1. Attachment
2. Penetration
3. Replication of Nucleic acid
4. Maturation
5. Release of new virions.

22.  Virusareinfectiousparticleswhichreplicatesonlyinside
thehost.
 Itattacks plants,animalsandbacteriacells.
 Theyreplicationexibitinlyticandlysogeniccycles.
 Theyexhibitbothlivingandnon-livingcharacters.
 Theycanbecalledas“Organismattheedgeofthelife”.

23. Reference
N.J.Dimmock, A.J.Eastan and K.N.Leppard.
2007.
Introduction to Modern Virology, Vol 6th : 1- 15
Alan.J.Cann.2005. Principles of Modern
Virology,
4thedition: 1-7
John.B.Carter and Ventic .A.Saunders.2007.

24. THANKYOU