Viruses are obligate intracellular parasites that lack cell organelles and exist in two phases - an extracellular phase where they possess few enzymes, and an intracellular phase where they induce host cells to synthesize viral components. They have a nucleocapsid containing either DNA or RNA surrounded by a protein capsid. Viruses replicate through either lytic or lysogenic cycles, with the lytic cycle involving adsorption to and penetration of host cells, synthesis of viral components, assembly of new virus particles, and cell lysis. Viruses can be grown in tissue culture or through plaque assays using bacterial hosts.

1. VIRUSVIRUS

2. Few definitions
• Virus- obligatory parasite, lacking cell organelles
• Virology- study of viruses
• Virologist- the one who studies viruses
Size – 10-400nm
Exception- pox virus- can be seen under light
microscope

3. General Properties
• Simple, lacking cell organelles
• Presence of either DNA or RNA
• Obligatory in nature (can not reproduce without
host)
• Exist in two phases-extracellular and intracellular
• Extracellular phase- possess few enzymes
• Intracellular phase- has replicating nucleic acid,
induce host metabolism to synthesize viral
components

4. Structure

5. • Nucleocapsid- Either DNA or RNA within protein
coat called capsid
• Capsid has many copies of one or a few types of
proteins
• These proteins are called protomers

6. Symmetry of virus

7. Helical symmetry
• Like hollow tubes with protein walls(rigid)
• Eg TMV(tobacco mosaic virus)
• Protomers or capsomers in spiral arrangement
• So becomes a tube like structure(15-18nm in
diameter by 300nm long)
• An RNA as genome, wound in spiral and lies within
groove formed by the protein subunits

9. Icosahedral symmetry
• Regular polyhedron
• 20 equilateral
triangular faces and
12 vertices
• nature’s favorite
shape
• Efficient way to close
a space
• Capsomers are ring
or knob like

10. Each capsomer is
made of five or six
protomers (pentamers
or hexamers)
Pentamers are at
vertices
Hexamers are at edges
and triangular faces

12. Complex virus-eg.pox virus &
bacteriphage

13. Largest of the animal
virus
400x240x200 nm
Complex
Ovoid to brick shaped
exterior
ds DNA with protein in
nucleoid(biconcave disk
like structure and two
elliptical and lateral
bodies lie between the
nucleoid and its outer
envelope which is a thick
layer covered by an array
of tubules or fibres

15. Viral envelopes
• An outer membrane
• Made up of lipid and carbohydrate of host
• Viral genes form peplopmes (spikes) on the
surface, important in virus attachment to the host
• Different in different viruses so can be used for the
identification
• Flexible so show variable shapes (also called
pleomorphic)

16. Viral envelopes and Enzymes

17. Viral envelopes and Enzymes
• Neuraminidase from spike in influenza virus. Can
release mature virion from the host cell
• Other spike is called haemagglutinin useful for
attachment
• These peplomers are glycoproteins
• M or matrix protein is non glycosylated protein
present on the inner surface of the envelope and
helps stabilize it
• RNA dependent RNA polymerase

18. Viral Genome
• ds DNA, ss DNA (double stranded and single
stranded)
• ds RNA, ss RNA
• Plus(+) strand- when RNA identical to viral mRNA
• Minus(-) strand- when RNA complementary to viral
mRNA
• Segmented genome-more than one RNA strands

19. Structure of phage T4

21. VIRUS REPRODUCTION
LYTIC CYCLE

24. Steps in reproduction
Adsorption and penetration-
• Bacteria have receptors for phages
• Eg. LPS, proteins, techoic acid, flagella, pili
• T even phages use CW and proteins as the
receptors
• Attachment starts when the base plates settles
down on the receptor firmly and tail fibrils
facilitate the attachment
• The tail sheath gets rearranged and becomes
shorter and wider
• The central tube or core is pushed through the CW
• The plate has lysozyme like activity

25. Steps in reproduction-contd.
• So helps in penetration
• Finally the linear DNA is extruded from the head,
through the tail tube and into the host cell through
a pore

27. Steps in reproduction
Synthesis of phage nucleic acid and proteins-
• Within 2 minutes after injection of T4 DNA into a
host E.coli starts synthesizing T4 mRNA
• Now there is synthesis of viral constituents
Assembly of phage particles-
Three proteins are required-
• Phage structural proteins
• Proteins that help in the assembly
• Proteins that are involved in the cell lysis and
phage release

30. Steps in reproduction
Release of phage particles-
Two proteins are required- T4 lysozyme, attacks
bacterial cell wall
and holin which creates holes in plasma membrane
Once 150 viral particles have accumulated inside the
host, lysis takes place

31. VIRUS REPRODUCTION
LYSOGENIC CYCLE by
temperate phages

33. Few definitions
• Lysogeny-relationship between a temperate phage
and its host
• Prophage- the form of phage in the host
• Lysogen or lysogenic bacteria-infected bacteria
• Lysogenic conversion- Corynebacterium
diphtheriae infected by beta phage and becomes
pathgenic

34. Cultivation of virus
(Chorioallantoic
membrane)

37. Tissue Culture

39. Cultivation of viruses in tissue culture
• Monolayer tissue culture
• Growth medium for cells in the tissue culture
• Antibiotic added to prevent bacterial growth
• Kept in a specialized plate
• Viruses inoculated in to layer of cells
• The cells are covered with a thin layer of agar
to limit virion growth

40. Detection of viral growth in tissue
culture
1. Rounding of cell
2. Detachment of cells
3. Formation of giant cells

41. Cytopathic effect (CPE)

43. Multinucleate giant cell(measles virus)

44. Growing Bacteriophages In The Laboratory
Cellular destruction and lysis called plaques

46. Cultivation of bacterial viruses
• The plaque method:
• Virus, bacteria, and agar mixed, plated and incubated.
• After replication the virus lyses the bacteria, forming
plaques, or clear zones.
• Each plaque is assumed to come from a single viral
particle.
• The titer (concentration of the stock solution) of the
virus is given in plaque forming units.