This document provides information about different types of viruses and how they are studied. It discusses viral structure, classification, methods of entry and reproduction. Specific viruses mentioned include bacteriophages, retroviruses, herpes simplex virus, and tobacco mosaic virus. Methods used to cultivate and study viruses include infecting laboratory animals, embryonated eggs, and tissue culture. The lytic and lysogenic cycles of bacteriophages are described in detail.

1. By SANCHIT DHANKHAR

2. T4 phage
 M13 (General properties and structure,classification,reproduction)
 Retrovirus
 Herpes simplex virus (Classification, reproduction )
 TMV (Morphology, taxonomy, and reproduction)

2

3. WHAT ARE
VIRUSES?
A virus is a non-cellular
particle made up of
genetic material and
protein that can invade
living cells.
3

4. VIRAL HISTORY

5. Beijerinck (1897)
coined the Latin name
“virus” meaning poison
He studied filtered plant
juices & found they
caused healthy plants to
become sick
5

6. Wendell Stanley (1935)
crystallized sap from
sick tobacco plants
He discovered viruses
were made of nucleic
acid and protein
6

7. Edward Jenner (1796)
developed a smallpox
vaccine using milder
cowpox viruses
Deadly viruses are said to
be virulent
Smallpox has been
eradicated in the world
today
7

8. VIEWING VIRUSES
Viruses are smaller than
the smallest cell
Measured in nanometers
Viruses couldn’t be seen
until the electron
microscope was invented
in the 20th century
8

9. 9

10. VIRAL
STRUCTURE

11. Non living structures
Noncellular
Contain a protein coat called the capsid
Have a nucleic acid core containing DNA or
RNA
Capable of reproducing only when inside a
HOST cell
11

12. Some viruses are enclosed
in an protective envelope
Some viruses may have
spikes to help attach to the
host cell
Most viruses infect only
SPECIFIC host cells
12
CAPSID
ENVELOPE
DNA
SPIKES

13. Viral capsids (coats)
are made of
individual protein
subunits
Individual subunits
are called
capsomeres
13
CAPSOMERES

14. Outside of host cells, viruses
are inactive
Lack ribosomes and enzymes
needed for metabolism
Use the raw materials and
enzymes of the host cell to be
able to reproduce
14
EBOLA VIRUS
HIV VIRUS

15. capsid nucleic acid
lipid
envelope
surface
proteins
capsid
nucleic acid
lipid envelope
Surface proteins capsid
surface
proteins
nucleic acid
helical
(rabies)
polyhedral
(foot-and-mouth
disease)
enveloped
(influenza)
15

16. CHARACTERISTICS
•Some viruses cause disease
•Smallpox, measles,
mononucleosis, influenza,
colds, warts, AIDS, Ebola
•Some viruses may cause some
cancers like leukemia
•Virus-free cells are rare
16
MEASLES

17. •Viruses come in a variety of shapes
•Some may be helical shape like the
Ebola virus
•Some may be polyhedral shapes
like the influenza virus
•Others have more complex shapes
like bacteriophages
17

18. 18

19. 19
20-sided with 12 corners
Vary in the number of
capsomers
Each capsomer may be
made of 1 or several
proteins
Some are enveloped

20. 20

21. 21
 Viral entry into host cells occurs through one of the following
methods:
Endocytosis
Direct fusion
Nucleic acid translocation

22. 22
 All of the virus is engulfed
and enclosed in a vacuole

23. 23
 Host cell membrane fuses with the virus

24. 24
Non enveloped virus injecting its
nucleic acid to the host cell

25. Viruses are obligate intracellular parasites, which
means they can replicate only within a host cell
Each virus has a host range, a limited number of
host cells that it can infect
25

26. Once a viral genome has entered a cell, the cell
begins to manufacture viral proteins
The virus makes use of host enzymes, ribosomes,
tRNAs, amino acids, ATP, and other molecules
Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses
26

27. Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle
27

28. The lytic cycle is a phage replicative cycle that
culminates in the death of the host cell
The lytic cycle produces new phages and lyses
(breaks open) the host’s cell wall, releasing the
progeny viruses
A phage that reproduces only by the lytic cycle is
called a virulent phage
Bacteria have defenses against phages, including
restriction enzymes that recognize and cut up
certain phage DNA
28

29. Attachment
1
29

30. Attachment
2
1
Entry of phage
DNA and
degradation
of host DNA
30

31. Attachment
2
1
3
Entry of phage
DNA and
degradation
of host DNA
Synthesis of
viral genomes
and proteins
31

32. Attachment
2
1
4
3
Entry of phage
DNA and
degradation
of host DNA
Synthesis of
viral genomes
and proteins
Assembly
Phage assembly
Head Tail Tail
fibers
32

33. Attachment
2
1
5
4
3
Entry of phage
DNA and
degradation
of host DNA
Release
Synthesis of
viral genomes
and proteins
Assembly
Phage assembly
Head Tail Tail
fibers
33

34. The lysogenic cycle replicates the phage genome
without destroying the host
The viral DNA molecule is incorporated into the
host cell’s chromosome
This integrated viral DNA is known as a prophage
Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
34

35. An environmental signal can trigger the virus
genome to exit the bacterial chromosome and
switch to the lytic mode
Phages that use both the lytic and lysogenic cycles
are called temperate phages
35

36. lytic cycle
is induced
or
Phage DNA
circularizes.
Certain factors
determine whether
lysogenic cycle
is entered
Lysogenic cycle
Prophage
Daughter cell
with prophage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage. 36

37. New phage DNA and proteins
are synthesized and assembled
into phages.
The cell lyses, releasing phages.
Phage
Phage
DNA
The phage
injects its DNA.
Bacterial
chromosome
Lytic cycle
lytic cycle
is induced
or
Phage DNA
circularizes.
Certain factors
determine whether
lysogenic cycle
is entered
Lysogenic cycle
Prophage
Daughter cell
with prophage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
37

38. The lytic cycle of the bacteriophage: a lunar analogy
Lytic Cycle Lysogenic Cycle
The lytic cycle causes the host bacterium to undergo
cell lysis, that is, cell destruction.
The lysogenic cycle does not cause cell lysis or cell
destruction.
The lytic cycle can lead to production of 100 to 200
progeny phages.
The DNA of the phage gets integrated into the
bacterial chromosome and no progeny are produced
mostly.
Lytic cycle cannot be converted into the lysogenic
cycle.
Lysogenic cycle can be converted into lysogenic cycle
when the host cell is exposed to chemical or physical
agents.
38

39.  Generally three methods are employed for the virus
cultivation
1. Inoculation of virus into animals
2. Inoculation of virus into embryonated eggs
3. Tissue culture
39

40.  Laboratory animals play an essential role in studies of viral
pathogenesis
 Live animals such as monkeys, mice, rabbits, guinea pigs, ferrets
are widely used for cultivating virus
 Mice are the most widely employed animals in virology
40

41.  The different routes of inoculation in mice are:
 intracerebral
 subcutaneous
 intraperitoneal
 or intranasal
 After the animal is inoculated with the virus suspension, the animal
is:
 observed for signs of disease
 visible lesions
 or is killed so that infected tissues can be examined for virus
41

42.  Prior to the advent of cell culture, animal viruses could be propagated only on whole
animals or embryonated chicken eggs
 Cell cultures have replaced embryonated eggs as the preferred type of growth
medium for many viruses
 Cell culture consists of cells grown in culture media in the laboratory
 These cultures can be propagated and handled like bacterial cultures; they are more
convenient to work with than whole animals or embryonated eggs
 Primary cell lines
 Secondary cell lines
42

43.  Goodpasture and Burnet in 1931 first used the embryonated hen’s egg
for the cultivation of virus
 The process of cultivation of viruses in embryonated eggs depends on
the type of egg being used
 Eggs provide a suitable means for:
 the primary isolation and identification of viruses
 the maintenance of stock cultures
 and the production of vaccines
43

44.  An embryo is an early developmental stage of animals marked by rapid differentiation
of cells
 Birds undergo their embryonic period within the closed protective case of an egg,
which makes an incubating bird egg a nearly perfect system for viral propagation
 It is an intact and self-supporting unit, complete with its own sterile environment and
nourishment
 It furnishes several embryonic tissues that readily support viral multiplication
 Defense mechanisms are not involved in embryonated eggs
 Cost- much less, Maintenance-easier, Less labor and Readily available
44

45.  Chicken, duck, and turkey eggs are the most common choices for inoculation
 The egg used for cultivation must be sterile and the shell should be intact and healthy
 Rigorous sterile techniques must be used to prevent contamination by bacteria and
fungi from the air and the outer surface of the shell
45

46. The egg must be injected through the shell,
usually by drilling a hole or making a small
window
The viral suspension or suspected virus-
containing fluid is injected into the fluid of the
egg
The exact tissue that is inoculated is guided
by the type of virus being cultivated and the
goals of the experiment
46

47.  An embryonated egg offers various sites for the
cultivation of viruses
 The different sites of viral inoculation in
embryonated eggs are:
1. Chorioallantoic membrane(CAM)
2. Amniotic Cavity
3. Allantoic Cavity
4. Yolk sac
47

48.  The chosen route of inoculation and age of the embryo are determined by the given
virus selectivity for a certain membrane or developmental stage of the embryo
 For example Infectious bronchitis virus is propagated in the yolk sac of a 5-6 day old embryo
 whereas Rous-sarcoma virus is inoculated on the chorioallantoic membrane of a 9-11 day
old embryo and will produce pocks 5-10 days post-infection
48

49.  This method has been widely used in veterinary virology
 Many viruses grow readily or can be adapted to grow on the CAM
 Viruses produce visible foci or ‘pocks’, inclusion bodies, oedema or other
abnormalities
 Each infectious virus particle forms one pock
 Viruses which can be grown include:
 Herpes viruses
 and poxviruses
49

50.  The virus is introduced directly into the amniotic fluid that bathes the developing embryo
 The volume of fluid in the infected amniotic sac is small (1-2 ml)
 The amniotic route is recommended for the primary isolation of human viruses:
 mumps virus
 and influenza A, B and C viruses
 has little application in veterinary virology
 Newly isolated influenza viruses may require several passages before they adapt to growth by
other routes, such as allantoic
50

51.  Many viruses such as Newcastle disease virus can grow readily
 Other viruses such as influenza, may require repeated amniotic passages before
becoming adapted to the egg and grown in the allantoic cavity
 Allantoic inoculation is a quick and easy method that yields large amounts (8–15 ml)
of virus-infected egg fluids
51

52.  It is also a simplest method for growth and multiplication of virus
 Mostly mammalian viruses are isolated using this method
 Immune interference mechanism can be detected in most of avian viruses
 This method is also used for the cultivation of some bacteria like Chlamydiae and
Rickettsiae
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53. THANKYOU
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