Viral classification and Types of Replication in virus Rakshith K, DVM
Precise presentation on Viral classification and Types of replication in Virus.
Entry of virus
Spread of virus
General steps in a virus replication cycle
Attachment, Penetration, Uncoating, Multiplication
Multiplication of Single-Stranded RNA (ss RNA) Viruses
Multiplication of Double-Stranded RNA (ds RNA) Viruses
Multiplication of Single-Stranded DNA (ss DNA) Viruses
Multiplication of Double-Stranded DNA (ds DNA) Viruses
Release of new virions
Common viral diseases of Bovines
Viral classification and Types of Replication in virus Rakshith K, DVM
Precise presentation on Viral classification and Types of replication in Virus.
Entry of virus
Spread of virus
General steps in a virus replication cycle
Attachment, Penetration, Uncoating, Multiplication
Multiplication of Single-Stranded RNA (ss RNA) Viruses
Multiplication of Double-Stranded RNA (ds RNA) Viruses
Multiplication of Single-Stranded DNA (ss DNA) Viruses
Multiplication of Double-Stranded DNA (ds DNA) Viruses
Release of new virions
Common viral diseases of Bovines
Largest viruses that infect vertebrates
Can be seen under light microscope
Poxvirus diseases are characterized by skin lesions – localized or generalized
Important diseases caused by poxviruses are-
Smallpox
Monkeypox
Cowpox
Tanapox
Molluscum contagiosum
General Characters and Classification of Viruses. Includes ICTV classification and Baltimore classification of viruses. A brief explanation of the Viral structure and Lifecycle.
Poxviruses are brick or oval-shaped viruses with large double-stranded DNA genomes. Poxviruses exist throughout the world and cause disease in humans and many other types of animals. Poxvirus infections typically result in the formation of lesions, skin nodules, or disseminated rash.
Cholera is a serious bacterial disease that usually
causes severe diarrhea and dehydration. The disease is typically spread through contaminated water.
Modern sewage and water treatment have effectively eliminated cholera in most countries. It’s still a problem in countries like Asia, America and Africa. Mostly in India.
Countries affected by war, poverty, and natural disasters have the greatest risk for a cholera outbreak.
Taxonomy:
class : Gamma Proteobacteria
Order: Vibrionales
Family: Vibrionaceae
Genus: Vibrio
Species: v.cholerae, v.parahaemolyticus,
v. vulnificus, v. alginolyticus
MORPHOLOGY:
Gram negative, actively motile, short, rigid curved bacilli
Resembling letter “V”
about 34 genus
most common in water
1.5µ X 0.2 -0.4 µ in size
polar flagellum , strongly aerobic
Smear – fish in stream appearance
PATHOGENESIS:
Source: Ingestion of contaminated water, food,
fruits and vegetables etc.,
Incubation periods: 1-5 days
Symptoms: Watery diarrhoea, vomiting, thirst, dehydration, muscle cramps
Complications: muscular pain, renal failure, pulmonary edema, cardiac arrhythrnias
DIAGNOSIS:
Specimen: stool sample, water sample(envt)
Microscopy: a) Hanging drop : +ve
b) Gram stain :-ve
Culture: Mac conkey Agar :colourless to light pink
TCBS : yellow colonies
Serology: serological tests are no diagnostic value
TREATMENT:
Adequate replacement of fluids and electrolytes.
Oral tetracycline reduces the period of vibrio excreation.
PREVENTION:
Drink and use bottled water
Frequent washing
Sanitary environment
Defecate in water
Cook food thoroughly
A picornavirus is a virus belonging to the family Picornaviridae, a family of viruses in the order Picornavirales. Vertebrates, including humans, serve as natural hosts. Picornaviruses are nonenveloped viruses that represent a large family of small, cytoplasmic, plus-strand RNA viruses with a 30-nm icosahedral capsid.
Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy
Largest viruses that infect vertebrates
Can be seen under light microscope
Poxvirus diseases are characterized by skin lesions – localized or generalized
Important diseases caused by poxviruses are-
Smallpox
Monkeypox
Cowpox
Tanapox
Molluscum contagiosum
General Characters and Classification of Viruses. Includes ICTV classification and Baltimore classification of viruses. A brief explanation of the Viral structure and Lifecycle.
Poxviruses are brick or oval-shaped viruses with large double-stranded DNA genomes. Poxviruses exist throughout the world and cause disease in humans and many other types of animals. Poxvirus infections typically result in the formation of lesions, skin nodules, or disseminated rash.
Cholera is a serious bacterial disease that usually
causes severe diarrhea and dehydration. The disease is typically spread through contaminated water.
Modern sewage and water treatment have effectively eliminated cholera in most countries. It’s still a problem in countries like Asia, America and Africa. Mostly in India.
Countries affected by war, poverty, and natural disasters have the greatest risk for a cholera outbreak.
Taxonomy:
class : Gamma Proteobacteria
Order: Vibrionales
Family: Vibrionaceae
Genus: Vibrio
Species: v.cholerae, v.parahaemolyticus,
v. vulnificus, v. alginolyticus
MORPHOLOGY:
Gram negative, actively motile, short, rigid curved bacilli
Resembling letter “V”
about 34 genus
most common in water
1.5µ X 0.2 -0.4 µ in size
polar flagellum , strongly aerobic
Smear – fish in stream appearance
PATHOGENESIS:
Source: Ingestion of contaminated water, food,
fruits and vegetables etc.,
Incubation periods: 1-5 days
Symptoms: Watery diarrhoea, vomiting, thirst, dehydration, muscle cramps
Complications: muscular pain, renal failure, pulmonary edema, cardiac arrhythrnias
DIAGNOSIS:
Specimen: stool sample, water sample(envt)
Microscopy: a) Hanging drop : +ve
b) Gram stain :-ve
Culture: Mac conkey Agar :colourless to light pink
TCBS : yellow colonies
Serology: serological tests are no diagnostic value
TREATMENT:
Adequate replacement of fluids and electrolytes.
Oral tetracycline reduces the period of vibrio excreation.
PREVENTION:
Drink and use bottled water
Frequent washing
Sanitary environment
Defecate in water
Cook food thoroughly
A picornavirus is a virus belonging to the family Picornaviridae, a family of viruses in the order Picornavirales. Vertebrates, including humans, serve as natural hosts. Picornaviruses are nonenveloped viruses that represent a large family of small, cytoplasmic, plus-strand RNA viruses with a 30-nm icosahedral capsid.
Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy
Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity,
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
The Roman Empire’s society was hierarchical, with a rigid class system. At the top were the patricians, wealthy elites who held significant political power. Below them were the plebeians, free citizens with limited political influence, and the vast numbers of slaves who formed the backbone of the economy. The family unit was central, governed by the paterfamilias, the male head who held absolute authority.
Culturally, the Romans were eclectic, absorbing and adapting elements from the civilizations they encountered, particularly the Greeks. Roman art, literature, and philosophy reflected this synthesis, creating a rich cultural tapestry. Latin, the Roman language, became the lingua franca of the Western world, influencing numerous modern languages.
Roman architecture and engineering achievements were monumental. They perfected the arch, vault, and dome, constructing enduring structures like the Colosseum, Pantheon, and aqueducts. These engineering marvels not only showcased Roman ingenuity but also served practical purposes, from public entertainment to water supply.
Palestine last event orientationfvgnh .pptxRaedMohamed3
An EFL lesson about the current events in Palestine. It is intended to be for intermediate students who wish to increase their listening skills through a short lesson in power point.
Model Attribute Check Company Auto PropertyCeline George
In Odoo, the multi-company feature allows you to manage multiple companies within a single Odoo database instance. Each company can have its own configurations while still sharing common resources such as products, customers, and suppliers.
The Art Pastor's Guide to Sabbath | Steve ThomasonSteve Thomason
What is the purpose of the Sabbath Law in the Torah. It is interesting to compare how the context of the law shifts from Exodus to Deuteronomy. Who gets to rest, and why?
Synthetic Fiber Construction in lab .pptxPavel ( NSTU)
Synthetic fiber production is a fascinating and complex field that blends chemistry, engineering, and environmental science. By understanding these aspects, students can gain a comprehensive view of synthetic fiber production, its impact on society and the environment, and the potential for future innovations. Synthetic fibers play a crucial role in modern society, impacting various aspects of daily life, industry, and the environment. ynthetic fibers are integral to modern life, offering a range of benefits from cost-effectiveness and versatility to innovative applications and performance characteristics. While they pose environmental challenges, ongoing research and development aim to create more sustainable and eco-friendly alternatives. Understanding the importance of synthetic fibers helps in appreciating their role in the economy, industry, and daily life, while also emphasizing the need for sustainable practices and innovation.
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
For more information, visit-www.vavaclasses.com
Students, digital devices and success - Andreas Schleicher - 27 May 2024..pptxEduSkills OECD
Andreas Schleicher presents at the OECD webinar ‘Digital devices in schools: detrimental distraction or secret to success?’ on 27 May 2024. The presentation was based on findings from PISA 2022 results and the webinar helped launch the PISA in Focus ‘Managing screen time: How to protect and equip students against distraction’ https://www.oecd-ilibrary.org/education/managing-screen-time_7c225af4-en and the OECD Education Policy Perspective ‘Students, digital devices and success’ can be found here - https://oe.cd/il/5yV
The Indian economy is classified into different sectors to simplify the analysis and understanding of economic activities. For Class 10, it's essential to grasp the sectors of the Indian economy, understand their characteristics, and recognize their importance. This guide will provide detailed notes on the Sectors of the Indian Economy Class 10, using specific long-tail keywords to enhance comprehension.
For more information, visit-www.vavaclasses.com
Ethnobotany and Ethnopharmacology:
Ethnobotany in herbal drug evaluation,
Impact of Ethnobotany in traditional medicine,
New development in herbals,
Bio-prospecting tools for drug discovery,
Role of Ethnopharmacology in drug evaluation,
Reverse Pharmacology.
How to Make a Field invisible in Odoo 17Celine George
It is possible to hide or invisible some fields in odoo. Commonly using “invisible” attribute in the field definition to invisible the fields. This slide will show how to make a field invisible in odoo 17.
CLASS 11 CBSE B.St Project AIDS TO TRADE - INSURANCE
General properties of viruses
1. General Properties of Viruses
Virology
Virology is the bioscience for study of viral nature,and the relationship
between viruses and hosts
Definition of Virus
Viruses may be defined as acellular organisms whose genomes
consist of nucleic acid, and which obligately replicate inside host cells
using host metabolic machinery and ribosomes to form a pool of
components which assemble into particles called VIRIONS
2.
Smallpox was endemic in
China by 1000BC. In response,
the practice of variolation was
developed. Recognizing that
survivors of smallpox
outbreaks were protected from
subsequent infection,
variolation involved inhalation
of the dried crusts from
smallpox lesions like snuff, or
in later modifications,
inoculation of the pus from a
lesion into a scratch on the
forearm of a child.
3.
Viruses are inert (nucleoprotein ) filterable Agents
Viruses are obligate intracellular parasites
Viruses cannot make energy or proteins independent of a
host cell
Viral genome are RNA or DNA but not both
Viruses lack the enzymes necessary for protein and nucleic
acid synthesis
Viruses do not have the genetic capability to multiply by
division
Viruses occupy the twilight zone that separates the ‘living’
from the ‘nonliving’
4. Properties of bacteria and viruses
Cellular
Growth
Binary
organisation on
fission
inanimate
media
Bacteria
Mycoplasma
Rickettsiae
Chlamydiae
Viruses
Both
DNA
and
RNA
Ribosome
Sensitivity
to
antibiotics
Sensitivity
to
Interferons
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+
-
-
+
+
-
+
+
-
+
+
-
+
+
-
+
+
5. Medical importance of viruses
Viral
diseases range from minor ailments such as the common cold
to terrifying diseases such as rabies or AIDS
They may be sporadic like mumps, endemic like infectious hepatitis,
epidemic like dengue fever or pandemic like influenza
They may be localised to circumscribed areas (as some
arbovirus diseases) or worldwide (as Herpes simplex)
Viruses can cause cancer in animals and birds, as well as in
humans
6. Morphology
Size
Methods of analysis
1. Passing through collodion membrane filters of graded porosity
(gradocol membranes)
2. Ultracentrifuge
3. Electron microscope
4. X-ray crystallography
7. Size of Viruses
A small virus has a diameter of about 20 nm.
Example: Parvovirus
A large virus have a diameter of up to 400 nm.
Example: Poxviruses
8.
9.
10. Structure and shape
Virion
The complete infectious unit of virus particle
Capsid
The protein shell, or coat, that encloses the nucleic acid genome.
Functions:
a. Protect the viral nucleic acid.
b. Participate in the viral infection.
c. Antigenic and specific for each virus type
d. Provides structural symmetry to the virus particle
Nucleocapsid
The capsid with the enclosed nucleic acid
14. Cubic or icosahedral symmetry
An icosahedron is a polygon with 12 vertices or corners and
20 facets or sides
Two types of capsomers constitute the icosahedral capsid
1. Petagonal capsomers at the corners (pentons)
2. Hexagonal capsomers making up the sides (hexons)
16. Envelope
A lipid-containing membrane that surrounds some viral
particles
It is derived from the plasma membrane of the host cell
during there release by budding from the cell surface
Viruses-encoded glycoproteins are exposed on the
surface of the envelope (peplomers)
A virus may have more than one type of peplomer
Envelopes confer chemical, antigenic and biological
properties on viruses
Not all viruses have the envelope, and viruses can be
divided into 2 kinds: enveloped virus and nonenveloped
(naked) virus
18. Shape of viruses
Overall shape of the virus particle varies in different groups of viruses
Spherical-shape
Filament-shape
Rod-shape
Brick-shape
Bullet-shape
Tadpole-shape
19. Viral Hemagglutination
A large number of viruses agglutinate erythrocytes from different
species
Hemagglutination by the influenza virus is due to the presence of
hemagglutinin spikes on the surface of the virus
In the haemagglutination test RBCs are added to serial dilutions of
viral suspension, the highest dilution that produces haemagglutination
provides the haemagglutination titer (HA units)
The haemagglutination test can be carried out in test tubes or
special plastic trays
The test is a convenient method for detection and assay of the
influenza virus and also serves to titrate killed influenza vaccine
20. As hemagglutination is specifically inhibited by the antibody to
virus, hemagglutination inhibition assay is useful for determining
antiviral antibody
Elution
In myxoviruses, neuraminidase (RDE) acts on the receptor and
destroys it and this leads to the reversal of hemagglutination and
the release of the virus from the red cell surface
21. Replication of viruses
The viral multiplication cycle can be divided into six sequential phases,
though the phases may sometimes be overlapping
1. Adsorption or attachment
2. Penetration
3. Uncoating
4. Biosynthesis
5. Maturation
6. Release
22. Adsorption
Virions may come into contact with cells by random collision but
adsorption takes place only if there is an affinity between the two
Differences in susceptibility to viral infection are to a large extent
based on the presence or absence of receptors on cells
Penetration
Virus particles may be engulfed by a mechanism resembling
phagocytosis, a process known as ‘viropexis’
In the case of enveloped viruses, the viral envelope may fuse with
the plasma membrane of the host cell and release the nucleocapsid
into the cytoplasm
23. Uncoating
Process of stripping the the virus of its outer layers and capsid
so that the nucleic acid is released into the cell
With most viruses, uncoating is effected by the action of lysosomal
enzymes of the host cell
Biosynthesis
During this phase, viral nucleic acid, capsid protein, enzymes
necessary in the various stages of viral synthesis, assembly
and release will be synthesised
Certain ‘regulatory proteins’ which serve to shut down the normal
cellular metabolism and direct the sequential production of viral
components are also synthesised
The site of viral synthesis depends on the type of virus
24. Biosynthesis consists essentially of the following steps
1.Transcription of mRNA from the viral nucleic acid
2. Translation of the mRNA into ‘early proteins’. These are enzymes
which initiate and maintain synthesis of virus components
3.Replication of viral nucleic acid
4. Synthesis of late or structural proteins, which are the components
of daughter virion capsids
The critical step in viral biosynthesis is the transcription of mRNA
from the viral nucleic acid
The mechanisms of nucleic acid synthesis differ in the different
type of viruses
25. Maturation
Assembly of daughter virions follows the synthesis of nucleic acid & proteins
Virion assembly may take place in the host cell nucleus (Herpes and
adenoviruses) or cytoplasm (picorna and poxviruses)
Naked viruses are present intracellularly as fully developed virions but
in the case of enveloped viruses, only the nucleocapsid is complete
Release
Progeny of bacterial viruses release by the lysis of the infected bacterium.
In the case of animal viruses, release usually occurs without cell lysis
Myxoviruses are released by budding from the cell membrane
over a period of time
In the case of some viruses (varicella), transmission occurs directly
from cell to cell and poliovirus causes profound damage to the host cell
and may be released by cell lysis
26. Stages in the infection of a host’s cell and replication of a virus
27. Abnormal replicative cycles
Incomplete viruses
Incomplete viruses are seen in large proportions, when cells are
infected with a high dose (MOI) of the influenza virus
This is the result of defective assembly
The virus yield will have a high hemagglutinin titer but low infectivity
and this is known as the ‘von Magnus phenomenon’
Abortive infection
In nonpermissive cells, viral components may be synthesised but
maturation or assembly is defective, and either no release occurs
or the progeny is noninfectious
28. Defective viruses
Some viruses are genetically defective in that when they infect cells,
they are unable to give rise to fully formed progeny
Yield of progeny virions occurs only if the cells are simultaneously
infected with a helper virus, which can supplement the genetic
deficiency
(eg; hepatitis D, adeno-associated satellite viruses, which replicate
only in the presence of their helper viruses – hepatitis B and
adenoviruses, respectively)
29. Cultivation of viruses
Systems for the propagation of viruses
Human volunteers
Animal inoculation
Embryonated eggs
Tissue culture
a) Organ culture
b) Explant culture
c) Cell culture
30. Animal inoculation
Mice, Infant (suckling) mice, rats, monkeys, chikens, guinea pigs,
rabbits, ferrets are used for inoculation
Growth of the virus in inoculated animals may be indicated
by death, disease or visible lesions
Animal inoculation is also used for the study of pathogenesis,
immune response, epidemiology and oncogenesis
Disadvantages of animal inoculation are that immunity may interfere
with viral growth and that animals often harbour latent viruses
31. Embryonated eggs
The embryonated egg offers several sites for the cultivation of viruses
Inoculation on the chorioallantoic membrane (CAM) produces visible
lesions (pocks); pock-forming viruses eg: variola or vaccinia
Inoculation into the allantoic cavity provides a rich yield of influenza
and some paramyxo viruses; hence employed for vaccine production
Inoculation into the amniotic sac is employed for the primary isolation
of the influenza virus
Yolk sac inoculation is used for the cultivation of some viruses,
chlamydiae and rickettsiae
34. Tissue culture
Organ culture
Small bits of organs can be maintained in vitro for days and weeks,
preserving their original architecture and function
Useful for the isolation of some viruses which appear to be highly
specialised parasites of certain organs (eg: Tracheal ring culture –
corona virus, a respiratory pathogen)
Explant culture
Fragments of minced tissue can be grown as ‘explants’ embedded in
plasma clots or may be cultivated in suspension (eg: adenoid tissue
explant cultures – adenoviruses)
35. Cell culture
Although embryonated eggs and laboratory animals are very useful for
isolation viruses, cell culture is the sole system for virus isolation in
most laboratories
Based on their origin, chromosomal characters and the number of
generations through which they can be maintained, cell cultures
are classified into three types
1. Primary cell cultures
2. Diploid cell cultures (semi-continuous)
3. Continuous cell lines
36. Primary cell cultures
Prepared directly from animal or human tissues and can be
subcultured only once or twice (eg: monkey kidney, human
embryonic kidney cell cultures)
Useful for the isolation of viruses and their cultivation for vaccine
production
Diploid cell cultures
Derived from human fetal tissue and can be subcultured 20-50 times
(human diploid fibroblasts – MRC-5, WI-38 (derived from normal
embryonic lung tissue)
They are useful for the isolation of some fastidious pathogens
They are also useful for the production of viral vaccines
(eg: poliomyelitis, rubella, rabies, CMV, VZV)
37. Continuous cell lines
These are cells of a single type, usually derived from cancer cells
and capable of continuous serial cultivation indefinitely
(eg: HeLa, HEp-2, Vero, KB cell lines)
Some continuous cell lines in common use
HeLa
Human carcinoma of cervix cell line
HEp-2
Human epithelioma of larynx cell line
Vero
Vervet monkey kidney cell line
KB
Human carcinoma of nasopharynx cell line
McCoy
Human synovial carcinoma cell line
BHK-21
Baby hamster kidney cell line
38. Detection of virus growth in cell cultures
Cytopathic effect
Many viruses cause morphological changes in cultured cells, these
changes can be readily observed by microscopic examination and
these changes are known as ‘cytopathic effects’ (CPE)
Cytopathic effects may be cytocidal (cell death) or non-cytocidal
Non-cytocidal effects include acidophilic or basophilic inclusion bodies
in the nucleus or cytoplasm, or both; cell fusion, and transformation
Cytopathic effects can be so characteristic of individual viruses that
they can often be used to identify viruses
(Measles – syncytium formation; adenovirus – large granular clumps)
40. Metabolic inhibition
When viruses grow in cell cultures, cell metabolism is inhibited and
there is no acid production
Hemadsorption
When hemagglutinating viruses grow in cell culture, their presence
can be indicated by the addition of guinea pig erythrocytes to the
cultures
Interference
The growth of a non-cytopathogenic virus in cell culture can be tested
by the subsequent challenge with a known cytopathogenic virus
Transformation
Oncogenic viruses induce cell transformation and loss of contact
inhibition, so that growth appears in a piled-up fashion producing
microtumors
Immunofluorescence
41. Viral assay
The virus content of a specimen can be assayed in two ways
1. Total virus particles: Electron microscopy and haemagglutination
2. Assay of infectivity (with reference to the infectious virions only)
a)Quantal assays
Only indicate the presence or absence of infectious viruses but using
serial dilutions of virus suspensions and with the aid of statistical
methods, reasonably accurate estimates of infectivity can be obtained
End points used for infectivity titration are the death of the animal,
production of hemagglutinin in allantoic fluid or the appearance of
CPE in cell cultures
The titers are expressed as the ‘50 percent infectious dose’ (ID50)
or LD50
42. b) Quantitative
assays
Similar to the estimation of bacterial viable counts by colony
counting
Two methods are available;
1.Plaque assay
A viral suspension is added to a monolayer of cultured cells in a bottle
or petri dish, and after adsorption, the medium is removed and
replaced with a solid agar gel
Each infectious viral particle gives rise to a localised focus of infected
cells that can be seen with the naked eye. Such foci are known
as ‘plaques’
2. Pock assay
44. Viral Genetics
Two main mechanisms for genetic modification in viruses are mutation
and recombination
Mutation
The frequency of mutation in viruses is about 10-4 to 10-8
Mutations may be lethal or sub lethal, may occur spontaneously
or induced by mutagens
Mutations affecting virulence, host range, antigenicity and pock
plaque morphology are of clinical and laboratory interest
Mutants which are able to grow under certain conditions (permissive
conditions) and can not grow or lethal under some specific conditions
(restrictive conditions) are called as ‘conditional lethal mutants’
45. Interaction between viruses (Genetic)
Reassortment
Found in viruses which have segmented genomes (eg: influenza,
bunya and arena viruses)
In a single cell infected with two related viruses, there is an exchange
of segments with production of reassortants
It is probably one of the ways by which the pandemic strains of
the influenza virus originate in nature
Reactivation
when a cell is infected with an active virus and a different but related
inactive virus, recombination may occur and as result of this the
progeny possessing one or more genetic traits of inactive virus
may be produced. This process is called cross reactivation or
marker rescue
46. Interaction between viral gene products (nongenetic)
1. Phenotypic mixing
When two different viruses infect the same cell, some mix up may
take place during assembly, so that progeny genome of one virus
may be surrounded by a capsid belonging partially or entirely to
other virus
This altered phenotype is not a stable variation
When the genome of one virus is surrounded by the entire capsid of
the other virus, it is called ‘transcapsidation’
47. 2. Genotypic mixing
It occurs when more than one complete genomes are accidentally
surrounded by a single virus capsid
There is no recombination between the different genomes so that the
two kinds of viral progeny are formed on passage
3. Complementation
One virus provides the gene products to the second virus in which
the latter is defective, this allows the second virus to replicate
48. 4. Interference
Infection of a cell by one virus inhibits simultaneous or subsequent
infection by other virus
The most important mediator of interference is interferon
Interference may also be produced by destruction of cell receptors
by an active or inactive virus
Viral interference has been applied in the field in controlling
poliomyelitis outbreaks by introducing vaccine strain into
the population
5. Enhancement
Mixed infection of cells may sometimes lead to increased virus yield
or greater CPE
49. Classification and nomenclature of viruses
Viruses are classified on the basis of biological, physical and
chemical properties
Viruses are broadly classified into DNA and RNA viruses and then
further divided into families, subfamilies, genera and species
54. Viroids
Viroids are single-stranded circular RNA molecules that lacks a
protein coat and they are mainly plant pathogens
Prions
Prions are infectious proteins without any detectable nucleic acid
They are highly resistant to physical and chemical agents
They produce slow infections with very long incubation period
Diseases caused by prions in animals include, scrapie of sheep and
goats,mink encephalopathy, bovine spongiform encephalopathy
Responsible for Kuru, Creutzfeldt-Jakob disease and some other
chronic neurological degenerative diseases of humans