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Medical VirologyIntroduction to Basics
Dr.T.V.Rao MD
Dr.T.V.Rao MD 1
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History Virology
Smallpox was endemic inChina by 1000BC. In response,the practice ofvariolation wasdeveloped. Recognizing that
survivors of smallpoxoutbreaks were protected fromsubsequent infection,variolation involved inhalationof the dried crusts fromsmallpox lesions like snuff, orin later modifications,inoculation of the pus from alesion into a scratch on theforearm of a child.
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Virus infections are Universal .
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Introduction to Virology
A virus is an obligate intracellular
parasite containing genetic material
surrounded by protein Virus particles can only be
observed by an electronmicroscope
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Introduction to Virology
Recognizing the shape, size, and
structure of different viruses is critical to
the study of disease Viruses have an inner core of nucleic acid
surrounded by protein coat known as an
envelope
Most viruses range in sizes from 20 250
nanometers
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Viral Properties
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 have a naked capsid or envelope withattached proteins
Viruses do not have the genetic capability to multiplyby division.
Viruses are non-living entities
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Viruses are Ultramicroscopic
7
Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997
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The size of viruses
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VIRAL STRUCTURESOME
TERMINOLOGY
virus particle = virion
protein which coats the genome =
capsid
capsid usually symmetrical
capsid + genome = nucleocapsid
may have an envelope
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Virion The complete
infectious unit
of virus particle Structurally
mature,
extracellularvirus particles.
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Virion
Capsid
Viral core
envelope
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Virion Structure
Nucleic Acid
Spike
Projections
Protein
Capsid
Lipid Envelope
Virion
Associated
Polymerase
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Distinguishing characteristics of viruses
Obligate intracellular parasites
Extreme genetic simplicity
Contain DNA or RNA
Replication involves disassembly
and reassembly Replicate by "one-step growth
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How are viruses named?
Based on:
- the disease they causepoliovirus, rabies virus
- the type of diseasemurine leukemia virus
- geographic locationsSendai virus, Coxsackie virus
- their discoversEpstein-Barr virus
- how they were originally thought to be contracteddengue virus (evil spirit), influenza virus (the influence of bad air)
- combinations of the aboveRous Sarcoma virus
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White, DO and Fenner, FJ.Medical Virology, 4th Ed. 1994
Virus particle = virion
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5 BASIC TYPES OF VIRAL STRUCTURE
HELICAL ENVELOPED HELICAL
ENVELOPED ICOSAHEDRAL
COMPLEX
ICOSAHEDRAL
Adaptedfrom Schaechter et al., Mechanisms of Microbial Disease
nucleocapsidicosahedralnucleocapsid
nucleocapsid
helicalnucleocapsid
lipid bilayer
lipid bilayer
glycoprotein spikes= peplomers
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Icosahedral
Adeno-associated Virus (AAV)AdenovirusB19Coxsackievirus - ACoxsackievirus - BCytomegalovirus (CMV)Eastern Equine EncephalitisVirus (EEEV)Echovirus
Epstein-Barr Virus (EBV)Hepatitis A Virus (HAV)Hepatitis B Virus (HBV)Hepatitis C Virus (HCV)Hepatitis Delta Virus (HDV)
Hepatitis E Virus (HEV)
Herpes Simplex Virus 1 (HHV1)Herpes Simplex Virus 2 (HHV2)Human Immunodeficiency Virus (HIV)Human T-lymphotrophic Virus (HTLV)Norwalk Virus
Papilloma Virus (HPV)Polio virusRhinovirusRubella VirusSaint Louis Encephalitis VirusVaricella-Zoster Virus (HHV3)
Western Equine Encephalitis Virus(WEEV)Yellow Fever Virus
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Viral Structure
Varies in size, shape and symmetry
VIP for classification
3 types of capsid symmetry:
Cubic (icosahedral)
Has 20 faces, each an equilateral triangle. Eg. adenovirus
Helical
Protein binds around DNA/RNA in a helical fashion eg. Coronavirus
Complex
Is neither cubic nor helical eg. poxvirus
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The Baltimore classification systemBased on genetic contents and replication strategies of
viruses. According to the Baltimore classification, viruses aredivided into the following seven classes:
1. dsDNA viruses
2. ssDNA viruses
3. dsRNA viruses4. (+) sense ssRNA viruses (codes directly for protein)
5. (-) sense ssRNA viruses
6. RNA reverse transcribing viruses
7. DNA reverse transcribing viruses
where "ds" represents "double strand" and "ss" denotes "single
strand".
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Virus Classification I- the Baltimore classification
All viruses must produce mRNA, or (+) sense RNA
A complementary strand of nucleic acid is () sense
The Baltimore classification has + RNA as its central point
Its principles are fundamental to an understanding of virus
classification and genome replication, but it is rarely usedas a classification system in its own right
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From Principles of Virology Flint et al ASM PressDr.T.V.Rao MD 23
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Virus classification II -the Classical system
This is a based on three principles -
1) that we are classifying the virus itself,not the host
2) the nucleic acid genome
3) the shared physical properties of the infectious agent(e.g capsid symmetry, dimensions, lipid envelope)
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Virus classification III -the genomic system
More recently a precise ordering of viruses
within and between families is possible based
on DNA/RNA sequence
By the year 2000 there were over 4000 viruses
of plants, animals and bacteria - in 71 families,
9 subfamilies and 164 genera
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Viral Structure - Overview
Fig 1. Schematic overview of the structure of animal viruses
** does not exist in all viruses
Nucleic acid
Capsid
Nucleocapsid
Envelope protein
Membrane proteinViral envelope**
Spike protein
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a) Crystallographic structure of a
simple icosahedral virus.b) The axes of symmetry
Icosahedral capsids
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C
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Cubic or icosahedral symmetry
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ICOSAHEDRAL SYMMETRY
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ICOSAHEDRAL SYMMETRY
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ICOSAHEDRAL SYMMETRY
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ICOSAHEDRAL SYMMETRY
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Adenovirus
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Adenovirus
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Helical symmetry
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Helical
California Encephalitis VirusCoronavirusHantavirus
Influenza Virus (Flu Virus)Measles Virus ( Rubeola)Mumps VirusPara influenza VirusRabies VirusRespiratory Syncytial Virus(RSV)
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Helical symmetry
How to
assemble
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In 1955, Fraenkel,
Conrat, and Williams
demonstrated that
tobacco mosaic virus
(TMV) spontaneously
formed when mixtures
of purified coatprotein and its
genomic RNA were
incubated together.
Helical symmetry
TMV, a filamentous virus
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Enveloped helical virus Enveloped icosahedral virus
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Properties of naked viruses
Stable in hostile environment
Not damaged by drying, acid, detergent, and heat
Released by lysis of host cells
Can sustain in dry environment
Can infect the GI tract and survive the acid and bile
Can spread easily via hands, dust, fomites, etc
Can stay dry and still retain infectivity Neutralizing mucosal and systemic antibodies are
needed to control the establishment of infection
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Naked viruses( Non Enveloped )
Adeno-associated Virus (AAV)AdenovirusB19
Coxsackievirus - ACoxsackievirus - BEchovirusHepatitis A Virus (HAV)
Hepatitis E Virus (HEV)Norwalk Virus
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COMPLEX SYMMETRY
POXVIRUS FAMILY
surface view cross section
White, DO and Fenner, FJ.
Medical Virology, 4th
Ed. 1994 Dr.T.V.Rao MD
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ENVELOPE
OBTAINED BY BUDDING THROUGH ACELLULAR MEMBRANE (except poxviruses)
POSSIBILITY OF EXITING CELL WITHOUT
KILLING IT CONTAINS AT LEAST ONE VIRALLY CODED
PROTEIN
ATTACHMENT PROTEIN
LOSS OF ENVELOPE RESULTS IN LOSS OFINFECTIVITY
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RNA or DNA
segmented or non-segmented
linear or circular
single-stranded or double-stranded
if single-stranded RNA
is genome mRNA (+) sense or
complementary to mRNA (-) sense
CLASSIFICATION
NUCLEIC ACID
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Genome
The genome of a virus can be either DNA orRNA
DNA-double stranded (ds): linear or circular
Single stranded (ss) : linear or circular
RNA- ss:segmented or non-segmented
ss:polarity+(sense) or polarity(non-sense)
ds: linear (only reovirus family)Dr.T.V.Rao MD 45
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DNA RNA
double-stranded single-strandeddouble-
strandedsingle-stranded
linear
circular linear
circular linear linear (circular)*
singl
e
sing
le
multip
le
singl
e
sing
le
multip
le
singl
e
multipl
e
(+)sense (-)sense
sing
le
multip
le
sing
le
multip
le
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Viral genome strategies
dsDNA (herpes, papova, adeno, pox)
ssDNA (parvo)
dsRNA (reo, rota)
ssRNA (+) (picorna, toga, flavi, corona)
ssRNA (-) (rhabdo, paramyxo, orthomyxo,
bunya, filo)
ssRNA (+/-) (arena, bunya)
ssRNA (+RTase) (retro, lenti)
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HERPESVIRIDAE
HEPADNAVIRIDAE
ENVELOPED
PAPILLOMAVIRIDAE
POLYOMAVIRIDAE(formerly grouped together as the
PAPOVAVIRIDAE)
CIRCULAR
ADENOVIRIDAE
LINEAR
NON-ENVELOPED
DOUBLE STRANDED
PARVOVIRIDAE
SINGLE STRANDED
NON-ENVELOPED
POXVIRIDAE
COMPLEX
ENVELOPED
DNA VIRUSES
Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991
All families shown areicosahedral except for
poxviruses
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DNA i
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DNA viruses
From Principles ofVirology Flint et alASM Press
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FLAVIVIRIDAETOGAVIRIDAE
RETROVIRIDAE
ICOSAHEDRAL
CORONAVIRIDAE
HELICAL
ENVELOPED
ICOSAHEDRAL
PICORNAVIRIDAECALICIVIRIDAE
ASTROVIRIDAE
NONENVELOPED
SINGLE STRANDED
positive sense
BUNYAVIRIDAEARENAVIRIDAE
ORTHOMYXOVIRIDAEPARAMYXOVIRIDAE
RHABDOVIRIDAEFILOVIRIDAE
SINGLE STRANDED
negative sense
REOVIRIDAE
DOUBLE STRANDED
RNA VIRUSES
ENVELOPED
HELICAL ICOSAHEDRAL
NONENVELOPED
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BASIC STEPS IN VIRAL LIFE CYCLE
ADSORPTION
PENETRATION
UNCOATING AND ECLIPSE
SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN
ASSEMBLY (maturation)
RELEASE
Dr.T.V.Rao MD
RECEPTOR VIRUS
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RECEPTOR VIRUS
ICAM-1 polio
CD4 HIV
acetylcholine rabies
EGF vaccinia
CR2/CD21 Epstein-
Barr
HVEM herpes
Sialic acid Influenza,
reo, corona
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Virus Replication1 Virus attachment
and entry
1 2 Uncoating of virion
2
3 Migration ofgenome nucleic
acid to nucleus
3
4 Transcription5 Genome replication
4
5
6 Translation of virusmRNAs
6
7 Virion assembly7
8 Release of newvirus particles
8
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ADSORPTION
TEMPERATURE INDEPENDENT
REQUIRES VIRAL ATTACHMENTPROTEIN
CELLULAR RECEPTORS
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PENETRATION
- ENVELOPED VIRUSES
FUSION WITH PLASMA MEMBRANE
ENTRY VIA ENDOSOMES
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PENETRATION
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PENETRATION
herpesviruses, paramyxoviruses, HIV
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PENETRATION
- ENVELOPED VIRUSES
FUSION WITH PLASMA MEMBRANEENTRY VIA ENDOSOMES, FUSION WITH
ACIDIC ENDOSOME MEMBRANE
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VIRUS UPTAKE VIA ENDOSOMES
CALLEDVIROPEXIS / ENDOCYTOSIS /
PINOCYTOSIS
Dr.T.V.Rao MD
PENETRATION
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PENETRATION
NON-ENVELOPED VIRUSES
entry directly across
plasma membrane:
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Replicative cycle As obligate intracellular parasites, Virus must
enter and replicate in living cells in order to
reproduce themselves. This growth cycle
involves specific attachment of virus,penetration and uncoating, nucleic acid
transcription, protein synthesis, maturation
and assembly of the virions and theirsubsequent release from the cell by budding
or lysis
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UNCOATING
NEED TO MAKE GENOME AVAILABLE
ONCE UNCOATING OCCURS, ENTER ECLIPSEPHASE
ECLIPSE PHASE LASTS UNTIL FIRST NEWVIRUS PARTICLE FORMED
Dr.T.V.Rao MD
SYNTHESIS OF VIRAL NUCLEIC ACID
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SYNTHESIS OF VIRAL NUCLEIC ACID
AND PROTEIN
MANY STRATEGIES
NUCLEIC ACID MAY BE MADE INNUCLEUS OR CYTOPLASM
PROTEIN SYNTHESIS IS ALWAYS IN
THE CYTOPLASM
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ASSEMBLY AND MATURATION
NUCLEUS
CYTOPLASM
AT MEMBRANE
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RELEASE
LYSIS
BUDDING THROUGH PLASMA MEMBRANE
NOT EVERY RELEASED VIRION IS INFECTIOUS
Dr.T.V.Rao MD
Transmission of Viruses
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Transmission of Viruses Respiratory transmission
Influenza A virus
Faecal-oral transmission
Enterovirus
Blood-borne transmission
Hepatitis B virus
Sexual Transmission
HIV
Animal or insect vectors
Rabies virus
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Viruses enter the body of the host
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in a variety of ways, for example...
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The commonest forms of
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transmission are via...
INHALED DROPLETSin sneezing of coughing
for example the COMMON COLD
or INFLUENZA VIRUSES.
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or by
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or by...
drinking water or
eating raw food, for example,
HEPATITIS A and POLIOVIRUS.
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The commonest forms of
transmission are also via...
sexual intercourse for exampleHIV and HEPATITIS B and...
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also...
vertical transmission -
from mother to baby for example
HIV, HEPATITIS B and RUBELLA...
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l
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also...
bites of vector arthropods such as
mosquitoes for example YELLOW FEVER,
RIFT VALLEY FEVER and DENGUE.
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Most viral infections...
do not lead to such seriouscomplications and the host...
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get well after a period of sickness
to be immune for the rest of their lives.
Examples are MEASLES INFECTION,
RUBELLA or German measles,
MUMPS and man others...Dr.T.V.Rao MD 79
A b i h
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A bacteriophage
A bacteriophage is any one of a number of
viruses that infect bacteria. They do this by
injecting genetic material, which they carry
enclosed in an outer protein capsid. Thegenetic material can be ssRNA, dsRNA, ssDNA,
or dsDNA ('ss-' or 'ds-' prefix denotes single-
strand or double-strand) along with eithercircular or linear arrangement.
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S f B i h
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Structure of Bacteriophage
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Cl ifi i f B i h
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Classification of Bacteriophages
The dsDNA tailed phages, or Caudovirales,account for 95% of all the phages reported inthe scientific literature, and possibly make upthe majority of phages on the planet.However, other phages occur abundantly inthe biosphere, with different virions, genomesand lifestyles. Phages are classified by the
International Committee on Taxonomy ofViruses (ICTV) according to morphology andnucleic acid.
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Sub-viral agents
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Sub-viral agents
Satellites Contain nucleic acid
Depend on co-infection with a helper virus
May be encapsidated (satellite virus)
Mostly in plants, can be human e.g. hepatitis delta virus
If nucleic acid only = virusoid
Viroids Unencapsidated, small circular ssRNA molecules that replicate
autonomously
Only in plants, e.g. potato spindle tuber viroid
Depend on host cell polII for replication, no protein or mRNA
Prions No nucleic acid
Infectious protein e.g. BSE
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Vi id & P i
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Viroids & Prions
Viroids ss RNA genome and the smallest known pathogens. Affects plants
Prions
Infectious particles that are entirely protein. No nucleic acid Highly heat resistant Animal disease that affects nervous tissue Affects nervous tissue and results in
Bovine spongiform encepahltits (BSE) mad cow disease, scrapie in sheep kuru & Creutzfeld-Jakob Disease (CJD) in humans
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Vi id
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Viroids
Viroids are small (200-400nt),circular RNA molecules with a rod-like secondary structure whichpossess no capsid or envelope whichare associated with certain plantdiseases. Their replication strategylike that of viruses - they are obligateintracellular parasites.
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Dependovirus /Virusoids
Viroids are small (200-400nt), circular
RNA molecules with a rod-like secondary
structure which possess no capsid orenvelope which are associated with
certain plant diseases. Their replication
strategy like that of viruses - they areobligate intracellular parasites.
(Prions)
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(Prions)
Prions are rather ill-defined infectiousagents believed to consist of a single type ofprotein molecule with no nucleic acidcomponent. Confusion arises from the fact
that the prion protein & the gene whichencodes it are also found in normal'uninfected' cells. These agents areassociated with diseases such asCreutzfeldt-Jakob disease in humans,scrapie in sheep & bovine spongiformencephalopathy (BSE) in cattle.
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Programme Created by Dr.T.V.Rao MD for
Medical and Paramedical Students in the
Developing World