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Dr.T.V.Rao MD 1
Medical VirologyIntroduction to Basics
Dr.T.V.Rao MD
Dr.T.V.Rao MD 2
History Virology• 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.
Virus infections are Universal …….
Dr.T.V.Rao MD 4
Introduction to Virology• A virus is an obligate intracellular
parasite containing genetic material surrounded by protein
• Virus particles can only be observed by an electron microscope
Dr.T.V.Rao MD 5
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
Dr.T.V.Rao MD 6
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 with
attached proteins• Viruses do not have the genetic capability to multiply
by division. • Viruses are non-living entities
Dr.T.V.Rao MD
Viruses are Ultramicroscopic
7
Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997
Dr.T.V.Rao MD 8
The size of viruses
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VIRAL STRUCTURE – SOME TERMINOLOGY
• virus particle = virion• protein which coats the genome =
capsid• capsid usually symmetrical• capsid + genome = nucleocapsid• may have an envelope
Dr.T.V.Rao MD 10
Virion• The complete
infectious unit of virus particle
• Structurally mature, extracellular virus particles.
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Virion
Capsid
Viral core
envelope
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Virion StructureNucleic Acid
Spike Projections
ProteinCapsid
Lipid Envelope
VirionAssociatedPolymerase
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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”
Dr.T.V.Rao MD 14
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 contracted
dengue virus (“evil spirit”), influenza virus (the “influence” of bad air)
- combinations of the aboveRous Sarcoma virus
Dr.T.V.Rao MD 15
White, DO and Fenner, FJ. Medical Virology, 4th Ed. 1994
Virus particle = virion
Dr.T.V.Rao MD 16
5 BASIC TYPES OF VIRAL STRUCTURE
HELICAL ENVELOPED HELICAL
ENVELOPED ICOSAHEDRAL
COMPLEX
ICOSAHEDRAL
Adapted from Schaechter et al., Mechanisms of Microbial Disease
nucleocapsid icosahedral nucleocapsid
nucleocapsid
helical nucleocapsid
lipid bilayer
lipid bilayer
glycoprotein spikes= peplomers
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Dr.T.V.Rao MD 19
Icosahedral• Adeno-associated Virus
(AAV)AdenovirusB19Coxsackievirus - ACoxsackievirus - BCytomegalovirus (CMV)Eastern Equine Encephalitis Virus (EEEV)EchovirusEpstein-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 VirusPapilloma Virus (HPV)Polio virusRhinovirusRubella VirusSaint Louis Encephalitis VirusVaricella-Zoster Virus (HHV3)Western Equine Encephalitis Virus (WEEV)Yellow Fever Virus
Dr.T.V.Rao MD 20
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
Dr.T.V.Rao MD 21
The Baltimore classification systemBased on genetic contents and replication strategies of viruses. According to the Baltimore classification, viruses are divided into the following seven classes:
1. dsDNA viruses 2. ssDNA viruses 3. dsRNA viruses 4. (+) 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".
Dr.T.V.Rao MD 22
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 used as a classification system in its own right
Dr.T.V.Rao MD 23From Principles of Virology Flint et al ASM Press
Dr.T.V.Rao MD 24
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
CapsidNucleocapsid
Envelope protein
Membrane proteinViral envelope**
Spike protein
Dr.T.V.Rao MD 27
a) Crystallographic structure of a simple icosahedral virus.
b) The axes of symmetry
Icosahedral capsids
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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 Virus
CoronavirusHantavirusInfluenza 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 coat protein and its genomic RNA were incubated together.
Helical symmetry
TMV, a filamentous virus
Dr.T.V.Rao MD 39
Enveloped helical virus Enveloped icosahedral virus
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
Naked viruses( Non Enveloped )
• Adeno-associated Virus (AAV)AdenovirusB19Coxsackievirus - ACoxsackievirus - BEchovirusHepatitis A Virus (HAV)Hepatitis E Virus (HEV)Norwalk Virus
Dr.T.V.Rao MD 42
COMPLEX SYMMETRY
POXVIRUS FAMILY
surface view cross section
White, DO and Fenner, FJ. Medical Virology, 4th Ed. 1994
Dr.T.V.Rao MD 43
ENVELOPE• OBTAINED BY BUDDING THROUGH A
CELLULAR 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 OF INFECTIVITY
Dr.T.V.Rao MD 44
• 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
Dr.T.V.Rao MD 45
Genome• The genome of a virus can be either DNA or
RNA
• 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 46
DNA RNA
double-stranded single-strandeddouble-
strandedsingle-stranded
linear
circularlinear
circular linear linear (circular)*
single
single
multiple
single
single
multiple
single
multiple
(+)sense (-)sense
single
multiple
single
multiple
Dr.T.V.Rao MD 47
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)
48
HERPESVIRIDAEHEPADNAVIRIDAE
ENVELOPED
PAPILLOMAVIRIDAEPOLYOMAVIRIDAE(formerly grouped together as the PAPOVAVIRIDAE)
CIRCULAR
ADENOVIRIDAE
LINEAR
NON-ENVELOPED
DOUBLE STRANDED
PARVOVIRIDAE
SINGLE STRANDEDNON-ENVELOPED
POXVIRIDAE
COMPLEXENVELOPED
DNA VIRUSES
Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991
All families shown are icosahedral except for poxviruses
Dr.T.V.Rao MD
Dr.T.V.Rao MD 49
DNA viruses
From Principles of Virology Flint et al ASM Press
50
FLAVIVIRIDAETOGAVIRIDAE
RETROVIRIDAE
ICOSAHEDRAL
CORONAVIRIDAE
HELICAL
ENVELOPED
ICOSAHEDRAL
PICORNAVIRIDAECALICIVIRIDAEASTROVIRIDAE
NONENVELOPED
SINGLE STRANDEDpositive sense
BUNYAVIRIDAEARENAVIRIDAE
ORTHOMYXOVIRIDAEPARAMYXOVIRIDAE
RHABDOVIRIDAEFILOVIRIDAE
SINGLE STRANDEDnegative sense
REOVIRIDAE
DOUBLE STRANDED
RNA VIRUSES
ENVELOPED
HELICAL ICOSAHEDRAL
NONENVELOPED
Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991Dr.T.V.Rao MD
Dr.T.V.Rao MD 51
RNA viruses
From Principles of Virology Flint et al ASM Press
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BASIC STEPS IN VIRAL LIFE CYCLE
• ABSORPTION• PENETRATION• UNCOATING AND ECLIPSE• SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN• ASSEMBLY (maturation)• RELEASE
RECEPTOR VIRUS
ICAM-1 polio
CD4 HIV
acetylcholine rabies
EGF vaccinia
CR2/CD21 Epstein-Barr
HVEM herpes
Sialic acid Influenza, reo, corona
Dr.T.V.Rao MD 54
Virus Replication1 Virus attachment
and entry
1 2 Uncoating of virion
2
3 Migration ofgenome nucleicacid to nucleus
3
4 Transcription5 Genome replication
4
5
6 Translation of virusmRNAs
6
7 Virion assembly78 Release of new
virus particles
8
Dr.T.V.Rao MD 55
ADSORPTION
• TEMPERATURE INDEPENDENT• REQUIRES VIRAL ATTACHMENT
PROTEIN• CELLULAR RECEPTORS
Dr.T.V.Rao MD 56
PENETRATION - ENVELOPED VIRUSES
•FUSION WITH PLASMA MEMBRANE •ENTRY VIA ENDOSOMES
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PENETRATION
herpesviruses, paramyxoviruses, HIV
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PENETRATION - ENVELOPED VIRUSES
•FUSION WITH PLASMA MEMBRANE •ENTRY VIA ENDOSOMES, FUSION WITH ACIDIC ENDOSOME MEMBRANE
Dr.T.V.Rao MD 59
Dr.T.V.Rao MD 60
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VIRUS UPTAKE VIA ENDOSOMES
• CALLED –VIROPEXIS / ENDOCYTOSIS /
PINOCYTOSIS
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PENETRATIONNON-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 their subsequent release from the cell by budding or lysis
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Dr.T.V.Rao MD 67
UNCOATING
• NEED TO MAKE GENOME AVAILABLE
• ONCE UNCOATING OCCURS, ENTER ECLIPSE PHASE
• ECLIPSE PHASE LASTS UNTIL FIRST NEW VIRUS PARTICLE FORMED
Dr.T.V.Rao MD 68
SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN
• MANY STRATEGIES• NUCLEIC ACID MAY BE MADE IN
NUCLEUS OR CYTOPLASM• PROTEIN SYNTHESIS IS ALWAYS IN
THE CYTOPLASM
Dr.T.V.Rao MD 69
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 71
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 hostin a variety of ways, for example...
Dr.T.V.Rao MD 73
The commonest forms oftransmission are via...
INHALED DROPLETSin sneezing of coughing
for example the COMMON COLDor INFLUENZA VIRUSES.
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or by...
drinking water oreating raw food, for example,HEPATITIS A and POLIOVIRUS.
Dr.T.V.Rao MD 75
The commonest forms oftransmission are also via...
sexual intercourse for exampleHIV and HEPATITIS B and...
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also...
vertical transmission -from mother to baby for exampleHIV, HEPATITIS B and RUBELLA...
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also...
bites of vector arthropods such asmosquitoes for example YELLOW FEVER,
RIFT VALLEY FEVER and DENGUE.
Dr.T.V.Rao MD 78
Most viral infections...
do not lead to such seriouscomplications and the host...
Dr.T.V.Rao MD 79
get well after a period of sicknessto be immune for the rest of their lives.
Examples are MEASLES INFECTION,RUBELLA or German measles,MUMPS and many others...
Dr.T.V.Rao MD 80
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. The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA ('ss-' or 'ds-' prefix denotes single-strand or double-strand) along with either circular or linear arrangement.
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Structure of Bacteriophage
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Classification of Bacteriophages • The dsDNA tailed phages, or Caudovirales,
account for 95% of all the phages reported in the scientific literature, and possibly make up the majority of phages on the planet. However, other phages occur abundantly in the biosphere, with different virions, genomes and lifestyles. Phages are classified by the International Committee on Taxonomy of Viruses (ICTV) according to morphology and nucleic acid.
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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
Dr.T.V.Rao MD 84
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
Viroids
• Viroids are small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites.
Dependovirus /Virusoids
• Viroids are small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites.
(Prions)• Prions are rather ill-defined infectious
agents believed to consist of a single type of protein molecule with no nucleic acid component. Confusion arises from the fact that the prion protein & the gene which encodes it are also found in normal 'uninfected' cells. These agents are associated with diseases such as Creutzfeldt-Jakob disease in humans, scrapie in sheep & bovine spongiform encephalopathy (BSE) in cattle.
Dr.T.V.Rao MD 88
• Programme Created by Dr.T.V.Rao MD for Medical and Paramedical Students in the
Developing World• Email
• doctortvrao@gmail.com
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