Virus | CEE Academy

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HISTORY

Table 1: Virus: Important Discoveries

Event / Term	Scientist / Key Point
Term 'virus'	Louis Pasteur
First evidence of filterable virus	Dmitri Ivanovsky → tobacco mosaic disease
Contagium vivum fluidum	Martinus Beijerinck
Crystallization of virus	Wendell M. Stanley → Tobacco mosaic virus
Bacteriophage discovery	Frederick Twort + Félix d'Hérelle
Retrovirus + reverse transcriptase	Howard Temin + David Baltimore

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MAIN CHARACTERS

Table 1: Living vs Non-living Characters

Character	Feature
Non-living	• Acellular • No cytoplasm / ribosome / independent metabolism • Inactive outside host • Can be crystallized
Living	• Contains genetic material • Replicates inside living host cell • Mutation + heredity + evolution • Host specificity

Core Definition

Virus = obligate intracellular acellular infectious agent containing either DNA or RNA inside a protein coat ± envelope

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MORPHOLOGY

▢ Size:

Table 1: Virus Size

Type	Virus / Point
Usual range	20–300 nm
Smallest	Foot-and-mouth disease virus
Among smallest animal DNA viruses	Parvovirus
Largest classical virus	Poxvirus / Variola virus

▢ Shape:

Table 1: Virus Shape

Shape	Example
Brick-shaped	◉ Poxvirus / Smallpox virus:
Spherical / pleomorphic	◉ Influenza virus:
Bullet-shaped	◉ Rabies virus:
Rod-shaped / helical	◉ Tobacco mosaic virus:
Polyhedral / icosahedral	◉ Adenovirus:
Tadpole-like / complex	◉ Bacteriophage:

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STRUCTURE OF VIRION

▢ Basic Plan:

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Virion = complete mature infective virus particle
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Genome + capsid = nucleocapsid
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Nucleocapsid ± envelope ± matrix / tegument ± enzymes
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No cell membrane, cytoplasm, nucleus, mitochondria or ribosome

▢ Envelope:

Table 1: Viral Envelope

Feature	Detail
Presence	Only in enveloped viruses; absent in naked viruses
Origin	Host-cell lipid membrane acquired during budding
Components	Lipid bilayer + viral glycoprotein spikes / peplomers
Functions	• Attachment to host receptor • Entry by membrane fusion • Antigenic property • Release by budding
Sensitivity	Heat, drying, ether, alcohol, detergents, bile
Transmission tendency	Usually close contact, droplets, blood, sexual route; poor environmental survival
Examples	Influenza, HIV, Herpes, Hepatitis-B, Rabies, Coronavirus

Naked Viruses

No lipid envelope → relatively resistant to drying, acid, detergents and environmental conditions; examples: Adenovirus, Poliovirus, Papillomavirus, Parvovirus, Reovirus

▢ Capsid:

Table 1: Capsid

Feature	Detail
Definition	Protein shell surrounding viral genome
Subunits	Capsomeres
Functions	• Protects nucleic acid • Gives shape / symmetry • Helps attachment in naked viruses • Major antigenic component • Assists delivery of genome into host
Symmetry	Helical / Icosahedral / Complex

▢ Nucleic Acid:

Table 1: Viral Genome: General Features

Feature	Detail
Type	Either DNA or RNA; never both in one virion
Strands	Single-stranded or double-stranded
Form	Linear or circular
Segmentation	Segmented or non-segmented
RNA polarity	Positive-sense, negative-sense or ambisense
Function	Carries genetic information + controls replication and synthesis of viral proteins

▢ Additional Components:

Table 1: Other Viral Components

Component	Function / Example
Matrix protein	Links nucleocapsid with envelope; supports assembly
Tegument	Protein layer between capsid and envelope in herpesviruses
Spikes / peplomers	Attachment + antigenicity; e.g., influenza HA and NA
Viral enzymes	Needed when host cell cannot supply them; e.g., reverse transcriptase, RNA-dependent RNA polymerase, neuraminidase

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CAPSID SYMMETRY

Table 1: Types of Viral Symmetry

Type	Arrangement	Examples
Helical	Capsomeres arranged spirally around nucleic acid	TMV, Rabies, Influenza nucleocapsid
Icosahedral / Cubic	20 triangular faces + 12 vertices	Adenovirus, Poliovirus, Herpesvirus
Complex	Neither purely helical nor icosahedral	Bacteriophage, Poxvirus

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NUCLEIC ACID TYPES

Table 1: Viral Nucleic Acid Types

Type	Examples / Special Point
dsDNA	• Herpes simplex virus • Varicella-zoster virus • Epstein-Barr virus • Adenovirus • Poxvirus / Variola virus • Papillomavirus • Cauliflower mosaic virus
Partially dsDNA	• Hepatitis-B virus
ssDNA	• Parvovirus • Circovirus • Bacteriophage φX174
dsRNA	• Reovirus • Rotavirus • Wound tumour virus • Rice dwarf virus
+ssRNA	• Poliovirus • Rhinovirus • Hepatitis-A virus • Hepatitis-C virus • Hepatitis-E virus • Dengue virus • Yellow fever virus • Coronavirus • Tobacco mosaic virus
−ssRNA	• Influenza virus • Measles virus • Mumps virus • Rabies virus
Diploid +ssRNA with reverse transcriptase	• HIV / AIDS virus • Rous sarcoma virus
Circular −ssRNA defective virus	• Hepatitis-D virus → requires Hepatitis-B virus

Exam Rule

Most DNA viruses replicate in nucleus; Poxvirus replicates in cytoplasm. Most RNA viruses replicate in cytoplasm; Influenza and Retrovirus require nuclear phase.

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BACTERIOPHAGE

▢ Definition: Virus infecting bacteria

▢ Example: T-even phage / T4 bacteriophage infecting Escherichia coli

▢ General Features:

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Complex symmetry
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Head contains genome
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Tail apparatus attaches to bacterium + injects genome
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T4 phage genome = linear dsDNA
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Protein coat remains outside bacterial cell during penetration

▢ Structure:

Table 1: T4 Bacteriophage Structure

Part	Structure / Function
Head / Capsid	Icosahedral protein coat enclosing dsDNA
Genome	Linear double-stranded DNA
Neck / Collar	Connects head with tail
Tail sheath	Contractile protein sheath
Tail core / tube	Passage for injection of phage DNA
Base plate	Anchors phage to bacterial surface
Tail fibres	Recognize specific bacterial receptors + adsorption
Tail pins / spikes	Pierce bacterial cell wall

▢ Types:

Table 1: Types of Bacteriophages

Type	Also Called	Outcome
Virulent phage	Lytic phage	Immediate replication → host-cell lysis
Temperate phage	Lysogenic phage	Genome integrates as prophage; may later enter lytic cycle

Specificity

Tail fibres determine host specificity by binding receptors on bacterial surface.

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LIFE CYCLE OF VIRUS

▢ General Viral Replication:

Table 1: General Steps

Step	Process
1. Attachment / Adsorption	Viral attachment protein binds specific host-cell receptor
2. Penetration / Entry	Fusion, endocytosis or genome injection
3. Uncoating	Capsid removed → viral genome released; absent in bacteriophage because capsid remains outside
4. Biosynthesis / Eclipse	Early proteins + genome replication + late structural proteins
5. Assembly / Maturation	Genome packaged into capsid → new virions
6. Release	Cell lysis in many naked viruses / bacteriophages; budding or exocytosis in many enveloped viruses

▢ Lytic Cycle of Bacteriophage:

Table 1: Lytic Cycle

Step	Key Event
Adsorption	Tail fibres attach to specific receptors on bacterial cell wall
Penetration	Tail sheath contracts; phage DNA injected; capsid remains outside
Host takeover	Host DNA may be degraded; host machinery redirected
Biosynthesis	Phage DNA replicates + phage proteins synthesized
Assembly / Maturation	Head, tail and fibres assembled; DNA packed into head
Lysis / Release	Phage lysozyme breaks bacterial wall → progeny phages released

Terms

❖ Eclipse period: No complete infective virion detectable inside host

❖ Latent period: Time from adsorption to release of new virions

❖ Burst size: Number of phages released from one infected bacterium

▢ Lysogenic Cycle of Bacteriophage:

Table 1: Lysogenic Cycle

Step	Key Event
Adsorption + penetration	Temperate phage injects genome
Integration	Phage DNA integrates into bacterial chromosome
Prophage formation	Integrated phage DNA = prophage
Replication with host	Prophage copied during bacterial cell division without lysis
Induction	UV light, chemicals or stress activate prophage
Entry into lytic cycle	Phage DNA excises → biosynthesis → assembly → lysis

Lysogenic Conversion

Prophage genes may give new properties / toxin production to bacteria; examples: diphtheria toxin, cholera toxin, erythrogenic toxin.

▢ Lytic vs Lysogenic:

Table 1: Lytic vs Lysogenic Cycle

Feature	Lytic Cycle	Lysogenic Cycle
Phage type	Virulent	Temperate
Genome integration	Absent	Present as prophage
Host-cell lysis	Rapid	Absent initially
Progeny virions	Produced immediately	Produced after induction
Host survival	Cell dies	Cell survives and divides

▢ Flowchart: Attachment → Penetration → Uncoating → Biosynthesis → Assembly → Release

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TERMS

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Prion

▢ Definition: Proteinaceous infectious particle lacking nucleic acid

▢ Key Point: Abnormally folded protein inducing misfolding of normal host protein

▢ Properties:

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Smallest known infectious agent
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Resistant to routine sterilization, heat and nucleases
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Causes slowly progressive fatal neurodegeneration

▢ Diseases:

Table 1: Prion Diseases

Disease	Host / Key Point
Creutzfeldt-Jakob disease	Human → rapidly progressive dementia
Kuru / laughing disease	Human
Fatal familial insomnia	Human
Bovine spongiform encephalopathy / Mad cow disease	Cattle
Scrapie	Sheep + goats

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Viroid

▢ Definition: Small circular naked single-stranded RNA without capsid

▢ Host: Plants

▢ Diseases:

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Potato spindle tuber disease / PSTV

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Chrysanthemum stunt

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Citrus exocortis

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Cucumber pale fruit

Examples

❖ Potato Spindle Tuber Viroid: Stunting + elongated malformed tubers + cracking

❖ Citrus Exocortis: Bark scaling + stunted growth + reduced yield

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Virusoid

Small satellite RNA requiring a helper virus for replication and encapsidation

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Satellite virus

Virus dependent on helper virus; example: Hepatitis-D virus depends on Hepatitis-B virus

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Virion

Completely assembled mature infective virus particle outside host cell

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Provirus

Viral genome integrated into eukaryotic host DNA

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Prophage

Phage genome integrated into bacterial chromosome

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Nucleocapsid

Viral nucleic acid + capsid

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PLANT DISEASES CAUSED BY VIRUS

Table 1: Important Viral Diseases of Plants

Disease	Plant / Virus
Tobacco mosaic	Tobacco mosaic virus
Cauliflower mosaic	Cauliflower mosaic virus
Dahlia mosaic	Dahlia mosaic virus
Papaya leaf curl	Papaya
Potato leaf roll	Potato leafroll virus
Tomato leaf curl	Tomato leaf curl virus
Yellow vein mosaic	Bhindi / Okra
Vein banding mosaic	Potato
Bunchy top	Banana bunchy top virus
Tungro disease	Rice tungro virus complex

Commonly Confused Non-viral Diseases

❖ Grassy shoot of sugarcane: Phytoplasma

❖ Little leaf disease: Usually phytoplasma

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HIGH-YIELD ONE-LINERS

Table 1: CEE Rapid Revision

Question Point	Answer
Complete infective viral particle	Virion
Genome + capsid	Nucleocapsid
Capsid subunit	Capsomere
Virus infecting bacteria	Bacteriophage
Integrated phage DNA	Prophage
Integrated viral DNA in eukaryotic host	Provirus
Protein-only infectious agent	Prion
Naked infectious plant RNA	Viroid
Helper-virus-dependent RNA	Virusoid / satellite RNA
Phage causing immediate lysis	Virulent / lytic phage
Phage capable of lysogeny	Temperate phage
Release of enveloped virus	Usually budding
Release of phage / many naked viruses	Usually lysis
Virus with complex symmetry	Bacteriophage / Poxvirus
DNA virus replicating in cytoplasm	Poxvirus
RNA virus with important nuclear replication phase	Influenza virus
Virus containing reverse transcriptase	Retrovirus + Hepatitis-B virus

Q1.

Tobacco mosaic disease is caused by a:

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