· Virology: the study of virus.
· Father of virology: Stanley
· Link between living and non-living.
· Simplest and smallest.
· Non-cellular infectious entities.
· Also called a super parasite.
· Nucleo-proteinaceous entities (particle).
· Tobacco mosaic was recognized to be caused by the virus.


· First detected by W. Meyer.
· First discovered by D.J. Ivanosky (Russian scientist) and was TMV (Tobacco mosaic virus)
· Term “virus” was given by Louis Pasteur. But some authors believe Beijernick gave the term virus but in fact, he used the term “Contagium vivum fluid”
· Smallpox and polio are considered prehistoric diseases because they are known since 6th BC
· Smallpox was discovered by E. Jenner and polio by sabin et al.

Characteristics Feature of Virus

(a) Living properties
· Contain either DNA or RNA
· Obligate intracellular parasite
· Can multiply and mutate inside living host.
· Controls the enzymatic system of the host when they enter into the living host.
· Obligate parasite
· Infectious

(b) Non-Living properties

· Lack protoplasm – the physical basis of life
· Absence of enzyme system and energy storing system.
· Absence of respiration and metabolism
· They can be crystallized
· In active outside living cell and called a virion.
· Viruses can reproduce only inside living cells and they use host cells for their multiplication.
· Viruses are also called Biological puzzles.
· In Latin virus means 'poison'.
· They can not be termed as prokaryotes or eukaryotes.
· They lack cellular structure. In fact, the virus is a particle neither living nor non-living.
· It is a non-cellular infectious entity.
· Virus is the connecting link between living and non-living organisms.
· They reproduce only inside a living host.
· They are the smallest living organisms (20 nm to 300 nm), which are 50 times smaller than bacteria.
· Study of the virus is called virology.
· They have no metabolic activity of their own but when it enters inside the body of the host, it controls the enzyme-producing system or metabolic system of that living host.
· Viral genomes are nucleic acids either DNA or RNA which are surrounded by a proteinaceous coat.
· Tobacco mosaic was the first disease recognized to be caused by viruses.
· They can't reproduce outside living cells (photobiotic agent). They use the biosynthetic machinery of the host cell for their multiplication.
· Phytophage - plant virus
· Virus requires a vector for transfer from one host to another since they lack protoplasm and motility.
· Arbo virus - virus having arthropod as vector or intermediate host.

Viruses that attack:
· Virus which attacks bacteria is termed as Bacteriophage.
· Virus attacking algae is – Phycophages.
· Virus attacking fungi is – Mycophage
· Virus attacking BGA is – Cynophage
· Virus attacking E. Coli – Coliphage
· Virus attacking yeast – Zymophage

Various Shape of Virus:
Brick shaped – smallpox virus
Spherical – influenza virus
Rod-shaped – TMV
Tadpole like – Bacteriophage virus
Bullet-shaped – Rabies virus
· Viruses usually infect the whole plant except Apical meristem. So apical meristem of even virus-infected plant is free from viruses. So virus-free plants can be obtained by shoot tip culture.
· Lysozyme is the enzyme secreted by the tail fibre of bacteriophage.
· Smallest virus is coliphage F2 (about 2 nm)
· Smallest plant virus – satellite tobacco necrosis virus.
· Largest known plant virus – citrus Tri-stezea virus.
· Poxvirus is the largest and most complex animal virus.

· Virus range in size from 10 - 300 nm.
· Smallest virus: Coliphage F2 (2 nm)
· Smallest plant virus: Satellite tobacco necrosis virus (17 nm).
· Longest plant virus: Citrus Tristeza virus
· Smallest animal virus: Foot and mouth virus of cattle (10 nm)
· Largest and most complex animal virus: pox virus (400 nm)

· They have a simple nucleo-proteinous structure consisting of the central core of genetic materials either DNA or RNA.
· A proteinaceous coat (the capsid) made up of small units (the capsomers) surrounds the nucleoid (i.e.either DNA or RNA)
· Nucleic acid in a virus may be double-stranded (SS) or single-stranded (S).
· Virus lack cytoplasm (Physical basis of life) and enzyme-producing glands.

Fig: Showing viral cell

· It is the outer covering and protecting layer of virus made from viral protein.
· Each unit of protein is called a peplomer.
· Envelope may be absent in virus.
· If an Envelope is absent in a virus it is called a nacked virus eg. TMV.
· Surface of the envelope may be smooth or may possess outgrowth called spikes.

· Capsid is the proteinous sheath that coves the nucleoid. It lies just inside the envelope.
· It is made up of small proteinous unit known as capsomers which determines the types of virus. The number of capsomers in the virus is fixed. 
Eg TMV-2130 capsomers. Possess antigenic property.
Function: Capsid helps in host specification
· In some mammalian viruses, outside there is another membrane, which is known as the limiting layer or mantle.

Nucleic Acid (Infectious):
· Plant virus: Normally RNA virus
· Animal virus: Normally DNA virus
· Leucovirus: Has both RNA and DNA.

· Plant ds DNA virus:
Cauliflower mosaic virus
Dahlia Mosaic virus
Mirabilis Mosaic virus (@ CDMA)

· Animal ssRNA virus:
Mumps, measles, Yello fever virus, dengue fever Rous sarcoma virus, Retrovirus, Rhinovirus, Rubella influenza virus, poliomyelitis virus, SARS virus, Hepatitis virus (Except Hepatitis B), Japnese encephalitis virus. (@ MY RIPS)
· DNA (ssDNA): Coliphage S13, Parvovirus, Bacteriophage M13.
· RNA (dsRNA): Reo sarcoma virus, Wound tumour virus, Ricedwarf Virus, Mycophage, Penicillin virus, Rotavirus.

· RNA polymerase and Reverse transcriptase: For the replication of nucleic acid component.
· Neuraminidase: Found in Influenza and parainfluenza
Help in penetration.
· Lysozyme enzyme: Carried on the tail of bacteriophage.
Facilitates penetration of the phage into the host cell.
· Virion is the fully assembled mature particle of virus capable of causing infection.
· Animal virus usually contains DNA except for Rheovirus (dsRNA), and AIDS virus retrovirus (ss, RNA).
· Plant viruses are usually RNA except for cauliflower mosaic virus (ds DNA)
· Bacteriophage and Cynophages are mostly ds DNA while Mycophage consists of ds RNA.
· Stanley obtained the Tobacco mosaic virus (TMV) in crystalline form.
· Viruses are obligate parasites i.e.grows and reproduce only inside the living host.
· ssDNA viruses are Coliphage 174.


· Reverse transcription (also called Teminism in the honor of discoverer M.Temin): synthesis of ds DNA by ssRNA Eg: Retrovirus, Rous sarcoma virus.
· Virion: fully assembled matured particle of virus capable of causing infection. It contains both protein and nucleic acid.
· Viroids: Only nucleic acid present (RNA) naked virus.
· Virusoids: Fragment of RNA. (Normally non-infectious)
· Prions: Stanley B. Prusiner discovered certain infectious agents or slow viruses, which contains only protein is called a prion.
Prions Causes:
1. MCD (Mad cow disease)
2. Scrapie falling wool in sheep
3. Kuru-laughing disease
4. Creutzfeld jakob disease


Has two multiplication cycles.
(a) Lytic cycle: Virus is virulent (infectious type) and viral genome controls the metabolic activities of the host. In this cycle, the virus multiplies in the host cells leading to its lysis.

(b) Lysogenic: Viral DNA gets incorporated with the host DNA but remains inactive or it doesn't control the metabolic activity of the host.
· Such inactive phase is called a temperate phase or prophage of the virus.
Note: Prophase – stage of cell division.
Prophage – viral DNA incorporated with host DNA and remains at an inactive stage.
· AIDS virus infects CD4T cells or helper T cells.

· Interferon is a protein produced by virus-infected cells. Interferon binds to receptors of non-infected cells causing the cell to prepare for a possible attack by producing substances that interferes with viral replication.
· Viroids – Simplest infectious agent consists of only RNA genome without a protein coat.
Virusoids (satellite RNAs): Virusoids or incomplete viruses are structurally similar to viroids but located inside envelop of a true virus.
· Prions or slow virus: Infectious agents discovered by Stanley Prusiner, which are made up of proteins only (no DNA and RNA).
· A portion of the integrated viral DNA when expressed can transform a normal cell into a malignant one. This portion of DNA is called an oncogene.
NOTE: Oncogene – It’s a cancer-causing gene found in cells.

Generally, oncogene is found in the normal cell also but in an inactive form. If it gets active that results in cancer.

Some Viral Diseases with their Causative Agents

Viral disease Causative agent
Common cold Rhinovirus
Influenza Influenza virus
German measles (Rubella) Rubella virus
Measles Measles virus
Polio Poliovirus
Smallpox Variola virus
Chickenpox Varicella virus

Plant disease
Tobacco mosaic Tobacco mosaic virus
Papaya mosaic Papaya mosaic virus
Cucumber mosaic disease Cucumber mosaic virus
Tomato leaf curl disease Tomato leaf curl virus
· Arbovirus are arthropod-borne viruses.
· Edward Jenner discovered a vaccine against smallpox.

Oncovirus – These are the virus causing cancer.
· Hep. B. HIV and Herpes simplex virus are sexually transmitted viruses.
· Viruses are commonly transmitted in plants through sucking insects.

TMV (Tobacco Mosaic Virus):
· Discovered by DJ Ivanowski.
· Rod-shaped.
· It is having helical symmetry.
· Nucleic acid: ssRNA
· Naked (No envelope)
· Circular plant virus

Influenza Virus:
· Helical symmetry.
· 10% RNA and 90% protein.
· ‘Killed at 65ÂșC but active at low temperature.

· Genetic material dsDNA as in T2 bacteriophage and T4 bacteriophage.
Note: ssDNA in f × 174 bacteriophages, M13, and coliphage S13.
· DNA:protein = 50:50
· Naked
· Structurally tadpole-like with polygonal head and hexagonal tail.
· The number of fibres in bacteriophage is 6.
· Heaf and tail are joined by cylindrical tubular structure filled with lysozyme.
· Spike fibre which arises from each corner of the hexagonal endplate is only for attachment with the bacteria. 
· Most studied series of bacteriophage is T-series characterized by angular head and a contractile tail

Application of Bactriophage:
1. Used in studying viral infection mechanisms.
2. Used in control of certain bacterial diseases.
3. Purity of the holy Ganga is due to bacteriophages.

Fig: Showing TMV, bacteriophage and Influenza Viruses

Disease caused by a Viruses

A. Plant diseases:
 i. Tobacco mosaic disease.
 ii. Leaf curl of papaya.
iii. Potato leaf roll. 
iv. Grassy shoot of sugarcane.
v. Bunchy Top of banana
vi. Tomato leaf curl.
vii. Mosaic of sugarcane. 
viii. Tungro disease of Rice.

B. Animal Diseases:
i. Influenza
ii. Chickenpox.
iii. Smallpox
iv. Measles.
v. Kawasaki disease.
vi. Mumps
vii. Rabies 
viii. AIDS
ix. SARS.
x. Yello fever.
xi. Dengue fever 
xii. Polio.
xiii. All hepatitis virus
xiv. Conjunctivitis.
xv. Warts.   
xvi. Para-influenza.

High Yielding Points

1. Most of the animal viruses are DNA containing except:
MY RIPS: @RNA as genetic material
M = Mumps, Measles
Y = Yellow fever, Dengue fever
R = Rabies, Rhino, Reo, Retro
I = Influenza
P = Polio

2. Most of the plant viruses have RNA as the genetic materials except:
CDMA: @DNA as genetic material
C = Cauliflower mosaic virus
D = Dahlia mosiac virus
Ma = mirabilis mosaic virus

3. Double-stranded RNA viruses: @3RPMW
3R = Rotavirus, Reo sarcoma virus, Rice dwarf virus
P = Penicillin virus
M = Mycophage
W = Wound tumour virus

4. Single stranded DNA virus: @PCB
P = Parvo virus
C = Coliphage S13
B = Bacteriophage, Bacteriophage M13

5. Virus has firstly detected by Mayer.

6. Smallpox and polioviruses were known since prehistoric times.

7. Polio was the first virus to be cultured in vitro by JOHN ENDERS

8. Smallest plant virus: satellite tobacco necrosis virus (17nm)

9. Smallest animal virus: foot and mouth virus of cattle (10nm)

10. Longest plant virus: citrus Tristeza virus (200 × 12nm)

11. Largest and most complex animal virus: pox virus (400nm)

12. No. of capsomers in the virus are genetically fixed.

13. Retrovirus is always enveloped.

14. The virus without an envelope is naked. e.g.: TMV

15. The term virus was coined by: Pasteur

16. Types of the life cycle in Virus:
a. Lytic life cycle: Bacteriophage (takes 20-30 min) (T2 and T4 phage)
b. Lysogenic: l virus (l phage)

17. Various Shape of viruses:
Rod: TMV
Tadpole: Bacteriophage
Brick shape: Poxvirus
Spherical: Influenza
Bullet-shaped: Rabies
Icosahedral: Polio and Adeno

Also, Read Notes of Other Lessons of Botany:

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