We all agree that no human being on earth ends up his life without being caught by a common cold. There is a saying regarding common cold that cold retreats in a week if given medicine and retreats in seven days if not given any medicine. Perhaps the strongest enemy that man could not conquer yet is this small creature that causes cold. What causes common cold? Is there any link between the cause of cold and the cause of AIDS? Let us try to find answers.
Is there any connecting link between non-living and living world? We may not assume a creature which exhibits both living and non-living characters. In the previous chapter we have discussed about the smallest living cells, the bacteria in detail. There are certain infectious particles which are smaller than bacteria but show characters of both living and non-living. They are the “Viruses”.
What are viruses?
* Viruses are infectious particles that exhibit certain living characteristics.
* They are not composed of cells and smaller than the smallest bacteria.
* Viruses show living characters like
Presence of DNA/RNA
Genetic continuity through multiplication
Undergo mutations
* They also show non-living characters like
They do not exhibit life processes.
They remain inert lifeless molecules outside the host cell.
They cannot multiply outside the host cells.
* Hence viruses are better referred as virus particles which are chemically made up of a protein coat and a nucleic acid either DNA or RNA but never both together.
* The nucleic acid forms the genome and the protein coat is called capsid.
* The nucleic acid in a virus may be single stranded RNA or double stranded RNA, double stranded DNA or single stranded DNA.
* ssRNA is seen in viruses that infect plants.
* dsDNA is found in animal viruses and bacteriophages.
* Some viruses like HIV have two identical molecules of RNA.
* The capsid is made up of protein sub units called capsomeres.
* A ‘virion’ is a completely assembled virus outside the host cell.
* A ‘viroid’ is a tiny fragment of nucleic acid with 300 to 400 nucleotides without a protein coat. Viroids cause diseases in tomatoes, potatoes, cucumbers and citrus.
* Some ‘proteinaceous infectious particles’ called prions cause serious diseases in animals.
* Some viruses may also contain lipids and carbohydrates.
* Viruses are obligate parasites causing diseases in plants, animals and human beings. They also infect bacterial cells.
Important discoveries in virology:
1. Dmitri Ivanovsky - filterable agent caused Tobacco Mosaic Disease.
2. Martinus Beijerinck - Contagium vivum fluidum (contagious living fluid).
3. W.M. Stanley - Crystallized virus that causes Tobacco Mosaic Disease.
4. Fraenkel Conrat - Genetic material of TMV is RNA.
Classification of viruses:
International committee on Taxonomy of Viruses (ICTV) regulates the classification and nomenclature of viruses.
Viruses are usually named after the diseases they cause.
e.g.: Polio virus, Influenza virus.
AIDS virus belongs to the family Retroviridae, genus Lentivirus and species Human Immune deficiency virus.
The size of viruses may vary from 20nm to 300 nm.
Viruses are classified according to symmetry into the following types:
* Helical viruses – rod shaped – e.g.: Rabies virus and Tobacco mosaic virus.
* Polyhedral viruses – many sided – e.g.: Herpes simplex and Polio virus.
* Enveloped viruses – spherical – e.g.: Influenza virus.
Measles virus shows envelop with glycoprotein spikes that help in attachment to receptors on host cells.
* Complex viruses – polyhedral in head and helical in tail – e.g.: Bacteriophages.
Structure of TMV (Tobacco Mosaic Virus)
Tobacco mosaic virus causes mosaic disease in Tobacco.
It is rod shaped with helical symmetry.
The size is approximately 300 nm long and 18 nm in diameter with a molecular weight of
39 × 106 Daltons.
The capsid is made up of 2,130 capsomeres which are arranged in a helical manner around the hollow core of 4 nm.
Each capsomere is made up of a polypeptide chain with 158 amino acids.
The spirally coiled RNA contains 6500 nucleotides.
Structure of T4 Bacteriophage:
T4 bacteriophage resembles a tad pole in its shape.
It shows a head and a tail joined by a collar.
The head shows polyhedral symmetry and tail shows helical symmetry.
The head region encloses DNA.
The tail is covered by a tail sheath and ends with a base plate with tail pins and tail fibres.
The tail pins and fibres help in attachment to the host cell.
The tail sheath helps in injecting the viral DNA into the host cell.
Reproduction/Replication/Multiplication of viruses:
Viruses like bacteriophages multiply by any of the two methods lytic cycle and lysogenic cycle.
Lytic cycle:
Lytic cycle caused lysis or death of host cell.
T – even phages which cause lysis are called virulent phages.
Lytic cycle involves 5 steps:
1. Attachment or Adsorption
2. Penetration
3. Bio synthesis
4. Maturation
5. Release
1. Adsorption: It is the attachment of phage particle to the bacterial host cell on complementary receptor sites.
2. Penetration: The tail sheath of the phage contracts and injects the DNA into the bacterial cell. The capsid of the phage remains outside the host cell as a ghost.
3. Bio synthesis: As the phage DNA enters the host cells it uses the cell machinery and synthesises phage DNA and capsid proteins. But the complete virus particles do not appear hence called eclipse period.
4. Maturation: During this phase bacteriophage DNA and capsids assemble into complete virions.
5. Release: At this stage the viral enzyme lysozyme produced in the host cell dissolves the plasma membrane and breaks the cell wall to release the newly synthesized phage particles or virions. This phase is hence called lysis phase. The number of newly synthesized phage particles released from a single host cell is called “burst size”. (50 to 200).
Lysogenic cycle:
Lysogenic cycle does not cause lysis of death of host cell.
The phage DNA (of λ phage) on entering into host cell gets integrated with the host DNA and remains inactive. Such phages are called ‘temperate phages’.
The inserted phage DNA is called ‘prophage’.
The prophage replicates along with the host DNA but remains latent.
Rarely the phage DNA gets separated from the bacterial DNA and initiates lytic cycle.
Thus lysogenic cycle facilitates transduction of bacteria.
Viral diseases in Plants: (symptoms)
1. Chlorosis – Peach yellow disease
2. Mosaic – Tobacco mosaic disease
3. Vein clearing – Bhendi vein clearing
4. Malformation – Swollen shoot of cocoa
5. Breaking of flowers – Tulip mosaic break
Viral diseases in humans and animals:
Common cold
Hepatitis
Chicken pox
Influenza
Herpes
Warts
Polio
Rabies
AIDS
Ebola
Neonatal rubella
Cancer causing viruses (oncoviruses) – Epstein-Barr virus, Human Papilloma virus and Hepatitis B virus.
Prions cause mad cow disease (Bovine Spongiform Encephalitis) in cows and scrapie disease in sheep.
Creutzfeldt-Jakob disease in man is caused by mad cow disease causing prion.
Additional reading on viruses
* The word virus means “Poison”.
* Edward Jenner discovered vaccine for small pox.
* Louis Pasteur discovered vaccine for Rabies. He also proposed germ theory and opposed theory of spontaneous generation.
* Andre Lwoff defined virus as “A virus is a virus”.
* Gierrer and Schramm proved that the nucleic acid of a virus is pathogenic (disease causing).
* The viruses that infect yeasts are called zymophages.
* The viruses that attack fungi are called mycophages.
* The viruses that attack cyano bacteria are called cyanophages.
* The rhabdo virus that attacks animals are bullet shaped, while rhabdo virus that attacks plants are rod shaped.
* The Φ × 174, M13 bacteriophages have single stranded DNA (ssDNA).
* Wound tumour virus, Maize dwarf virus, Rice dwarf virus, Reo virus, Blue tongue virus, bacteriophage – Φ 6 contain double stranded RNA (dsRNA).
* Potato spindle tuber and citrus exocortosis diseases are caused by viroids.
* The word bacteriophage was proposed by Felix deHerilli.
* The coliphage λ contains a head composed of DNA but do not have contractile sheath on the tail region.
* Zephenima and Longidorus species of nematodes transmit viruses which are soil borne.
* The fungus Olphidium brassicae transmit TNV (Tobacco Necrosis Virus) through its zoospores.
