• Viruses (L., venom or poisonous fluid)
• Viruses are submicroscopic biological entities which lack cellular
organization (viz., plasma membrane and metabolic machinery)
• Possess their own genetic material.
• Essentially require host for their multiplication i.e., they are obligate
cellular parasites of either bacteria, plants or animals
• Size- range in between 30-300nm or 300-3000Ao. They can be observed by
electron microscope or X-ray crystallography.
• Basically an infectious virus particle (called virion) is composed of a core
of only one type od nucleic acid (DNA and RNA) which is wrapped in a
protective coat of protein called capsid.
• Capsid consists of numerous capsomers (capsomer is composed of one or
more polypeptide chains).
• Specific arrangement of capsomers determines the shape of virion.
• Viruses have following three different types of symmetry
1) Icosahedral symmetry - many viruses have spherical, cubical or polygonal
shape which is basically icosahedral or 20 sided. e.g., bacteriophage, turnip
yellow mosaic virus (TYMV)
2) Helical or cylindrical symmetry- tobacco mosaic virus (TMV),
bacteriophage M13 and influenza virus have such symmetry. Consists of
numerous identical capsomers arranged in a helix.
3) Complex symmetry -viruses with complex shaped capsid are of two shapes :
those without identifiable capsids (e.g., pox viruses such as vaccinia,
cowpox), rabies virus is bullet shaped. Some virus like influenza virus and
mump virus are surrounded by a 100-150Ao thick spiked membrane. This
membrane consist of lipid bilayer of plasma membrane.
6. Are viruses living entities?
• There is no clear answer to this question.
• If life is defined as being cellular then viruses are not alive.
• If life is defined as being capable of making new life through own
metabolic efforts then viruses are not living.
• If life is defined as being able to specify each new generation according to
its own genetic instructions then viruses are living systems.
7. Naming and classification
• Viruses are not named according to the method of binomial nomenclature (like
genus species name of plant, animal, bacteria and algae)
• Viruses are named randomly like according to disease caused (poliomyelitis
virus), host organism (bacteriophage), plant viruses, animal viruses or some
coded system T1, T2 phages, nature of nucleic acid (DNA viruses, RNA
viruses). T2 phage is ds DNA phage.
1) Bacterial Viruses or bacteriophages- viruses that parasitize the bacterial
cells, are called bacteriophages or phages (phage means to eat). The phages
have specific host. The most widely studied phages are T-even
bacteriophages such as T2, T4,T6 etc which infects E. coli are also known as
8. o T4 bacteriophage- contains DNA molecule of 60µm long, linear double
stranded and tightly packed within the head. DNA contain 1,66,000bp and
encoded 200 different proteins (protein involved in DNA replication and
assembly of head and tail) and enzymes like helicases, topoisomerase,
DNA polymerases, DNA ligases etc.
o During replication an unusual nitrogen base called
5-hydroxylmethylcytosine is incorporated in place of cytosine in the phage
DNA. This unusual base makes phage DNA incorporated in place of
o This unusual base makes phage DNA recognizable from that of host DNA
and selectively protects it from nuclease enzymes.
9. Life cycle of the bacteriophage
• Life cycle of the bacteriophage – bacteriophage may have the following two
types of life cycles.
i) Lytic cycle- in which viral infection is followed by lysis (bursting and death)
of the host cell and release of new infective phages. Phages with lytic cycle are
known as virulent phage e.g., virulent phage such as T4 and other T even
ii) Lysogenic cycle- in this type infectious rarely causes lysis or causes delayed
lysis e.g-P1 and lambda phages. A virus with lysogenic cycle called temperate
virus. In lysogenic phage cycle integration of viral genome into bacterial genome
is often termed as prophage or provirus
10. Plant viruses
• All plant viruses consist of ribonucleoprotein in their organization. (except few
exceptions like cauliflower mosaic virus (DNA))
• Important plant viruses are tobacco rattle virus (TRV), tobacco mosaic virus
(TMV), potato virus, turnip yellow virus (TYV).
• Citrus tristeza is longest among plant viruses
• Plant viruses mainly spread by insects such as aphids, leaf hoppers and beetles
• Tobacco mosaic virus is the most extensively studied plant virus. It was
discovered by Iwanowski and crystallized by Stanley
• TMV is rod shaped RNA (single stranded) virus and are cigarette like shaped
11. Animal viruses
• Animal viruses mainly infects animal cells and cause different diseases in
animals including man. Contains genetic material in the form of DNA or
• The protein coat or capsid of animal viruses is surrounded by an envelope.
• Poliomyelitis is a most extensively studied RNA containing animal virus.
• Common viral diseases of animals- common cold, influenza, mumps,
chicken pox, small pox, polio viral hepatitis. Etc
• Pox virus is largest virus among animal viruses
12. Life cycle of animal virus
• Like bacteriophages, animal viruses have two types of life cycle.
i) Permissive growth- permit an animal virus to multiply lytically and kill
the host cell.
ii) Non-permissive growth-which permits an animal virus to enter the host
cell but does not allow it multiply lytically, virus genome replicates in
controlled fashion without killing the cell.
13. • Spikes of animal viruses (specially RNA viruses) are made up of
• Spike like projections on the capsid are known as Peplomers
• Animal viruses like influenza viruses are surrounded by phospholipid
• Viroids are small RNA circles of 300-400 nucleotides long lacking AUG
• They are replicated autonomously despite the fact that they do not code for
• No protein coat no capsid exist as naked RNA molecule and passes plant to
plant when the surface of plant are damaged so there is no membrane
• The term viroid was coined by Diener (1971) who discovered first viroid
called Potato spindle tuber viroid (PSTV)
• Prions are rod shaped proteinaceous particle though to cause a number of
diseases in animals such as Scrapie disease
• Causes Kuru disease of brain in human being
• Prions are named by S. B. Prusiner (1984)
• They are resistant to heat, ionizing radiations, UV radiations, chemicals
like phenols and nuclease enzymes
• There is no clear evidence regarding prions replication or multiplication. It
is though that they performs reverse translation (turning Protein to RNA
and RNA to DNA)