Concept of virology Viruses Types of viruses Viral characteristics Virion Size and Shape Structure Replication Viral Variation Classification Presentation BEST OF LUCK

1. VIRUSES

2. VIRUSES
A virus is a super molecular complex that can replicate
within appropriate host cells. It is a sub-microscopic,
obligate, infective and filterable entity which is nucleo-
proteinic in nature. It is placed in mesobiotic group of plant
pathogens. They consist of DNA or RNA surrounded by a
protein capsid or membranous envelope.
Viroid
It is a smaller, low molecular weight ribo-nucleic acids that
can infect plant cells, replicate themselves and cause
diseases contain naked RNA having no protein coat.
Virion
A complete infectious virus particle is called virion.

3. Viral morphology/ Life Form
Virus consists of nucleic-acid and protein. Protein forms a
protective coat around the nucleic acid called capsid. If lipid
layer is present in protein coat of virus it is called envelop.
Viruses are called obligate parasites because these replicate
(multiply) only within living host but not on artificial media.
Prion
It is the viral protein which is infectious in nature (similar to
virus, lacking nucleic acid.
Morphological forms of viruses
1 Rod-shaped e.g. TMV
2 Flexible thread e.g. Maize Dwarf Viruses
3 Geminate or Circular e.g. CLCuV
4 Rhabdo-shaped e.g. Cucumber Mosaic Viruses

4. RNA DNA
Potyvirus
Badnavirus
Tobamovirus
Geminivirus

5. Physical properties
Non-cellular
Sub-microscopic having different number of particle type
(mono, bi, tri or multiple )
Live inside the host but some can remain infective outside the
host e.g. TMV can remain infective for 50 years outside the
host.
pH stability
composition of protein
Chemical properties
Type of nucleic acid (RNA or DNA, ss or ds, +tive or –tive
strand)
A plant virus may consist of 5-40% nucleic acid and
60-95% protein

6. CLASSIFICATION OF VIRUSES
• All viruses are belonged to this phylum. It
has two sub phyla:
1. Sub phylum Deoxyvira
2. Sub phylum Ribovira

7. • The sub phylum includes viruses with DNA
core. It has three classes.
• Class Deoxyhelicase
• Class Deoxycubica
• Class Deoxybinala
1. Sub phylum Deoxyvira

8. • This class includes DNA viruses.
• They have helical symmetry.
• This class has only one order Chaetovirales
• It has single family Poxviridae
Class Deoxyhelicase

9. • This class contains DNA viruses.
• They have cubical symmetry.
• It has two orders:
• (a) Haplovirales: They are without envelop.
It has six families.
• (b) Peplovirales: They are with envelop. It
has only one family, Herpesviridae
Class Deoxycubica

10. • The class possesses DNA containing binal
viruses.
• They have single order Urovirates.
• It has only one family Phagoviridae.
Class Deoxybinala

11. • The sub phylum includes viruses with RNA
core. It has two classes.
• Class Ribohelica
• Class Ribocubica
2. Sub phylum Ribovira

12. • This class includes RNA viruses.
• They have helical symmetry.
• This class include two orders.
• Rhabdivirales:
• They are without envelopes.
• It has six families.
• Sagovirales:
• They have envelope.
• It has three families.
Class Ribohelica

13. • The RNA viruses which can convert their
single stranded RNA in to double stranded
DNA are called retroviruses.
• These viruses has special enzyme called
reverse transcriptase.
• This enzyme is responsible for the
conversion of RNA in to DNA.
• This DNA infects the host cells.
• It can also incorporate in to the host genome
as Provirus.
Retroviruses

14. • This retrovirus can pass to the daughter
cells. Therefore, some of the retroviruses
can convert normal cells in to cancer cells.
• Most of the retroviruses are specific.
• They infect only those host cells which
possess required receptors.
• These receptors allow the adsorption and
penetration of these viruses.
Cont…..

15. Two types of retroviruses are most common.
1. RNA tumor viruses
2. HIV Viruses
Types of Retroviruses

16. • These are single stranded RNA retroviruses
or oncoviruses.
• Their diameter is about 100nm.
• Their shape is spherical.
• They are enveloped by plasma membrane of
the host.
• They produce tumor in a number of animals
like fowl, rodents and cats.
1. RNA tumor viruses

17. • The most familiar virus is the Human
immunodeficiency virus (HIV).
• It causes acquired Immune deficiency
syndrome (AIDS).
HIV Virus

18. • Each type of virus infects only a limited range of
host. It is called its host range.
• Viruses identify their host by a lock and key.
• There are specific receptor molecules on the host
cells.
• Similarly, some proteins are present on surface of
virus.
• These receptors and proteins are involved in lock
and key attachment.
HOST RANGE

19. • Some viruses have broad host range.
• They infect on several species of host.
• For example, rabies virus can infect number of
species.
• Some viruses have narrow host range.
• They infect only single specie.
• For example, phage virus infects only E.coli.
Cont….

20. • There are following stages of infection and replication of
viruses.
• 1. Penetration:
• It is the first step of infection.
• Mechanism of penetration is different in different species.
• For example, phage uses their tail apparatus to inject DNA in
to bacterium.
• Their tails have special enzyme lysozyme.
• It breaks the cell wall and cell membrane of bacterium.
• In some cases, complete virion enters into the host like AIDS
virus.
Stages of virus replication

21. • Viral DNA controls the metabolic
machinery of the host.
• It reprogrammes the viral genes and
manufactures its capsids.
• Most DNA viruses use DNA polymerase of
the host cell to synthesize its new DNA.
• DNA viruses contain their enzymes to
initiate replication with in the host.
• Host has no enzyme to copy RNA.
2.Replication

22. • The virus uses resources of host for viral
production.
• Host provides the nucleotides for nucleic
acid synthesis.
• Virus uses its enzymes, ribosomes, tRNAs,
amino acids and other machinery.
Cont…

23. • Viral nucleic acid molecules and capsids
produced.
• Their assembly in to new viruses is a
spontaneous process. This process is called
self assembly.
• RNA and capsomeres of TMV can be
separated in the laboratory. If its contents
are mixed it spontaneously reassemble
itself.
3. Assembly of virion:

24. • Hundred or thousands of viruses emerge from host cell.
• The host cell is often destroyed in the process.
• The destruction of host cells cause some symptoms of
viral diseases.
• For example, cold and influenza viruses destroy destroy
the mucous membrane in respiratory passage.
• It causes symptoms of cold and influenza.
• The newly formed viruses infect other cells and in this
way a new cycle started.
4. Release of virions:

26. Transmission / Entry of viruses into the host/ Dissemination
Because plant cell walls are so thick compared to the sizes of the
viruses infecting them, plant viruses have not evolved mechanisms
similar to those of other microorganisms for entering their host cells.
The only ways that viruses can enter plant cells to cause a primary
infection are via:
Through wounds purely mechanical injury that breaches the cell wall
and the plasma membrane of underlying cells;
Through natural openings e.g. stomata
Through vector Injection directly into cells through the piercing
mouthparts of sap-sucking insects or nematodes;

27. Through other pathogens Carriage into plant tissue on or in
association with cells bodies of a fungal, bacterial and nematode
parasite
Through seed Vertical transmission through infected seed or by
vegetative propagation;
Transmission via pollen;
Pruning and Grafting of infected tissue onto healthy tissue.