2. Objectives
• Explain concept of viruses, in relation to other microorganisms
• Describe important morphological and chemical features of viruses
• Understand the salient features of viral replication cycle
• Understand methods of virus cultivation
• Describe role of viruses in cancer
4. Introduction
• Viruses are obligate intracellular parasites of human,
animals, plants, bacteria.
• They are 20-300 nm in size.
• Ultramicroscopic (seen only under electron
microscope)
5.
6. Introduction
• Filterable (Pass through bacteria proof filter)
• They do not have their own metabolic machinery.
• They can not be cultivated in nutrient media.
• They reproduce only in living cells
7. Structure of virus
• They do not have cellular structure. - acellular
• They are nucleocapsids / nucleoproteins.
• They have only one type of nucleic acid, either DNA or RNA.
8. Structure of virus
• Nucleic acid is enclosed in capsid.
• Function of capsid:
• Protection to NA
• Attachment to host cell surface
• Capsid is made up of protein.
• Capsid is made up of morphological subunits called
capsomeres.
13. Icosahedral Symmetry
• Spherical viruses
• It is a regular polyhedron with 12 vertices, 20
triangular faces and 30 edges.
• Faces – equilateral triangles
• Ex. – Poliovirus and Adenovirus.
14. Helical symmetry
• Rod shaped
• Its NA is covered by a capsid containing capsomeres arranged in
regular helix.
• Ex. – Measles, Mums, Influenza, Rabies virus and TMV.
15. Complex symmetry
A. Ex. - Poxviruses
• They consist of many layers of proteins and
lipoproteins.
• They are brick shaped.
B. Ex. - Bacteriophages
• Binal symmetry
• They have icosahedral head attached to
helical tail.
17. Nucleic acid of viruses
• They have only one type of nucleic acid.
• Viruses have either DNA or RNA never both.
• NA may be single or double stranded.
• RNA may be single or double stranded
• DNA may be single or double stranded.
19. Nucleic acid of viruses
• NA may be circular or linear.
• Proportion of NA in viruses varies from 1 % -
(Influenza virus) to 50 % (certain bacteriophages)
• No. of genes in viruses vary from 3 - 4 (picorna virus)
to several hundred (poxviruses)
• All viruses are haploid except retroviruses are diploid.
(Contain 2 identical ssRNA)
• Some viruses have segmented NA (Influenza virus – 8
segments)
21. Envelope
• Some viruses have envelope around capsid.
• Envelope: lipoprotein
• Lipid: derived from host cell membrane
• Protein: virus coded
• Glycoprotein projections may be present on surface
of envelope
• Ex. - Influenza virus: Hemagglutination
prickles (spike like) and neuraminidase
(knob like )
25. Bacteriophage
• These are viruses that infect bacteria. (Bacteria eater)
• Discovered by Twort and d’Herelle in 1917.
• There two main types of Bacteriophages.
• Virulent or Lytic bacteriophages
• Avirulent or Temperate bacteriophages
27. Bacteriophage
• Bacteriophages have binal symmetry.
• They have elongated polyhedral (Icosahedral) head.
• Head is attached to tail (Helical) by collar.
• Head encloses DNA.
28. Bacteriophage
• Tail has inner hard core tube which is covered by
contractile sheath.
• Contractile sheath contain 24 protein rings.
• At the end of tail base plate is present.
• Base plate is hexagonal.
• Tail fibers and tail pins are attached to 6 corners of
base plate.
29. Effect of physical and chemical agents
• Viruses are sensitive to physical and chemical agents
• They are easily killed by heat: 60oC for 30min
• Viruses are stored at: – 40oC to -70oC
• They are killed by – hydrogen peroxide,
formaldehyde, chlorine, iodine, ethylene oxide.
• Enveloped viruses are disrupted by lipid solvents
• Antibiotics have no effect on viruses
30. Gratitude is the fairest
blossom which springs from
the soul.
Henry Ward Beecher
Adenovirus – mild RT infection – common cold, ds DNA, iscosahedral,50 types , isolated from adenoids for first time
Common cold – 200 different types of viruses 30-35 % - Rhinivirus (ssRNA)
Parvovirus – cause infection to variety of animals from insects to mammals.
Adenovirus – isolated from adenoids - variety of diseases from mild respiratory tract (common cold ) to life threatening multi organ diseases
Small pox – Variola virus --- deadly highly contagious diseases, infected 300 million people in 20th century . 1980 global irrdication
Reovirus – icosahedral - ex. Rotavirus . Parvovirus – cause infection to variety of animals from insects to mammals.