virology lecture

1. COURSE CODE: 320
Pre-requisite: 301
Ms. Zainab Masood
VIROLOGY & PARASITOLOGY
REFERENCE TEXT BOOK:
Howley, P.M., Roizman , B., Straus, S.E., Martin , M.A., Griffin,
D.E., 2001. Fundamental Virology, Lippincott Williams &
Wilkins.

2. TODAY’S TOPIC
INTRODUCTION TO VIRUSES
• What are viruses
• Structure of viruses
• Viral characteristics
• Nature of animal viruses

4. WHAT ARE
VIRUSES?
“'A PIECE OF BAD NEWS
WRAPPED UP IN A PROTEIN

5. • Viruses are literally “parasitic” chemicals, segments of
DNA or RNA wrapped in a protein coat.
• Obligate intracellular parasites
• Acellular
• No ATP generating system
• No Ribosomes or means of Protein Synthesis

6. • Louis Pasteur and Edward Jenner developed the
first vaccines to protect against viral infections
but
• they did not know that viruses existed
• First evidence of the existence of viruses, In
1892, Dmitry Ivanovsky
• Martinus Beijerinck introduced the term "virus"
• considered to be the beginning of virology

7. • virus is an infectious agent of small size and simple
composition
• They range in size from about 20 to 300
nanometres in diameter
• (1 nanometre = 10 -9 meters).

8. Nucleic Acid
Spike
Projections
Protein
Capsid
Lipid Envelope
Virion
Associated
Polymerase

9. • Nucleic acid------Circular/linear
• Nucleic acid------ Segmented/nonsegmented
DNA RNA
Nucleic acid
SS DS SS DS

10. Conti……
• Protein coat (capsid)
• Viral proteins
Matrix protein: associated with inner layer of envelop
Fusion proteins: found on envelop surface, allow
viruses to fuse with uninfected cells

11. Conti…….
• Non structural viral proteins: enzymes present in
core of some viruses
• Envelope (lipid bilayer, proteins and glycoproteins)
• Peplomers (Spike projections)

12. • DNA dependent DNA polymerase (DNA polymerase)
An enzyme that uses DNA as a template for producing
DNA
• DNA dependent RNA polymerase (RNA polymerase)
An enzyme that uses DNA as a template for producing
RNA
• RNA dependent RNA polymerase (reverse
transcriptase)
An enzyme that uses RNA as a template for producing
DNA.

13. • The nucleic acid is either linear or circular.
• Almost all viruses are haploid except
retrovirus family, which is diploid.

14. • Nucleic acid is surrounded by a protein coat called
capsid made up of subunits called capsomers.
• The structure composed of nucleic acid genome
and capsid proteins is called nucleocapsid.

15. 1. Icosahedral: in which capsomers are arranged in
20 triangles that form a symmetric figure.
2. Helical: in which capsomers are arranged in
hollow coil that appears rod-shaped. The helix
can be either rigid or flexible.

16. conti….
3. Complex symmetry:
• The virus particle does not confirm either cubic or
helical symmetry e.g. pox virus
• All human viruses that have a helical nuleocapsid
are enclosed by an outer membrane called an
envelop

18. Cubic/icosahydral symmetry

19. Helical symmetry

20. Helical symmetry ( influenza & rabies viruses ).

21. Complex symmetry ( Poxviruses ).

23. • The capsid proteins protect the genome DNA or
RNA from degradation by nucleases.
• The surface proteins mediate the attachment of the
virus to specific receptors on host cell surface. This
specific interaction is the major determinant of
species and organ specificity.
• Outer viral proteins are also important antigens
that induce neutralizing antibody and activate
cytotoxic T cells to kill virus-infected cells.

24. Used to describe subcategory of virus based on its
surface antigens.
• Some viral proteins are structural e.g., capsid
proteins.
• Some proteins are enzymes e.g., polymerases

25. • Envelope is a lipoprotein membrane composed of
lipid derived from host cell membrane and protein
that is virus specific.
• Matrix proteins mediate the interaction between
the capsid proteins and the envelope.
• The viral envelop is acquired as the virus exits from
the cell in a process called budding.

26. conti……
• All viruses that are transmitted by the fecal-oral
route do not have an envelope.
• Enveloped viruses are most often transmitted by
direct contact, such as blood or by sexual
transmission.

28. Classification of Viruses
• Classification of viruses is based on
1. Morphology – structure of capsid
– presence or absence of envelope
2. Size of the virion
3. Type of host/host structures the virus infected
• - Bacteriophages: infect bacterial cells
• - Plant viruses infect plant cells

29. • Animal viruses are sub grouped by the tissues they
attack:
1. Dermotrophic: if they infect the skin
2. Neurotrophic: if they infect nerve tissue
4. Genome composition – DNA / RNA
– ds/ss DNA and ds/ss RNA

30. 5. Groupings by Transmission Mechanism
• Enteric viruses: fecal-oral route
• Respiratory viruses: aerosols
• Zoonotic agents
• Biting
• Respiratory route
• Sexually-transmitted

31. • Transfection
Transfection is the process of deliberately introducing
naked or purified nucleic acids into eukaryotic cells
• Transformation
• Transformation is the genetic alteration of a cell
resulting from the direct uptake and incorporation of
exogenous genetic material from its surroundings
through the cell membrane.

32. • Permissive host
Cells which support complete virus life cycle with
production of infectious virus particles
• Nonpermissive host
Cells which allow only part of virus life cycle

33. • Productive infection
Infection that results in production of infective virus
particles
• Non productive infection
Infections that have no infectious virus, cells may be
transformed

34. • Cytopathic effects
virus-induced damage to the cell that alters its
microscopic appearance
Inclusion bodies- compacted masses of viruses or
damaged cell organelles

35. Figure 6.16