Morphology and structure of viruses; origin of viruses; viral classification; Viral properties

1. VIRUSES: BASICSVIRUSES: BASICS
SHYAM KUMAR MISHRA
(IOM)

2. Classification scheme
Microbial world
Organisms Infectious agents
(Living) (Nonliving)
Bacteria Archae Eukarya
(Prokaryotes)
Algae Protozoa Fungi Helminthes
Viruses Viroids PrionsVIRUSESVIRUSES

3. BeijerinckBeijerinck
19211921
Virus (Latin- poison and
other noxious substances,
first used in English in 1392)
"Organisms at the edge of
life”
At the twilight zone
separating the “living” from
the “nonliving”.
Living chemicals ????
Introduction

5. Introduction
 VirusesViruses - Obligate intracellular infectious agents
(20-400 nm in diameter) containing only one kind
of nucleic acid (DNA or RNA as their genome)
 1000 nm = 1 µm
 Morphological definitions:
Capsid- Protein shell or coat that encloses the
nucleic acid genome
Capsomers- Morphologic units composing the
capsid; Shapes - vary (spherical/wedge)

7.  Envelope- A lipid containing membrane that
surrounds some virus particles and are acquired
during viral maturation by a budding process
through a cellular membrane of host cell
 Peplomers- Virus encoded glycoprotein exposed
on the surface of the envelope (also known as
spikes)
 Nucleocapsid- Capsid together with the
enclosed N/A

8.  VirionVirion – Complete virus particle
Herpes, Orthomyxo viruses Nucleocapsid + Envelope
Papova viruses, Picorna viruses Nucleocapsid
Intactness of structure and the property of
infectiousness

9. Origin of viruses
Regressive hypothesis (degeneracy
hypothesis)
Viruses are remnants of cellular organisms.
Progressive hypothesis or Cellular
origin hypothesis (vagrancy
hypothesis)
Some viruses may have evolved from bits of DNA / RNA that
"escaped" from the genes (plasmids or transposons) of a larger
organism.
Coevolution hypothesis (virus-first
hypothesis)
Viruses may have evolved from complex molecules of protein and
nucleic acid at the same time as cells first appeared on earth.

11. Morphological Properties
 Size:
 20-400 nm in diameter
 Shape:
 Mostly roughly spherical
 Rabies virus – bullet shaped
 Pox virus – brick shaped
 TMV – rod shaped

12.  Structure/ Chemical composition:
 NUCLEIC ACID CORE
 VIRAL CAPSID
 ENVELOPE
 VIRAL PROTEINS

13.  NUCLEIC ACID CORE (GENOME):
 StructureStructure: contains either DNA or RNA
 SizeSize: 3-3000 kbp
 StrandednessStrandedness: either single-stranded (RNA viruses
except Reo-) or double-stranded (DNA viruses
except Parvo-)
 SegmentationSegmentation: NA as single or several segments
(Influenza virus has 8 RNA molecules, each
carrying different genetic information. Such viruses
are termed segmented viruses)

14.  VIRAL CAPSID:
COMPOSITION
 CapsomersCapsomers: morphological subunits
FUNCTIONS
 Protection from physical, chemical and
enzymatic damage
 Recognition of the host cell- (Viral attachment
protein-VAP and cellular receptor)

15. Symmetry
IcosahedralIcosahedral
HelicalHelical
HelicalHelical
Complex

16.  ENVELOPE:
 Lipid bilayer (derived from host cell membrane) and
virus-coded gp (peplomers) as spikes
 Matrix proteins: internal virion proteins that links the
internal nucleocapsid
 Glycoproteins: Transmembrane proteins, anchored to
the membrane; also includes transport channels (ion-
channels)
 Influenza virus- Hemagglutinin and Neuraminidase

17. ENVELOPE FUNCTIONS
 Promote interaction with nucleocapsid proteins (final
stage of assembly)
 Act as VAPs to cellular receptors
 Major viral antigens
 Interaction (spikes) with neutralizing antibodies
Tegument: Herpesvirus nucleocapsid are enveloped but
have a featureless layer of tegument interposed
between the nucleocapsid and envelope.
Function ????

18.  VIRAL PROTEINS:
 Structural proteinsStructural proteins: essential for the formation
of new viral particles.
 Some are associated with NA
 Some form protective layers (capsid and envelope)
 VAP
 Antigenicity
 EnzymesEnzymes: essential for initiation of replication
 Polymerases, Proteases, Endonucleases

20. Chemical Properties - Resistance
 Temperature:
 Enveloped viruses are more heat-labile
 Inactivated at 560
C-30’ (exception- HBV) and few sec at 1000
C.
 Some may be preserved by lyophilization
 Salts:
 1 M salt solution e.g., MgCl2, MgSO4
 pH:
 Usu. stable between pH 5 and 9
 Enteroviruses – resistant to acidic conditions

21.  Radiation: UV and ionising radiation inactivates viruses
 Photodynamic Inactivation
 Vital dyes- Toluidene blue; Visible light - Inactivation
 Disinfectants
 Most viruses are destroyed by oxidising agents such as
chlorine, iodine and hydrogen peroxide. However, majority of
viruses are resistant to phenol; chlorination does not always
inactivate enteroviruses or hepatitis viruses, particularly if
present with organic or faecal material.

22. Viral hemagglutination
 Hirst (1941)
 When viral particles are added to a red cell suspension, the
virus particles are adsorbed to each red cell, bridge
between them and bind them together forming a lattice
Hemagglutinin + N-acetyl neuraminic acid
(VIRUS + RBC)
 Agglutination occurs rapidly at 0o
C
 Eluted at 37o
C
 About 107
influenza virions are needed to cause
hemagglutination visible to the naked eye.

24.  Neuraminidase (Receptor destroying enzyme)
Virus – No alteration
Destroy receptor on RBCs
Destruction of the surface receptors result in the reversal of
haemagglutination and the release of the virus from the
RBC cell surface.
 Epithelial cells of the respiratory tract have the same
properties as those of RBCs so far as virus adsorption is
concerned.
 Influenza virus normally remain confined to the respiratory
tract because the protease enzyme that cleave HA are
common only at those sites.

25.  Hemagglutination inhibition
 Virus + Specific antiserum = Hemagglutination
 Influenza, Parainfluenza viruses – Fowl, Guinea pig, Human
RBCs
Adenovirus – Monkey, Mice
Rabies virus – Pigeon

26. Classification-Basis
Morphology
Genome properties
Physicochemical properties
Protein properties
Strategy of replication
Antigenic properties
Biologic properties
Host specificity

27. Animal or plant virus
(N=4000)
56 Families
233 genera

28. ThankThank
youyou